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An alpha-amino acid contains an alpha carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable side chain (R group). At ordinary biological pH, the amino and carboxyl groups are generally ionized.
Except for glycine, standard amino acids have a chiral alpha carbon. Absolute configuration is assigned by the Cahn-Ingold-Prelog R/S system; most proteinogenic L-amino acids are S, while cysteine is R because sulfur changes priority ordering.
L/D notation describes relative configuration to glyceraldehyde and is not a statement about optical rotation. Ribosomal proteins are built predominantly from L-amino acids.
Glycine has two hydrogens attached to its alpha carbon, so the alpha carbon is not stereogenic. Its small side chain also gives glycine unusual conformational flexibility in proteins.
An amino acid can simultaneously contain a positively charged ammonium group and a negatively charged carboxylate group. Such a dipolar ion is a zwitterion and may have net charge zero despite containing formal charges.
When pH is below a group’s pKa, its protonated form predominates; when pH is above pKa, its deprotonated form predominates. The net charge of an amino acid is the sum of charges on all ionizable groups.
The isoelectric point (pI) is the pH at which the molecule’s average net charge is zero. For an amino acid without an ionizable side chain, pI is the average of the two pKa values surrounding the zwitterionic form; acidic or basic side chains change which pKa values are used.
Aspartate and glutamate have acidic carboxylate side chains; lysine, arginine, and histidine have basic nitrogen-containing side chains. Their protonation and charge depend on pH and side-chain pKa.
Hydrophobic side chains interact poorly with water and tend to be buried in soluble proteins, while polar or charged hydrophilic side chains interact favorably with water and often appear on exposed surfaces. Membrane proteins can reverse this pattern in transmembrane regions.
Amino-acid side chains contain functional groups such as hydroxyls, thiols, amides, carboxylates, amines, guanidinium groups, imidazole rings, aromatics, and hydrocarbons. These groups govern hydrogen bonding, acid-base chemistry, nucleophilicity, oxidation-reduction, and post-translational modification.
Two cysteine thiol groups can be oxidized to form a covalent disulfide bond; the linked pair is called cystine. Reduction reverses the linkage. Disulfides can stabilize protein tertiary or quaternary structure, especially in oxidizing extracellular environments.
A peptide bond is an amide linkage formed between the alpha-carboxyl group of one amino acid and the alpha-amino group of another, with loss of water in the formal condensation description. Polypeptides have an N-terminus and a C-terminus and are conventionally written N to C.
Resonance gives the peptide C-N bond partial double-bond character, making the peptide unit approximately planar and restricting rotation around that bond. Most backbone flexibility comes from rotations about adjacent bonds.
Hydrolysis cleaves peptide bonds by adding the components of water, ultimately yielding shorter peptides or free amino acids. The uncatalyzed reaction is slow under physiological conditions, while proteases or harsh acid/base conditions accelerate cleavage.
The standard proteinogenic amino acids are distinguished by their side chains. MCAT-level fluency includes recognizing names and structures and, when used in sequences or data tables, conventional three-letter and one-letter abbreviations.
A peptide has an N-terminus, a C-terminus, and any ionizable side chains. Its net charge at a stated pH is the algebraic sum of the predominant charges on those groups.
Primary structure is the linear amino-acid sequence of a polypeptide, joined by peptide bonds and written from the N-terminus to the C-terminus. Sequence constrains all higher levels of structure. Disulfide bonds are covalent cross-links, but in MCAT structure-level questions they are usually analyzed as contributors to tertiary or quaternary structure rather than as the defining feature of primary structure.
Secondary structure consists of local, regular backbone conformations stabilized mainly by hydrogen bonds between peptide carbonyl oxygens and amide hydrogens. Major patterns include alpha helices, beta sheets, and turns.
An alpha helix is typically a right-handed coil stabilized by intrachain backbone hydrogen bonds. Side chains project outward, and helix stability is influenced by residue identity, charge pattern, and helix-breaking residues.
Beta sheets consist of extended beta strands stabilized by interstrand backbone hydrogen bonds and may be parallel or antiparallel. Turns reverse chain direction and often accommodate residues such as glycine or proline.
Tertiary structure is the overall three-dimensional arrangement of a single polypeptide, including packing of secondary structures and long-range side-chain interactions. It is stabilized by hydrophobic interactions, hydrogen bonds, ionic interactions, van der Waals forces, and sometimes disulfide bonds.
Quaternary structure describes the number, identity, and spatial arrangement of multiple polypeptide subunits in a functional protein complex. Subunits may be identical or different and associate through noncovalent interactions and sometimes disulfides.
A domain is a compact structural and often functional unit within one polypeptide that can sometimes fold semi-independently. A motif is a recurring smaller sequence or structural pattern associated with a function or fold.
Proline’s side chain bonds to the backbone nitrogen, restricting backbone rotation and removing the usual amide hydrogen. It can disrupt alpha helices, promote turns, and impose conformational constraints.
Cystine disulfide linkages can lock distant parts of one chain together or connect separate chains, thereby stabilizing tertiary or quaternary structure.
Nonpolar groups tend to cluster away from water in soluble proteins. Folding releases ordered water from hydrophobic surfaces, often increasing solvent entropy and favoring a compact hydrophobic core.
Protein folding is the process by which a polypeptide adopts a functional conformational ensemble determined primarily by sequence and environment. Folding reflects a balance of enthalpic interactions and entropy changes in both the chain and solvent.
Denaturation disrupts native secondary, tertiary, and/or quaternary structure through heat, extreme pH, detergents, chaotropes, organic solvents, or reducing conditions. It usually leaves peptide-bond primary structure intact unless conditions also cause hydrolysis.
Water molecules organize around exposed polar, charged, and hydrophobic surfaces. Burial of hydrophobic surface during folding can release constrained water, increasing solvent entropy, while the polypeptide itself loses conformational entropy.
Protein function depends on three-dimensional arrangement of chemical groups. Mutations or environmental changes can alter folding, dynamics, binding sites, catalysis, localization, or assembly even when most of the sequence is unchanged.
A protein’s isoelectric point is the pH at which its total net charge is zero, reflecting all ionizable groups and their environments. Below pI a protein tends to be net positive; above pI it tends to be net negative.
Electrophoresis separates charged molecules by movement through an electric field. Migration depends on net charge, field direction, frictional resistance, matrix properties, and whether the protein remains native or is denatured.
SDS denatures proteins and coats polypeptide chains with a roughly uniform negative charge-to-mass ratio, so migration through polyacrylamide depends primarily on polypeptide size. Reducing agents can additionally cleave disulfide bonds.
Native PAGE preserves much of protein conformation and charge, so migration reflects size, shape, and charge. Isoelectric focusing separates proteins in a pH gradient until each reaches its pI, where net migration ceases.
Proteins can bind ligands through complementary shape, charge, hydrogen bonding, hydrophobic interactions, and other noncovalent forces. Binding is reversible unless a covalent mechanism is specifically involved and can be characterized by affinity and saturation.
Transport proteins bind specific solutes and carry them between locations or states without chemically transforming them. Reversible binding, conformational change, and competition can govern loading and release.
Immune proteins perform recognition, binding, signaling, transport, and effector functions. Antibodies exemplify highly specific antigen binding, while many other immune proteins form receptors, complement components, cytokines, and structural complexes.
Motor proteins convert chemical energy, commonly from ATP hydrolysis, into directed mechanical work through cycles of binding and conformational change. Directionality and track specificity depend on protein structure and cellular context.
Some amino-acid sequences and assemblies provide mechanical strength, elasticity, scaffolding, or resistance to deformation. Their function depends on repeated motifs, cross-links, and higher-order organization.
Enzymes accelerate biological reactions by providing a lower-activation-energy pathway and stabilizing transition states. They affect reaction rate but do not change the overall free-energy change or equilibrium constant and are regenerated after the catalytic cycle.
Enzymes are classified by the reactions they catalyze. The traditional MCAT-level framework uses oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases; class names often indicate the transferred group or reaction. Modern IUBMB nomenclature also recognizes translocases, but the six-class framework remains the appropriate baseline for interpreting the 2020 AAMC outline unless a passage supplies newer terminology.
Activation energy is the energy barrier between reactants and the transition state. Enzymes lower this barrier by orienting substrates, stabilizing charge development, providing favorable microenvironments, straining bonds, or forming transient intermediates.
An enzyme recognizes one substrate or a related group of substrates through complementary interactions in the active site. Specificity can be chemical, regioselective, and stereoselective, while some enzymes retain measurable activity on alternative substrates.
The active site is a three-dimensional region formed by residues that may be distant in primary sequence. It binds substrate and positions catalytic groups in a microenvironment distinct from bulk solution.
The lock-and-key model emphasizes preexisting complementarity, whereas induced fit emphasizes conformational changes in enzyme and/or substrate upon binding that create a catalytically competent state. Real enzymes may combine both features.
Enzymes use combinations of acid-base catalysis, covalent catalysis, electrostatic or metal-ion catalysis, proximity/orientation, and transition-state stabilization. The same active site can employ several strategies during one reaction.
A cofactor is a nonprotein component required for activity, often a metal ion or an organic molecule. It can participate in charge stabilization, redox chemistry, group transfer, substrate orientation, or structural stabilization.
Coenzymes are organic cofactors that carry electrons or chemical groups during catalysis. They may bind transiently as cosubstrates or tightly as prosthetic groups and are regenerated in the broader metabolic network.
Many water-soluble vitamins, especially B vitamins, are precursors to coenzymes used in electron transfer, acyl transfer, one-carbon transfer, amino-group transfer, carboxylation, and rearrangement reactions. Deficiency can reduce activities of multiple enzymes sharing the derived coenzyme.
Vitamin-derived coenzymes expand the chemistry available to enzyme active sites by carrying electrons or transferable groups. The relevant testable relationship is usually the coenzyme’s reaction role, not a clinical deficiency catalogue.
Increasing temperature generally increases molecular collision rates and catalytic rate up to a range where loss of native structure becomes important. Beyond that range, denaturation can reduce activity.
pH alters protonation of catalytic residues, substrates, and binding groups; ionic strength and salts can alter electrostatic interactions and protein stability. Enzymes often show a characteristic activity range rather than one universally fixed optimum.
Enzyme kinetics measures reaction rate as a function of substrate, enzyme, inhibitors, and conditions. Initial rates are often used to minimize complications from substrate depletion, product accumulation, and reverse reaction.
For a simple enzyme following Michaelis-Menten behavior, initial velocity rises hyperbolically with substrate concentration and approaches Vmax as active sites become saturated.
Vmax is the limiting initial rate at saturating substrate and is proportional to the total concentration of active enzyme when catalytic conditions are unchanged.
Km is the substrate concentration at which v0 equals half Vmax in the Michaelis-Menten model. Under specific mechanistic conditions it can approximate a dissociation constant, but it is fundamentally a kinetic parameter.
kcat is the maximum number of substrate molecules converted per active site per unit time under saturating substrate. kcat/Km compares catalytic performance at low substrate concentration and is often termed catalytic efficiency.
A double-reciprocal plot linearizes the Michaelis-Menten equation, with y-intercept 1/Vmax, x-intercept −1/Km, and slope Km/Vmax. It can illustrate inhibitor patterns but overweights low-substrate data.
Cooperativity occurs when binding or activity at one site changes the behavior of other sites, commonly in multimeric proteins. Positive cooperativity produces a sigmoidal response and can create switch-like sensitivity over a concentration range.
Feedback regulation occurs when a downstream product or state alters an earlier step in a pathway, often inhibiting a committed or rate-controlling enzyme when product is abundant. Feedback can also be activating in some networks.
A competitive inhibitor reduces substrate access to the free enzyme, often by binding the active site. In the ideal reversible model, apparent Km increases while Vmax is unchanged because sufficiently high substrate can outcompete inhibitor.
An uncompetitive inhibitor binds only the enzyme-substrate complex in the ideal model. Both apparent Km and Vmax decrease by the same factor, giving parallel Lineweaver-Burk lines.
A mixed inhibitor binds both free enzyme and enzyme-substrate complex with different affinities. Vmax decreases; apparent Km increases if inhibitor favors free enzyme and decreases if it favors ES.
Pure noncompetitive inhibition is the special mixed case in which inhibitor binds free enzyme and ES equally. Vmax decreases while apparent Km remains unchanged in the ideal model.
Allosteric regulation occurs when a ligand binds at a regulatory site and shifts the conformational ensemble, changing activity or affinity at another site. Effectors may activate or inhibit, and cooperative responses are common.
Reversible covalent modifications such as phosphorylation can alter enzyme conformation, localization, interactions, stability, or activity. Dedicated enzymes install and remove the modification, enabling regulated switching.
A zymogen is an inactive or less-active enzyme precursor activated by specific proteolytic cleavage. Cleavage changes primary structure and often causes an essentially irreversible activation step that must be controlled by synthesis, localization, inhibitors, or degradation.
Cells regulate enzyme activity through substrate availability, product removal, reversible inhibitors, allosteric effectors, covalent modification, proteolytic activation, enzyme abundance, localization, and environmental conditions. Multiple controls can act simultaneously on different timescales.
A nucleoside consists of a nitrogenous base linked to a pentose sugar; a nucleotide is a nucleoside with one or more phosphate groups.
Purines are two-ring bases (adenine and guanine); pyrimidines are one-ring bases (cytosine, thymine, and uracil).
Nucleotides in a strand are joined by 3-prime-to-5-prime phosphodiester bonds, producing a directional sugar-phosphate backbone.
DNA generally contains deoxyribose and thymine and is usually double-stranded; RNA contains ribose and uracil and is often single-stranded with extensive secondary structure.
Canonical DNA consists of two antiparallel strands coiled into a double helix with bases stacked internally and sugar-phosphate backbones externally.
In DNA, adenine pairs with thymine through two hydrogen bonds and guanine pairs with cytosine through three; in RNA, adenine commonly pairs with uracil.
The linear base sequence stores information; complementary pairing permits one strand to serve as a template for synthesis of another polymer.
Denaturation separates complementary strands by disrupting noncovalent interactions without ordinarily cleaving the phosphodiester backbone.
Complementary single strands can reassociate when conditions permit; hybridization is sequence-specific pairing between nucleic-acid strands.
Each daughter DNA duplex contains one parental strand and one newly synthesized strand.
Replication begins at defined origins and proceeds bidirectionally through replication forks; eukaryotic chromosomes use many origins to complete replication efficiently.
Helicase separates strands, topoisomerase relieves torsional strain, and single-strand binding proteins stabilize exposed templates.
DNA polymerases require a preexisting 3-prime hydroxyl; primase synthesizes short RNA primers de novo.
DNA polymerases select complementary deoxyribonucleotides and extend DNA only 5-prime-to-3-prime by adding to a 3-prime hydroxyl; many also proofread.
Because templates are antiparallel and polymerases synthesize 5-prime-to-3-prime, one new strand is made continuously toward the fork and the other discontinuously away from it.
Lagging-strand DNA is synthesized as Okazaki fragments; RNA primers are removed and replaced, and DNA ligase seals remaining backbone nicks.
Removal of the terminal RNA primer on a linear chromosome leaves an end that conventional polymerase cannot fully replace, causing progressive shortening without a compensating mechanism.
Telomerase is a ribonucleoprotein reverse transcriptase that uses its internal RNA template to extend chromosome ends, enabling subsequent DNA synthesis.
Many replicative polymerases detect mispaired 3-prime termini, remove the incorrect nucleotide with proofreading exonuclease activity, and resume synthesis.
Mismatch repair detects replication errors that escaped proofreading, removes a segment from the newly synthesized strand, and resynthesizes it.
Base excision repair removes a chemically altered or inappropriate base, creates an abasic site, and replaces the affected nucleotide using glycosylase, endonuclease, polymerase, and ligase activities.
Nucleotide excision repair removes a short oligonucleotide containing a bulky, helix-distorting lesion, then fills and seals the gap.
Double-strand breaks can be repaired by template-guided homologous recombination or by direct end joining, which is generally more error-prone.
In ordinary cellular gene expression, DNA is transcribed into RNA and RNA is translated into protein; reverse transcription is a recognized exception to the usual DNA-to-RNA direction.
mRNA is read in nonoverlapping groups of three nucleotides called codons, each specifying an amino acid or stop signal.
Most amino acids are encoded by more than one codon, so the code is degenerate but unambiguous: each codon normally specifies only one amino acid.
A tRNA anticodon pairs antiparallel with an mRNA codon; flexibility at the third codon position permits some tRNAs to recognize multiple synonymous codons.
AUG usually serves as the start codon and encodes methionine; UAA, UAG, and UGA are stop codons recognized by release factors rather than tRNAs.
A missense substitution changes an amino acid, a nonsense substitution creates a premature stop, a silent substitution preserves the amino acid, and an indel not divisible by three causes a frameshift.
mRNA carries a coding sequence from DNA to ribosomes and includes untranslated regions and signals affecting translation and stability.
tRNA is an adaptor with an anticodon and a 3-prime amino-acid attachment site; aminoacyl-tRNA synthetases charge each tRNA with the appropriate amino acid using ATP.
rRNA provides structural and catalytic functions in ribosomes; some RNA molecules, including rRNA and self-splicing RNAs, act as ribozymes.
RNA polymerase binds promoter-associated machinery, initiates RNA synthesis, elongates 5-prime-to-3-prime using a DNA template, and terminates at appropriate signals.
RNA polymerase reads the template strand 3-prime-to-5-prime; the RNA sequence matches the coding strand 5-prime-to-3-prime except U replaces T.
A eukaryotic primary transcript is commonly modified by 5-prime capping, 3-prime cleavage/polyadenylation, and intron removal before export and translation.
The 5-prime cap and 3-prime poly(A) tail protect eukaryotic mRNA, promote export and translation, and influence stability.
Introns are removed from pre-mRNA and exons are joined by the spliceosome, a complex containing snRNAs and proteins assembled as snRNPs.
Alternative selection of splice sites allows one pre-mRNA to generate multiple mature mRNAs and protein isoforms.
Introns can harbor regulatory elements and noncoding RNAs, facilitate alternative splicing, and permit exon shuffling over evolutionary time.
Ribosomes contain small and large subunits; tRNAs move through A, P, and E sites as the ribosome reads mRNA 5-prime-to-3-prime and synthesizes protein N-terminus to C-terminus.
Initiation factors assemble the small subunit, initiator tRNA, mRNA, and large subunit at the correct start codon.
During elongation, aminoacyl-tRNA selection, peptidyl transfer, and translocation repeat; GTP-driven factors assist fidelity and movement.
A stop codon in the A site is recognized by a release factor, which promotes hydrolysis of the completed polypeptide from the tRNA and ribosome disassembly.
Multiple ribosomes can translate one mRNA simultaneously as a polysome; in prokaryotes, translation can begin while transcription is still occurring.
New polypeptides may be cleaved, folded, covalently modified, or targeted to specific compartments after or during translation.
Proteolytic cleavage can remove signal peptides or activate precursor proteins and zymogens.
Covalent modifications such as phosphorylation, glycosylation, acetylation, methylation, ubiquitination, and lipidation can alter activity, interactions, stability, or localization.
Eukaryotic DNA wraps around histone octamers to form nucleosomes, compacting DNA while regulating accessibility.
Euchromatin is generally less condensed and more transcriptionally accessible; heterochromatin is generally more condensed and less active.
DNA twisting and unwinding generate supercoiling; topoisomerases alter DNA topology to relieve or introduce supercoils.
Genomes contain unique sequences and repeated sequences ranging from tandem repeats to dispersed elements.
The centromere is a specialized chromosomal region where the kinetochore forms and spindle microtubules attach during chromosome segregation.
Telomeres are repetitive DNA-protein structures at linear chromosome ends that protect ends from degradation and mistaken recognition as breaks.
An operon places multiple functionally related genes under shared regulatory DNA and can produce a polycistronic mRNA.
In negative control, a repressor decreases transcription by binding regulatory DNA; an inducer or corepressor can alter repressor activity.
In positive control, an activator increases transcription by helping recruit or stabilize RNA polymerase at a promoter.
The lac operon is induced when lactose-derived allolactose reduces repressor binding. It is also positively regulated by CAP-cAMP: low glucose raises cAMP, promotes CAP binding near the promoter, and increases transcription. Maximal expression therefore requires lactose present and glucose low.
The trp operon is typically repressible: tryptophan acts as a corepressor, enabling repressor binding when the end product is abundant.
Promoters, enhancers, silencers, and other cis-elements regulate transcription through binding of trans-acting factors and the transcriptional machinery.
Chromatin-remodeling complexes and histone modifications alter nucleosome positioning or interaction, changing DNA accessibility.
DNA methylation, especially at promoter-associated CpG regions in many eukaryotic contexts, can reduce transcription and contribute to heritable expression states.
Gene amplification increases copy number within cells; duplication creates additional gene copies that can diverge over evolutionary time.
Noncoding RNAs can regulate transcription, RNA stability, translation, splicing, and chromatin state; small RNAs may guide sequence-specific silencing.
Cells regulate gene output by controlling RNA processing, export, localization, stability, and translation efficiency.
Proto-oncogenes normally promote regulated growth or survival; activating mutation, amplification, or inappropriate expression can convert them into oncogenes.
Tumor suppressors restrain proliferation, promote repair, or trigger apoptosis; loss of function can permit abnormal growth and often requires inactivation of both cellular copies.
Restriction endonucleases recognize specific DNA sequences and cleave phosphodiester backbones, producing sticky or blunt ends useful for DNA manipulation.
Gene cloning inserts a DNA fragment into a vector that can replicate in a host; expression requires appropriate regulatory sequences and compatible cellular machinery.
A genomic library represents genomic DNA, including introns and regulatory/intergenic sequences; a cDNA library is reverse-transcribed from mature RNA and reflects genes expressed in the source cells.
Reverse transcriptase synthesizes DNA from an RNA template; subsequent synthesis can produce double-stranded cDNA.
PCR uses repeated denaturation, primer annealing, and extension by a thermostable DNA polymerase to amplify a defined DNA region.
RT-PCR first converts RNA to cDNA; quantitative PCR monitors product accumulation to estimate starting template abundance.
Negatively charged nucleic acids migrate toward the positive electrode through a gel; smaller fragments generally move farther under comparable conditions.
Southern blotting separates DNA fragments, transfers them to a membrane, and detects a target sequence with a labeled complementary probe.
DNA sequencing determines nucleotide order. In chain-termination sequencing, incorporation of labeled dideoxynucleotides terminates extension because they lack a 3-prime hydroxyl.
Gene expression can be assessed by measuring RNA abundance with complementary probes or sequencing; each method compares transcript presence or quantity rather than protein activity directly.
Gene function can be inferred by perturbing expression or sequence and observing phenotype, ideally with controls and rescue experiments.
Labeled nucleic-acid probes identify complementary targets; temperature, salt, and sequence match determine hybridization stringency.
Stem cells can self-renew and differentiate; potency describes the range of cell types they can generate, from totipotent to pluripotent to multipotent.
DNA technology supports diagnostics, recombinant pharmaceuticals, gene therapy, forensic identification, environmental applications, and agriculture.
Gene therapy introduces, replaces, edits, or regulates genetic material in target cells to produce a therapeutic effect; delivery efficiency, specificity, persistence, and immune response constrain outcomes.
Responsible use of DNA technology requires attention to informed consent, privacy, equitable access, off-target effects, ecological impact, dual use, and distinctions between somatic and heritable modification.
DNA is the principal hereditary material in cellular organisms because it stores sequence information, can be copied with high fidelity, and can direct phenotype through gene expression.
Transformation is the stable acquisition of a heritable phenotype after uptake of genetic material from an external source.
Selective isotopic or molecular labeling can track whether DNA or protein enters cells and is transmitted to progeny.
Strong evidence identifies a candidate molecule, selectively perturbs it, and links that perturbation to loss or gain of a heritable phenotype.
A gene is a functional hereditary unit, a locus is its chromosomal position, and an allele is an alternative sequence variant at that locus.
Genotype is the allelic composition of an organism; phenotype is the observable or measurable outcome produced by genotype in an environmental context.
A diploid organism is homozygous at a locus when its two homologous alleles are the same and heterozygous when they differ.
Wild type denotes the reference or commonly observed phenotype or allele in a defined population or experimental system; mutant denotes a variant relative to that reference.
In a simple diploid model, a dominant allele determines the heterozygote phenotype, whereas a recessive allele is phenotypically evident only when no dominant allele is present.
Complete dominance occurs when the heterozygote phenotype is indistinguishable from one homozygote.
Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygotes.
Codominance occurs when both allelic products are distinctly expressed in the heterozygote.
A locus may have more than two alleles in a population even though a diploid individual carries no more than two at that locus.
Penetrance is the proportion of individuals with a specified genotype who display the associated phenotype.
Expressivity is the degree or pattern of phenotypic manifestation among individuals who express a genotype.
A leaky mutation retains partial gene-product function, producing a less severe or conditional phenotype than a complete loss-of-function allele.
The two alleles at a diploid locus segregate into different gametes during meiosis, so each gamete normally receives one allele.
Alleles at different loci assort independently when the loci are on different chromosomes or sufficiently far apart on the same chromosome.
The product rule combines independent events, the sum rule combines mutually exclusive routes, and conditional probability updates outcomes given known information.
A testcross mates an individual with a dominant phenotype but unknown genotype to a homozygous recessive tester to reveal the unknown genotype through offspring.
The parental (P) generation produces the first filial (F1) generation; intercrossing or selfing F1 individuals produces F2.
Hybridization crosses genetically distinct individuals or populations; viability and fertility of hybrids determine whether gene flow can occur across those groups.
A gene pool is the total set of alleles present in a breeding population.
Meiosis reduces chromosome number from diploid to haploid and generates genetically variable gametes, allowing fertilization to restore the species-typical diploid number.
Mitosis usually produces two genetically similar cells with conserved ploidy, whereas meiosis produces four haploid products after homolog pairing, recombination, and two divisions.
Homologous chromosomes carry the same loci but may contain different alleles; sister chromatids are replicated copies of one chromosome.
During prophase I, homologous chromosomes pair by synapsis and are stabilized by the synaptonemal complex, enabling aligned recombination.
A tetrad or bivalent is a paired set of homologous replicated chromosomes containing four chromatids.
Crossing over exchanges corresponding DNA segments between nonsister chromatids of homologous chromosomes during prophase I; chiasmata are visible manifestations of crossover connections.
A single crossover between two loci typically produces two recombinant and two parental chromatids within one tetrad.
Two crossovers can restore the parental arrangement of outer markers while changing an internal marker, causing some recombination events to be missed in two-point mapping.
Each homolog pair orients independently at metaphase I, producing 2^n possible chromosome combinations in gametes before considering crossing over.
Linked genes lie on the same chromosome and tend to be inherited together unless crossing over separates them.
Recombination frequency equals recombinant offspring divided by total offspring, multiplied by 100%; for short intervals, 1% recombination approximates 1 map unit or centimorgan.
Sex-linked traits are determined by genes on sex chromosomes, most commonly X-linked loci with distinctive transmission patterns.
The Y chromosome contains relatively few genes; Y-linked traits are transmitted from affected fathers to all sons and no daughters.
In the common human XY system, eggs carry X and sperm carry X or Y; the presence and function of Y-linked sex-determining pathways influence gonadal development.
Extranuclear genes, especially mitochondrial genes, are inherited through cytoplasmic organelles and therefore often show maternal inheritance in animals.
Random union of genetically varied gametes multiplies the number of possible zygotic genotypes.
Nondisjunction is failure of homologs in meiosis I or sister chromatids in meiosis II to separate properly, producing gametes with abnormal chromosome numbers.
Independent assortment, crossing over, mutation, and random fertilization create variation among offspring; selection and drift then alter variant frequencies across generations.
A mutation is a change in DNA sequence; it can arise spontaneously or from mutagen exposure and may occur in germline or somatic cells.
A base substitution replaces one nucleotide pair and may be silent, missense, nonsense, or affect regulatory/splicing sequences.
Insertions or deletions add or remove DNA; within a coding region, changes not divisible by three shift the reading frame.
An inversion reverses a chromosome segment; a translocation moves or exchanges a segment to a nonhomologous chromosome or new location.
Incorrect base pairing during replication can become a permanent mutation if not corrected before the next round of replication.
The fitness effect of a mutation depends on environment, genetic background, dominance, and life stage; many mutations are effectively neutral.
An inborn error of metabolism is an inherited defect in an enzyme or pathway that causes substrate accumulation, product deficiency, or toxic side products.
Mutagens increase mutation frequency; carcinogens increase cancer risk. Many carcinogens are mutagenic, but the categories are not identical.
Mutations can arise spontaneously from replication chemistry or endogenous damage, or be induced by external mutagens; in either case, the resulting sequence change is not directed toward an organism’s adaptive needs.
A transcription error produces an incorrect RNA molecule and a translation error produces an incorrect polypeptide, but neither is a heritable DNA mutation unless the underlying DNA sequence is also changed.
Genetic drift is random change in allele frequencies due to finite population sampling, with stronger effects in small populations.
A bottleneck sharply reduces population size, leaving a nonrepresentative subset of the original gene pool and often reducing genetic diversity.
The founder effect occurs when a new population is established by a small, nonrepresentative sample of a source population.
Gene flow transfers alleles between populations through migration and reproduction, often reducing divergence between populations.
Inbreeding is mating between relatives and increases homozygosity without by itself changing allele frequencies in the idealized first-order model.
Outbreeding is mating between less-related individuals or populations and often increases heterozygosity; very divergent crosses can sometimes reduce fitness by disrupting local adaptation or coadapted gene combinations.
A polymorphism is the coexistence of two or more common, heritable variants in a population.
Hardy-Weinberg equilibrium is a null model requiring random mating, very large population size, no selection, no mutation, and no migration for the locus considered.
For two alleles, p + q = 1 and p² + 2pq + q² = 1, representing AA, Aa, and aa genotype frequencies under equilibrium.
Observed genotype counts can be compared with Hardy-Weinberg expectations calculated from observed allele frequencies; a statistical test can assess whether deviations exceed sampling variation.
Two-point mapping estimates distance between two loci; three-point mapping can infer gene order and reveal double crossovers by comparing parental, single-crossover, and double-crossover classes.
Biometry applies statistical reasoning to genetic data, including expected ratios, sampling error, confidence, and hypothesis testing.
Evolution at the population level is change in allele frequencies across generations.
Natural selection is nonrandom differential survival or reproduction associated with heritable phenotypic variation.
Evolutionary fitness is relative contribution of an individual or genotype to the next generation, often measured by viable fertile offspring.
If heritable variants produce different numbers of surviving offspring, their allele frequencies can change across generations.
Selection is usually analyzed through differential success of genes, individuals, or kin; group-level selection can occur when groups differ in persistence or reproduction, but group-benefit claims require evidence beyond individual advantage.
A lineage is evolutionarily successful when its genetic contribution becomes more represented in the next generation’s gene pool.
An adaptation is a heritable trait that increases fitness in a particular environment; specialization is adaptation to a narrower ecological role or resource set.
Speciation is the evolution of reproductive isolation such that populations no longer exchange genes effectively.
Prezygotic barriers prevent mating or fertilization; postzygotic barriers reduce hybrid viability or fertility after fertilization.
Allopatric speciation begins with geographic separation; sympatric speciation develops reproductive isolation without geographic separation, often through ecological divergence or chromosome changes.
Sequence differences can accumulate over time and, when mutation rates are sufficiently regular and calibrated, can provide a molecular estimate of divergence time.
Mutation creates new alleles, recombination reshuffles them, selection changes frequencies nonrandomly with fitness, drift changes frequencies stochastically, and gene flow moves alleles among populations.
Thermodynamics describes energy changes by defining a system, its surroundings, and state functions such as internal energy, enthalpy, entropy, and Gibbs free energy.
Energy is conserved; it can be transferred as heat or work and converted among chemical, thermal, mechanical, and electrochemical forms.
The total entropy of the universe tends to increase in spontaneous processes, even when a local biological structure becomes more ordered.
At constant temperature and pressure, Delta G = Delta H - TDelta S predicts thermodynamic favorability: negative is spontaneous, zero is equilibrium, and positive is nonspontaneous under the stated conditions.
Actual free-energy change depends on standard free energy and the reaction quotient: Delta G = Delta G degrees prime + RT ln Q for biochemical standard conditions.
At equilibrium Delta G = 0, Q = Keq, and Delta G degrees = -RT ln Keq (or biochemical Delta G degrees prime and K prime eq).
A system at equilibrium shifts to oppose a perturbation in concentration, pressure, or temperature, without changing the equilibrium constant unless temperature changes.
Endothermic/exothermic describe heat flow or Delta H, whereas endergonic/exergonic describe Delta G and spontaneity.
ATP contains adenine, ribose, and three phosphates; hydrolysis is favorable because products are stabilized by resonance, hydration, and reduced electrostatic repulsion.
Cells couple ATP hydrolysis to unfavorable reactions through a shared intermediate or coordinated mechanism so the summed Delta G is negative.
Oxidation is loss of electrons or hydrogen and often gain of oxygen; reduction is gain of electrons or hydrogen. Redox reactions pair an electron donor with an electron acceptor.
NAD+/NADH and NADP+/NADPH carry hydride equivalents, while FAD/FADH2 commonly participates as a tightly bound flavin cofactor accepting two electrons and two protons in stepwise or concerted redox chemistry.
Metabolic enzyme names often reveal reaction class: dehydrogenases are oxidoreductases, kinases are transferases, mutases and many epimerases are isomerases, hydrolases cleave bonds with water, lyases add or remove groups without hydrolysis or redox, and ligases join substrates using energy.
Carbohydrates are polyhydroxy aldehydes or ketones and their derivatives, classified by carbon number, carbonyl type, and number of sugar units.
D/L designation is determined by the configuration of the highest-numbered chiral carbon relative to glyceraldehyde in a Fischer projection.
Monosaccharides cyclize by intramolecular nucleophilic addition to form hemiacetals from aldoses or hemiketals from ketoses.
Six-membered pyranoses and five-membered furanoses adopt nonplanar conformations, with chair forms often minimizing steric strain.
Epimers are diastereomers that differ at exactly one stereocenter other than the distinction specifically created at the anomeric carbon.
Alpha and beta anomers differ at the anomeric carbon and interconvert through the open-chain form in solution, producing mutarotation when the anomeric center is free.
A glycosidic bond forms when an anomeric hydroxyl reacts with another nucleophile, creating an acetal/ketal linkage; hydrolysis cleaves the bond by adding water.
Monosaccharides are single sugar units; those capable of generating a free carbonyl under test conditions act as reducing sugars.
Disaccharides join two monosaccharides through defined glycosidic linkages; maltose and lactose retain a free anomeric carbon, whereas sucrose links both anomeric carbons and is nonreducing.
Polysaccharides differ in monomer linkage and branching: glycogen and starch use alpha glucose linkages for storage, while cellulose uses beta(1->4) glucose for structure.
Glycolysis occurs in the cytosol and converts one glucose to two pyruvate, with net production of 2 ATP and 2 NADH per glucose.
Phosphorylation of glucose to glucose-6-phosphate traps it in the cell and commits ATP while preserving branching options; hexokinase is broadly expressed, whereas liver/pancreatic glucokinase has higher Km and higher capacity.
PFK-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate and is the major committed, rate-regulated step of glycolysis.
GAPDH oxidizes glyceraldehyde-3-phosphate while reducing NAD+ and forming a high-energy acyl phosphate intermediate.
Phosphoglycerate kinase and pyruvate kinase transfer phosphate directly from metabolic intermediates to ADP, generating ATP independently of the ETC.
Pyruvate kinase catalyzes the irreversible conversion of phosphoenolpyruvate to pyruvate while generating ATP.
Pyruvate can be oxidized to acetyl-CoA, reduced to lactate, transaminated to alanine, or carboxylated to oxaloacetate depending on tissue and metabolic state.
Lactate fermentation oxidizes NADH to NAD+ while reducing pyruvate, allowing glycolysis to continue without net additional ATP beyond glycolysis.
Dietary starch and stored glycogen provide glucose units that enter central metabolism, often as phosphorylated intermediates after digestion or phosphorolysis.
Gluconeogenesis synthesizes glucose mainly in liver and kidney from lactate, glycerol, and glucogenic amino-acid carbon, consuming ATP/GTP and reducing power.
Gluconeogenesis bypasses irreversible glycolytic steps using pyruvate carboxylase plus PEP carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase.
The Cori cycle transfers lactate from anaerobic tissues to liver, where it is converted to glucose at an energetic cost and returned to peripheral tissues.
The cytosolic pentose phosphate pathway uses glucose-6-phosphate to produce NADPH, ribose-5-phosphate, and glycolytic intermediates without directly producing ATP.
PPP-derived NADPH supports reductive biosynthesis and maintains reduced glutathione, which helps detoxify reactive oxygen species.
The nonoxidative PPP interconverts pentose phosphates with fructose-6-phosphate and glyceraldehyde-3-phosphate, allowing cells to balance nucleotide precursor and NADPH needs.
A metabolic steady state maintains approximately constant intermediate concentrations while molecules continuously enter and leave pathways; it is not thermodynamic equilibrium.
Pathways are commonly controlled at irreversible, far-from-equilibrium steps, especially committed steps that direct a metabolite toward a particular route.
Opposing pathways are regulated reciprocally to prevent futile cycling, with energy status and hormones favoring either glucose use or glucose production.
Glycogenesis stores glucose as glycogen through activated UDP-glucose, glycogen synthase, and branching enzyme.
Glycogenolysis uses glycogen phosphorylase to release glucose-1-phosphate from alpha(1->4) bonds and debranching activity to resolve alpha(1->6) branches.
Insulin generally promotes glycogen synthesis, whereas glucagon in liver and epinephrine in liver/muscle promote glycogen breakdown through phosphorylation cascades; local metabolites provide tissue-specific allosteric control.
Relative ATP, ADP, and AMP levels signal cellular energy status; AMP can rise disproportionately when ATP falls because adenylate kinase interconverts adenine nucleotides.
Pathway flux depends on network context; changing one enzyme concentration may have little or large effect depending on its control coefficient and compensatory responses.
The mitochondrial pyruvate dehydrogenase complex irreversibly converts pyruvate to acetyl-CoA, producing CO2 and NADH and linking glycolysis to the TCA cycle.
The mitochondrial TCA cycle oxidizes the acetyl group of acetyl-CoA to CO2 while regenerating oxaloacetate and capturing energy in NADH, FADH2, and GTP.
Citrate synthase condenses acetyl-CoA with oxaloacetate to form citrate, coupling thioester hydrolysis to an effectively irreversible entry step.
Isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase release CO2 and produce NADH, coupling carbon oxidation to electron capture.
Succinyl-CoA synthetase generates GTP by substrate-level phosphorylation, while succinate dehydrogenase produces FADH2 and also functions as ETC Complex II.
TCA flux is stimulated by substrate availability and low-energy signals such as ADP/Ca2+ in appropriate tissues and inhibited by high ATP, NADH, citrate, or succinyl-CoA at key enzymes.
Anaplerotic reactions replenish TCA intermediates, whereas cataplerotic reactions withdraw them for biosynthesis; balanced replenishment is required to sustain oxidation.
Fatty acids are hydrocarbon chains with a terminal carboxyl group; chain length and degree/configuration of unsaturation influence packing, melting behavior, and oxidation yield.
Bile salts emulsify dietary lipids, pancreatic enzymes hydrolyze them, and intestinal cells reassemble and package lipids into chylomicrons for lymphatic and blood transport.
During fasting or stress, stored triacylglycerols are hydrolyzed to free fatty acids and glycerol; fatty acids travel bound to albumin, while glycerol can support hepatic gluconeogenesis.
Fatty acids are activated to fatty acyl-CoA at an ATP-equivalent cost and long-chain acyl groups enter the mitochondrial matrix through the carnitine shuttle.
Each beta-oxidation cycle oxidizes a fatty acyl-CoA, producing one FADH2, one NADH, one acetyl-CoA, and an acyl-CoA shortened by two carbons, except the final cleavage yields two acetyl-CoA.
Unsaturated fatty acids require auxiliary enzymes to reposition or reduce double bonds, and preexisting double bonds can bypass FAD-dependent oxidation, lowering energy yield relative to comparable saturated chains.
Odd-chain fatty acids end with propionyl-CoA, which can be converted to succinyl-CoA and contribute carbon to the TCA cycle and potentially gluconeogenesis.
During prolonged fasting, low-carbohydrate states, or uncontrolled diabetes, liver mitochondria convert excess acetyl-CoA to ketone bodies that are exported and oxidized by extrahepatic tissues.
Fatty acid synthesis occurs mainly in the cytosol in the fed state, uses acetyl-CoA-derived malonyl-CoA, fatty acid synthase, ATP, and NADPH, and is not simply beta oxidation in reverse.
Unlike nucleic acids and proteins, lipids and many polysaccharides are synthesized by enzyme-guided pathways without copying a sequence template.
Dietary and cellular proteins are hydrolyzed to amino acids that enter a shared pool used for protein synthesis, specialized molecules, or catabolism; amino acids are not stored as a dedicated polymeric energy reserve.
Aminotransferases transfer amino groups between amino acids and alpha-keto acids, commonly using PLP, allowing carbon skeletons and nitrogen to be handled separately.
Oxidative deamination, especially of glutamate, releases ammonium while regenerating an alpha-keto acid and reducing an electron carrier.
The liver converts toxic nitrogen to urea through a mitochondrial/cytosolic cycle, consuming high-energy phosphate and combining nitrogen from free ammonia and aspartate.
Amino-acid carbon skeletons enter metabolism as pyruvate, acetyl-CoA/acetoacetate, or TCA intermediates; glucogenic amino acids can support net glucose production, whereas purely ketogenic amino acids yield acetyl-CoA or ketone precursors.
The inner mitochondrial membrane separates matrix from intermembrane space, contains the ETC and ATP synthase, and is normally highly impermeable to protons.
Electrons from NADH and FADH2 move through mitochondrial carriers to oxygen, the terminal electron acceptor, which is reduced to water; released energy drives proton pumping.
Complex I accepts electrons from NADH, transfers them through flavin/iron-sulfur centers to ubiquinone, and pumps protons.
Complex II transfers succinate-derived electrons to ubiquinone without pumping protons; ubiquinol carries electrons to Complex III, which passes them to mobile cytochrome c and pumps protons.
Complex IV transfers electrons from cytochrome c to oxygen, forms water, and contributes to proton translocation/consumption that strengthens the gradient.
The proton-motive force combines membrane potential and pH gradient across the inner mitochondrial membrane, storing electrochemical free energy.
ATP synthase couples proton flow down the proton-motive force to rotational/conformational changes that catalyze ATP formation from ADP and inorganic phosphate.
Tight coupling links electron transport, proton gradient, and ATP synthesis; uncouplers dissipate the gradient, increasing fuel oxidation and heat while reducing ATP yield, whereas chain or ATP-synthase inhibitors have distinct patterns.
Oxidative phosphorylation yields approximately 2.5 ATP per matrix NADH and 1.5 per FADH2 in modern estimates; total glucose yield varies with shuttle and accounting assumptions.
Electron leakage from respiratory carriers can partially reduce oxygen to reactive oxygen species; antioxidant systems limit damage, but excess ROS can oxidize lipids, proteins, and DNA.
Mitochondrial outer-membrane permeabilization can release cytochrome c and activate the intrinsic apoptotic cascade, linking organelle integrity to regulated cell death.
Insulin signals nutrient abundance and generally promotes glucose uptake in insulin-responsive tissues, glycolysis, glycogen synthesis, fatty acid synthesis, and protein synthesis while suppressing hepatic glucose output and adipose lipolysis.
Glucagon signals low blood glucose and acts primarily on liver to stimulate glycogenolysis, gluconeogenesis, fatty acid oxidation, and ketogenesis while inhibiting glycolysis and glycogen synthesis.
Epinephrine rapidly mobilizes glycogen and fat during acute stress, acting on liver, muscle, and adipose through tissue-specific adrenergic signaling.
Cortisol is a steroid hormone that alters gene expression to support longer-term stress, increasing substrate availability through proteolysis, lipolysis, and hepatic gluconeogenic capacity while opposing some insulin actions.
The liver buffers blood fuels by storing and releasing glucose, performing gluconeogenesis, processing lipids, producing ketone bodies, and converting nitrogen to urea.
Tissues differ in transporters, enzymes, organelles, and priorities: muscle uses local glycogen, adipose stores/releases fat, brain relies mainly on glucose but adapts to ketones, and erythrocytes depend entirely on anaerobic glycolysis because they lack mitochondria.
Metabolic priorities shift from storage and glucose use after feeding, to hepatic glycogenolysis and gluconeogenesis in early fasting, to greater lipolysis, fatty acid oxidation, ketogenesis, and protein sparing during prolonged fasting.
Body mass reflects long-term energy intake, expenditure, storage, and neuroendocrine regulation; adipose-derived and gut/pancreatic signals influence hunger, satiety, and metabolic partitioning.
Respiratory quotient, CO2 produced divided by O2 consumed, provides a qualitative indicator of predominant fuel oxidation: approximately 1.0 for carbohydrate and 0.7 for fat.
The plasma membrane separates the cytoplasm from the extracellular environment while allowing regulated exchange, communication, adhesion, and maintenance of electrochemical gradients.
Phospholipids contain hydrophilic head groups and hydrophobic tails and spontaneously form bilayers in water because hydrophobic surfaces are excluded from water.
Biological membranes contain phospholipids and sterols; waxes are highly hydrophobic lipids used mainly as protective coatings rather than major plasma-membrane bilayer components.
Cholesterol fits between phospholipids and buffers membrane fluidity: it restrains phospholipid movement at high temperature and limits tight packing/solidification at low temperature while reducing permeability to small polar molecules.
Shorter tails and cis-unsaturated tails weaken packing and generally increase fluidity; longer and more saturated tails strengthen dispersion interactions and raise transition temperature.
The fluid mosaic model describes a dynamic lipid bilayer containing laterally mobile proteins, lipids, and carbohydrates with nonrandom organization and constrained domains.
The two membrane leaflets differ in lipid and protein composition; membrane proteins have fixed orientation, and extracellular carbohydrate groups remain on the noncytosolic face through trafficking.
Integral proteins interact strongly with the bilayer, often spanning it; peripheral proteins associate noncovalently with membrane surfaces or other proteins; lipid-anchored proteins attach through covalently linked lipids.
Oligosaccharides attached to lipids and proteins project mainly from the extracellular surface, forming a glycocalyx involved in protection, adhesion, and cell recognition.
Passive transport proceeds down a chemical or electrochemical potential gradient; active transport requires energy to move solute against its net gradient.
Small nonpolar molecules and some small uncharged molecules cross the lipid bilayer by simple diffusion down their concentration gradient without a transport protein.
Facilitated diffusion uses selective channels or carriers to move solute down its electrochemical gradient without direct energy input.
Primary active transport directly couples an energy source, commonly ATP hydrolysis, to movement of solute against its electrochemical gradient.
Secondary active transport couples downhill movement of one solute to uphill movement of another; symport moves them in the same direction, antiport in opposite directions.
The Na+/K+ ATPase typically exports 3 Na+ and imports 2 K+ per ATP, maintaining low intracellular Na+, high intracellular K+, osmotic balance, membrane potential, and secondary transport gradients.
Osmosis is net water movement across a selectively permeable membrane toward the side with higher effective concentration of nonpenetrating solute (higher osmotic pressure).
Osmolarity counts total dissolved particles, whereas tonicity predicts long-term cell-volume change and depends mainly on nonpenetrating solutes.
Colligative properties depend on the number of dissolved particles; for dilute solutions osmotic pressure is approximated by pi = iMRT.
Aquaporins are selective water channels that increase membrane water permeability without changing the equilibrium direction set by osmotic forces.
Endocytosis internalizes membrane and extracellular material in vesicles; forms include phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Exocytosis fuses intracellular vesicles with the plasma membrane, releasing lumenal cargo outside and adding vesicle membrane to the cell surface.
Membrane potential is the voltage difference across a membrane produced by unequal ion distributions and selective permeability.
An ion’s equilibrium potential is the voltage at which its electrical and chemical driving forces balance, producing no net flux for that ion through open selective channels.
Resting potential reflects multiple ion gradients weighted by relative permeability, often dominated by K+ leak in many cells, with gradients maintained by pumps.
Ion channels form selective hydrophilic pathways and can be gated by voltage, ligands, mechanical force, phosphorylation, or other signals.
Membrane receptors bind extracellular ligands and transmit information through conformational changes, second messengers, enzymatic activity, or interaction with intracellular proteins.
Sustained stimulation can reduce responsiveness through receptor phosphorylation, uncoupling, endocytosis, degradation, or recycling.
Tight junctions seal adjacent epithelial cells near the apical surface, restrict paracellular movement, and help maintain apical-basolateral polarity.
Desmosomes are strong cell-cell anchoring junctions that link cadherin-family adhesion proteins to intermediate filaments, distributing mechanical stress.
Gap junctions are intercellular channels formed by connexons that allow ions and small molecules to pass directly between neighboring cell cytoplasms.
Adherens junctions use cadherins linked to actin to form cell-cell adhesion belts important in tissue shape and morphogenesis.
Integrins are transmembrane adhesion receptors that connect extracellular-matrix proteins to the cytoskeleton and transmit mechanical and chemical signals bidirectionally.
The extracellular matrix contains proteins and polysaccharides that provide support, elasticity, adhesion cues, growth-factor reservoirs, and mechanical signals.
Epithelial tissues form sheets with apical-basal polarity, strong cell junctions, a basement membrane, and functions in protection, secretion, absorption, and transport.
The basement membrane is a specialized extracellular matrix beneath epithelia that supports adhesion, polarity, filtration, and tissue organization.
Connective tissues contain cells dispersed within abundant extracellular matrix and perform support, binding, storage, transport, defense, and repair.
Eukaryotic cells possess a membrane-bound nucleus, membrane-bound organelles, a complex cytoskeleton, and mitotic nuclear division.
Internal membranes create specialized chemical environments, concentrate enzymes/substrates, separate incompatible reactions, and increase membrane surface area.
The nucleus stores most eukaryotic DNA and organizes replication, transcription, RNA processing, and regulated nucleocytoplasmic exchange.
The nuclear envelope is a double membrane continuous with the ER; nuclear pore complexes regulate traffic between nucleus and cytoplasm.
The nucleolus is a non-membrane-bound nuclear region where rRNA is transcribed/processed and ribosomal subunits are assembled before export.
Mitochondria have an outer membrane, intermembrane space, highly folded inner membrane, and matrix; the inner membrane hosts the ETC and ATP synthase, while the matrix contains TCA and beta-oxidation enzymes.
Mitochondria contain their own circular DNA and ribosomes and divide by fission, but depend heavily on nuclear-encoded proteins and cellular control.
The rough ER is studded with ribosomes and receives proteins destined for secretion, membranes, or many endomembrane-system compartments during cotranslational translocation.
Smooth ER functions in lipid synthesis, detoxification, carbohydrate metabolism, and Ca2+ storage/release in specialized cells.
The ER synthesizes much membrane lipid and supports folding, disulfide formation, initial glycosylation, and quality control of secretory/membrane proteins.
The Golgi receives ER-derived cargo, modifies glycans and other groups, and sorts proteins/lipids into vesicles for secretion, plasma membrane, endosomes, or lysosomes.
Transport vesicles bud from donor membranes, select cargo, move along cytoskeleton, and fuse specifically with target membranes while preserving lumenal/cytosolic orientation.
Lysosomes are acidic membrane-bound organelles containing hydrolytic enzymes that degrade endocytosed material, macromolecules, and damaged cellular components.
Endosomes sort internalized receptors and cargo for recycling, return to membrane, transport to Golgi, or degradation in lysosomes.
Peroxisomes carry out oxidative reactions, including very-long-chain fatty-acid processing and detoxification of hydrogen peroxide using catalase.
Proteins contain targeting information that directs them to nucleus, ER, mitochondria, peroxisomes, or other destinations; loss or alteration of a signal changes localization and function.
The cytoskeleton is a dynamic network of microfilaments, microtubules, and intermediate filaments that supports shape, organization, transport, division, adhesion, and movement.
Actin monomers polymerize into polar microfilaments that undergo ATP-dependent dynamics and concentrate in the cell cortex.
Actin polymerization drives protrusion, and actin-myosin interactions produce contraction, including the contractile ring during cytokinesis.
Microtubules are hollow polar polymers of alpha/beta-tubulin that alternate between growth and shrinkage and organize from microtubule-organizing centers.
Motor proteins move vesicles, organelles, and macromolecular complexes along polarized microtubules, commonly with kinesins toward plus ends and dyneins toward minus ends.
Intermediate filaments are relatively stable, rope-like polymers that resist tensile stress and support cells, nuclei, and tissue integrity.
Motile eukaryotic cilia and flagella typically contain a 9+2 microtubule axoneme; dynein-driven sliding is converted into bending by structural constraints.
Primary cilia are usually nonmotile 9+0 projections that organize sensory and signaling pathways in many vertebrate cells.
The centrosome is a major animal-cell microtubule-organizing center containing a pair of centrioles and pericentriolar material; centrioles also contribute to basal bodies.
Cell theory states that living organisms are composed of cells, the cell is the basic unit of life, and new cells arise from pre-existing cells.
Prokaryotes lack a membrane-bound nucleus and the typical membrane-bound organelles of eukaryotes, while retaining a plasma membrane, cytoplasm, ribosomes, and DNA.
Bacteria and Archaea are distinct prokaryotic domains that differ in membrane lipids, cell-wall chemistry, and aspects of information-processing machinery.
Common bacterial morphologies include cocci (spherical), bacilli (rod-shaped), and spirilla or spiral forms; arrangements may reflect division planes and adhesion.
The main prokaryotic chromosome is located in a nucleoid region rather than a membrane-bound nucleus and is commonly circular, although exceptions exist.
Prokaryotes use 70S ribosomes composed of 30S and 50S subunits, whereas cytosolic eukaryotic ribosomes are 80S.
Prokaryotes do not undergo mitosis and lack the canonical mitotic spindle; chromosome segregation occurs during binary fission through other mechanisms.
Most bacteria possess a rigid cell wall containing peptidoglycan that resists osmotic lysis and helps determine shape.
Gram-positive bacteria have a thick peptidoglycan layer and no outer membrane; Gram-negative bacteria have a thin peptidoglycan layer plus an outer membrane containing lipopolysaccharide.
Some bacteria produce external polysaccharide or protein layers that aid adhesion, reduce desiccation, impede phagocytosis, and contribute to biofilms.
The bacterial plasma membrane controls transport and may house electron-transport chains and ATP-generating machinery because bacteria lack mitochondria.
Bacterial flagella are rotary appendages built from flagellin and powered by an ion gradient, unlike the microtubule-based bending of eukaryotic flagella.
Fimbriae commonly mediate attachment, whereas specialized sex pili facilitate cell-to-cell contact during conjugation.
Binary fission replicates the bacterial chromosome, segregates DNA, forms a septum, and produces two daughter cells without mitosis.
Under unrestricted conditions, a bacterial population doubles each generation, so N = N₀ × 2ⁿ, where n is the number of generations.
A closed-culture growth curve commonly includes lag, exponential, stationary, and death phases.
Bacteria vary in oxygen use: obligate aerobes require oxygen, obligate anaerobes are harmed by it, and facultative anaerobes can grow with or without it using different pathways.
Microbial relationships may harm the host (parasitism), benefit both partners (mutualism), or benefit one without clear effect on the other (commensalism).
Chemotaxis biases bacterial movement toward attractants or away from repellents by altering the frequency and duration of runs and tumbles.
Large populations, short generation times, mutation, and horizontal gene transfer allow bacterial populations to adapt rapidly under selection.
Resistance may arise through target modification, drug inactivation, reduced uptake, increased efflux, metabolic bypass, or acquisition of resistance genes.
Sterilization eliminates all viable microbes including resistant forms, whereas disinfection reduces pathogenic microbes on inanimate objects and antisepsis applies to living tissue.
Plasmids are independently replicating DNA molecules, often circular, that can carry selectable traits such as antibiotic resistance or metabolic functions.
Transformation is uptake of free DNA from the environment followed by maintenance or incorporation into the recipient genome.
Conjugation transfers DNA through direct cell-to-cell contact, commonly using a conjugative plasmid and pilus.
Transduction is bacteriophage-mediated transfer of bacterial DNA between cells.
Generalized transduction can transfer many bacterial genes after accidental packaging during a lytic cycle, whereas specialized transduction transfers genes near a prophage integration site after imprecise excision.
Transposons are mobile DNA elements that can change genomic location and may carry genes such as antibiotic-resistance determinants.
Vertical transfer passes DNA from parent to progeny, whereas horizontal transfer moves DNA between organisms through transformation, conjugation, or transduction.
Mechanisms can be distinguished using DNase, physical separation, phage exclusion, and cell-contact requirements.
Viruses are acellular infectious units consisting of a genome enclosed by a protein capsid and sometimes a lipid envelope; they lack ribosomes and independent metabolism.
A viral genome may be DNA or RNA, single- or double-stranded depending on the virus, but an individual virion carries a defined genome type.
The capsid is a protein shell that protects the viral genome, contributes to host-cell attachment in some viruses, and organizes virion architecture.
Enveloped viruses possess a host-derived lipid membrane containing viral proteins, whereas nonenveloped viruses lack that lipid layer and are often more resistant to detergents and drying.
A typical tailed bacteriophage has a capsid head containing nucleic acid, a tail apparatus, and receptor-binding fibers used to attach to bacteria and deliver the genome.
Viruses are generally smaller than bacteria, which are generally smaller than eukaryotic cells, although size ranges overlap at their extremes.
Viral host range and tissue tropism depend strongly on compatibility between viral attachment proteins and host receptors, plus intracellular permissiveness.
A generalized viral cycle includes attachment, entry or genome penetration, uncoating when applicable, genome replication and gene expression using host resources, assembly, and release.
Viruses are obligate intracellular parasites because they rely on host ribosomes, energy, metabolites, and often enzymes to produce progeny.
Viruses first bind specific receptors, then enter by fusion, endocytosis, or genome injection, followed by uncoating when the capsid enters the cell.
Viral genomes redirect host biosynthetic machinery while supplying or encoding functions the host lacks, such as certain viral polymerases.
Viral genomes and structural proteins assemble into particles, and some viruses require proteolytic or conformational maturation to become infectious.
Nonenveloped viruses often exit by cell lysis, whereas many enveloped viruses bud through host membranes and acquire their envelope during release.
A lytic phage rapidly produces progeny and lyses the host; a temperate phage may integrate as a prophage and replicate with the host before induction into a productive cycle.
Animal viruses vary in entry route, genome-replication strategy, site of replication, and release, but all must generate readable mRNA and assemble progeny.
Retroviruses carry an RNA genome and use reverse transcriptase to synthesize DNA, which can integrate into the host genome.
Retroviral DNA integrates into the host chromosome as a provirus, allowing persistence and host-driven transcription of viral RNA.
HIV is an enveloped retrovirus that infects cells expressing appropriate receptors, reverse-transcribes its RNA, integrates a provirus, and produces new virions through host and viral machinery.
Prions are infectious misfolded proteins that propagate by inducing conformational change in normal host proteins and contain no nucleic-acid genome.
Viroids are small, circular infectious RNA molecules that lack a protein capsid and primarily infect plants.
The eukaryotic cell cycle coordinates cell growth, DNA replication, chromosome segregation, and cytokinesis.
G0 is a quiescent, noncycling state; growth arrest can be reversible or terminal depending on cell type and signals.
Chromosome number is counted by centromeres, whereas DNA content and chromatid number change during replication and segregation.
Interphase prepares a cell for division through growth, DNA replication, centrosome duplication, and checkpoint surveillance.
During prophase chromosomes condense and the mitotic spindle begins forming; nuclear-envelope breakdown and kinetochore attachment are often assigned to prometaphase.
At metaphase, duplicated chromosomes align at the metaphase plate with sister kinetochores attached to opposite spindle poles.
Anaphase begins when cohesin is removed and sister chromatids move toward opposite poles.
During telophase, chromosomes reach poles, decondense, and become enclosed by re-forming nuclear envelopes.
Cytokinesis partitions the cytoplasm after nuclear division, typically by an actin-myosin contractile ring in animal cells.
Centrosomes organize microtubules and form spindle poles; animal centrosomes usually contain a pair of centrioles surrounded by pericentriolar material.
The centromere is a chromosomal region where the kinetochore assembles; kinetochores bind spindle microtubules, while cohesin holds sister chromatids together.
Chromosome movement results from regulated microtubule polymerization/depolymerization, motor proteins, and spindle-pole separation.
Checkpoints permit progression only when critical conditions such as DNA integrity, replication completion, and spindle attachment are satisfactory.
Cyclin-dependent kinases drive cell-cycle transitions when activated by stage-specific cyclins and regulatory modifications.
Cancer arises when accumulated alterations permit inappropriate proliferation, survival, invasion, and other malignant behaviors.
Proto-oncogenes normally promote regulated growth or survival; activating mutation, amplification, or overexpression can convert them into oncogenes.
Tumor suppressors restrain proliferation, preserve genome integrity, or promote apoptosis; loss of function removes these safeguards.
Apoptosis is an energy-dependent, regulated process that dismantles cells with limited leakage and inflammation.
Apoptosis can be initiated by intracellular stress and mitochondrial signals or by extracellular death-receptor signaling, converging on caspase cascades.
Apoptosis is regulated and generally noninflammatory, whereas necrosis classically follows severe injury with swelling, membrane rupture, and inflammation.
Many normal cells reduce proliferation when crowded and require attachment to extracellular matrix; transformed cells may lose these controls.
Meiosis produces haploid gametes and increases genetic variation while allowing fertilization to restore diploidy.
Spermatogenesis converts diploid spermatogonia into haploid sperm through mitosis, meiosis, and differentiation.
Oogenesis produces a large haploid ovum through unequal cytokinesis and prolonged meiotic arrest.
Gamete morphology reflects different roles: sperm are small and motile, whereas ova are large and provide cytoplasm, organelles, and early developmental resources.
Both gametes contribute a haploid nuclear genome, but the oocyte supplies most cytoplasm, organelles, maternal RNAs, and early proteins.
Fertilization fuses haploid gametes, restores diploidy, activates the oocyte, and initiates development.
Blocks to polyspermy prevent more than one sperm from contributing a haploid genome to the egg.
Implantation is attachment and invasion of the blastocyst into the receptive uterine endometrium.
The reproductive sequence proceeds from fertilization through cleavage, implantation, embryonic and fetal development, and birth.
Cleavage is a series of rapid mitotic divisions that partitions the zygote into smaller blastomeres without substantial overall growth.
The morula is a solid ball of blastomeres; compaction increases cell-cell adhesion and begins inside-outside differences.
Blastula formation creates a hollow or cavity-containing embryonic stage; in mammals the blastocyst contains an inner cell mass, trophoblast, and blastocoel.
Gastrulation reorganizes the embryo to establish the three primary germ layers and basic body axes.
Gastrulation uses coordinated movements such as invagination, involution, ingression, delamination, and epiboly to reposition cells.
Ectoderm gives rise primarily to epidermis and nervous-system structures.
Mesoderm forms much of the musculoskeletal, cardiovascular, connective-tissue, urogenital, and blood systems.
Endoderm forms epithelial linings of much of the digestive and respiratory tracts and associated organs.
Neurulation forms the neural tube from ectoderm under inductive influence of underlying tissues.
Neural crest cells are migratory ectoderm-derived cells that generate diverse structures, especially in the peripheral nervous system and craniofacial region.
Embryonic induction occurs when one tissue influences the developmental fate of another through signals and competence.
Extraembryonic membranes support exchange, protection, early blood formation, and placentation without becoming the embryo proper.
Dizygotic twins result from two separately fertilized oocytes, whereas monozygotic twins result when descendants of one fertilized egg separate into two embryonic lineages.
Determination is commitment to a developmental fate before all specialized features are visibly expressed.
Differentiation is acquisition of specialized structure and function through changes in gene expression and cellular organization.
Four broad animal tissue classes are epithelial, connective, muscle, and nervous tissue.
Developing cells coordinate fate and behavior through contact-dependent, paracrine, endocrine, and other signaling modes.
A morphogen is a signaling molecule whose concentration or exposure can specify different cell fates across a field.
Developmental cell migration uses cytoskeletal remodeling, adhesion changes, polarity, and guidance cues to move cells to new locations.
Potency describes the range of cell types a stem or progenitor cell can generate.
Stem cells balance self-renewal with production of differentiating descendants, often through asymmetric division or population-level regulation.
Development depends on spatially and temporally controlled gene expression driven by transcription factors, enhancers, chromatin state, and signaling.
Phenotype emerges from interaction between genotype and environmental inputs such as nutrition, hormones, toxins, temperature, and neighboring-cell signals.
Developmental apoptosis removes temporary structures, sculpts tissues, and adjusts cell populations.
Regenerative capacity varies among species and tissues according to stem-cell availability, signaling environment, immune response, and structural complexity.
Cellular senescence is a durable proliferation arrest triggered by replicative limits, stress, or damage, while cells remain metabolically active.
In many somatic cells, telomeres shorten with repeated division and can trigger DNA-damage responses and replicative senescence.
Aging is a progressive decline in physiological resilience arising from interacting mechanisms such as genomic damage, epigenetic change, proteostasis loss, mitochondrial dysfunction, stem-cell exhaustion, and altered signaling.
The nervous system detects stimuli, integrates information, and coordinates rapid responses that support homeostasis and adaptive behavior.
The central nervous system (CNS) consists of brain and spinal cord; the peripheral nervous system (PNS) connects the CNS to receptors, muscles, and glands.
Sensory neurons transduce and carry information toward the CNS; motor neurons carry commands from the CNS to effectors.
The somatic motor system controls skeletal muscle; the autonomic system regulates smooth muscle, cardiac muscle, and glands.
Sympathetic activity generally supports mobilization during challenge; parasympathetic activity generally supports rest, digestion, and conservation.
Autonomic motor pathways generally use a two-neuron chain. All autonomic preganglionic neurons release acetylcholine (ACh) onto nicotinic receptors in autonomic ganglia. Parasympathetic postganglionic neurons usually release ACh onto muscarinic receptors. Most sympathetic postganglionic neurons release norepinephrine onto adrenergic receptors, with important exceptions.
A reflex arc links receptor, afferent pathway, integration center, efferent pathway, and effector to produce a rapid response.
Spinal circuits mediate rapid reflexes, while brain pathways can modulate, suppress, or coordinate them.
Neural signals can control hormone release, and hormones can feed back on neurons and endocrine tissues.
The soma contains the nucleus and most organelles; dendrites usually receive and integrate synaptic input.
The axon conducts action potentials away from the soma; the axon hillock/initial segment is a common trigger zone.
Myelin electrically insulates axons and speeds conduction; Schwann cells myelinate PNS axons.
Nodes of Ranvier contain high densities of voltage-gated channels, allowing action potentials to regenerate discontinuously along myelinated axons.
The resting membrane potential arises from ion concentration gradients and selective membrane permeability, especially K+ leak, maintained indirectly by active transport.
The Nernst equation gives the membrane potential at which electrical and chemical forces on one ion balance.
Once threshold is reached, regenerative voltage-gated channel opening produces a stereotyped action potential.
Voltage-gated Na+ influx drives depolarization; Na+ channel inactivation and K+ efflux drive repolarization; persistent K+ conductance can cause after-hyperpolarization.
During the absolute refractory period, inactivated Na+ channels prevent another spike; during the relative period, stronger stimulation may overcome hyperpolarization.
The Na+/K+ ATPase uses ATP to move 3 Na+ out and 2 K+ in, maintaining gradients and contributing a small electrogenic effect.
EPSPs move membrane voltage toward spike threshold; IPSPs move it away or stabilize it through shunting.
Temporal summation combines repeated input from one source; spatial summation combines simultaneous inputs from multiple sources.
Stimulus intensity is commonly encoded by action-potential frequency and recruitment, because individual spike amplitude is all-or-none.
A chemical synapse includes presynaptic terminal, synaptic cleft, and postsynaptic receptors.
Action potentials open presynaptic voltage-gated Ca2+ channels; Ca2+ triggers exocytosis, and signaling ends by reuptake, enzymatic degradation, or diffusion.
Electrical synapses pass ionic current directly through gap junctions, enabling rapid and often bidirectional synchronization.
Glia support neurons through myelination, homeostasis, immune defense, nutrient support, and synaptic regulation.
Voltage-gated channels change open probability in response to membrane voltage.
Ligand-gated channels open or close when a chemical ligand binds, producing rapid postsynaptic responses.
Receptor enzymes have extracellular ligand-binding domains and intracellular catalytic activity or enzyme association, often initiating phosphorylation cascades.
GPCRs activate heterotrimeric G proteins, which regulate effectors and second messengers.
Adenylyl cyclase produces cAMP, which activates PKA and downstream phosphorylation responses.
Gq can activate phospholipase C, producing IP3 and DAG; IP3 releases Ca2+ from ER stores and DAG helps activate PKC.
Signaling cascades amplify small inputs and are limited by phosphatases, GTP hydrolysis, receptor internalization, degradation, and feedback.
Endocrine glands secrete hormones into extracellular fluid and blood to act on distant target cells.
Major endocrine sources include hypothalamus, pituitary, thyroid, parathyroids, adrenals, pancreas, gonads, pineal gland, and endocrine tissues in other organs.
The anterior neck contains the thyroid, which produces thyroid hormones and calcitonin, and posterior parathyroid glands, which produce parathyroid hormone (PTH).
The adrenal cortex produces steroid hormones such as cortisol and aldosterone; the adrenal medulla releases catecholamines such as epinephrine and norepinephrine.
Pancreatic islets contain endocrine cells that secrete insulin, glucagon, and other regulatory hormones into blood.
Ovaries and testes produce sex steroids and peptide signals that regulate reproduction, development, and secondary sex characteristics.
The pineal gland secretes melatonin and participates in circadian timing influenced by light-dark information.
Several organs not classically labeled endocrine glands also release hormones, including kidney, heart, and gastrointestinal tissues.
The hypothalamus links neural input to endocrine output through releasing hormones and direct axonal projections to the posterior pituitary.
Peptide hormones are synthesized as gene products, stored in vesicles, released by exocytosis, and usually bind cell-surface receptors.
Steroid hormones are cholesterol-derived, lipid-soluble signals often synthesized on demand and transported bound to carrier proteins.
Amino-acid-derived hormones include catecholamines and thyroid hormones, which differ sharply in solubility and receptor location.
Terpenes are built from isoprene units; terpenoids are modified derivatives and contribute to biologically active lipids and steroid precursors.
Water-soluble hormones generally circulate dissolved, whereas lipid-soluble hormones commonly bind carrier proteins.
A target cell responds because it expresses the appropriate receptor and signaling machinery.
Water-soluble hormones bind cell-surface receptors and alter intracellular activity through channels, G proteins, enzymes, and second messengers.
Lipid-soluble hormones cross membranes and bind cytosolic or nuclear receptors that regulate transcription.
Second messengers such as cAMP, IP3, DAG, and Ca2+ relay and amplify signals from activated receptors.
End products commonly inhibit upstream hypothalamic and pituitary signals, stabilizing hormone output.
Positive feedback amplifies a process until a defined endpoint, as in oxytocin-driven labor or the preovulatory LH surge.
Cells can alter receptor number or sensitivity in response to chronic hormone levels, changing responsiveness.
Neuroendocrine cells receive neural input and release hormones into blood, bridging electrical and endocrine communication.
The respiratory system exchanges O2 and CO2, contributes to acid-base regulation, supports thermoregulation, and limits entry of particulates and pathogens.
Air moves through conducting airways to alveoli, where thin respiratory surfaces contact pulmonary capillaries.
Contraction of the diaphragm and external intercostals increases thoracic volume and lowers intrapulmonary pressure, drawing air inward; quiet expiration is usually passive.
Lung compliance describes ease of expansion; elastic recoil promotes expiration; alveolar surface tension favors collapse and is reduced by surfactant.
O2 and CO2 diffuse down partial-pressure gradients across the alveolar-capillary membrane.
At constant temperature, dissolved gas concentration is proportional to its partial pressure and solubility coefficient.
Brainstem respiratory centers adjust ventilation in response primarily to CO2-derived changes in pH, with additional input from O2-sensitive peripheral chemoreceptors.
Ventilation alters CO2, shifting the carbonic-acid/bicarbonate equilibrium and thereby changing blood pH.
Nasal hairs, mucus, cilia, cough, and immune cells trap and remove inhaled particles and pathogens.
Airway vascular beds and evaporation exchange heat and water; panting increases evaporative heat loss in many animals.
The circulatory system transports gases, nutrients, wastes, hormones, immune cells, ions, fluids, and heat.
The right heart pumps deoxygenated blood through pulmonary circulation; the left heart pumps oxygenated blood through systemic circulation.
Atrioventricular and semilunar valves open and close in response to pressure differences, preventing backflow.
Systole is ventricular contraction and ejection; diastole is ventricular relaxation and filling.
Cardiac output equals heart rate times stroke volume: CO = HR × SV.
Arteries have thicker, more elastic and muscular walls; veins operate at lower pressure and often contain valves and serve as capacitance vessels.
Arterioles are major resistance vessels; changes in smooth-muscle tone strongly alter regional flow and arterial pressure.
For steady incompressible flow, volumetric flow rate is conserved; velocity varies inversely with total cross-sectional area.
Capillary exchange occurs by diffusion and bulk flow driven by hydrostatic and oncotic pressures.
Arterial pressure depends on cardiac output and systemic vascular resistance; pulse pressure is systolic minus diastolic pressure.
Blood contains plasma plus formed elements: erythrocytes, leukocytes, and platelets; hematocrit is the fraction of blood volume occupied by red cells.
Erythrocytes are produced in bone marrow under erythropoietin influence and removed mainly by spleen and liver macrophages.
Hemoglobin binds O2 cooperatively; blood O2 content depends mainly on hemoglobin concentration and saturation, while affinity affects loading and unloading.
CO2 travels dissolved, bound to proteins, and mainly as bicarbonate formed through carbonic anhydrase reactions.
Hemostasis includes vasoconstriction, platelet plug formation, and a coagulation cascade that generates fibrin.
Blind-ended lymphatic capillaries collect interstitial fluid, which passes through vessels and nodes before returning to venous blood.
Lymphatics return excess fluid and escaped plasma proteins to blood, limiting edema.
Intestinal lacteals absorb chylomicrons and transport dietary lipids through lymph before they enter blood.
Innate immunity is rapid and broadly patterned; adaptive immunity is antigen-specific, clonally expanded, and generates memory.
Phagocytes engulf microbes and debris; macrophages also secrete signals and present antigen.
Activated B cells can differentiate into plasma cells that secrete antibodies and memory B cells that support future responses.
Helper T cells coordinate immune responses; cytotoxic T cells kill infected or abnormal host cells; memory T cells persist.
An antigen contains epitopes recognized by antibodies or lymphocyte receptors; antibodies bind specific molecular features through variable regions.
Antibodies contain two heavy and two light chains, variable antigen-binding regions, and constant regions that mediate effector functions.
MHC molecules display peptide antigens to T cells; class I generally presents endogenous peptides to CD8 T cells, while class II presents exogenous peptides to CD4 T cells.
Antigen selects and activates rare lymphocyte clones with matching receptors, producing effector and memory cells.
Immune tolerance limits responses to self; failures can produce autoimmune disease.
Bone marrow produces blood cells and supports B-cell development; thymus supports T-cell maturation; spleen filters blood; lymph nodes filter lymph and organize immune encounters.
Vaccination exposes the adaptive immune system to antigenic information without the full disease burden, generating memory that accelerates later responses.
The digestive tract mechanically and chemically processes food, absorbs nutrients and water, and eliminates indigestible material.
Saliva lubricates food and begins digestion through enzymes such as salivary amylase while facilitating swallowing.
The esophagus transports a bolus to the stomach through coordinated peristaltic contractions.
The stomach stores and churns food, acidifies contents, begins protein digestion, and protects itself with mucus, bicarbonate, tight junctions, and rapid epithelial renewal.
The liver produces bile, processes absorbed nutrients, regulates blood glucose, synthesizes plasma proteins, and detoxifies compounds.
Bile salts emulsify dietary lipids, increasing surface area for lipase; bile is made by liver and stored/concentrated in gallbladder.
The exocrine pancreas secretes digestive enzymes and bicarbonate into the small intestine.
The small intestine receives chyme, pancreatic secretions, and bile; it completes much digestion in a near-neutral environment.
Villi and microvilli increase surface area; capillaries absorb many water-soluble nutrients, while lacteals receive chylomicrons.
The large intestine absorbs water and electrolytes, compacts feces, and houses microbial communities that metabolize substrates and produce some vitamins.
The rectum stores feces before elimination; internal and external sphincters provide involuntary and voluntary control.
The enteric nervous system coordinates local motility and secretion and is modulated by autonomic input.
GI hormones coordinate acid secretion, pancreatic secretion, bile delivery, motility, and satiety in response to luminal contents.
Kidneys regulate fluid volume, osmolarity, electrolytes, acid-base balance, blood pressure, and nitrogenous-waste excretion.
The cortex contains renal corpuscles and convoluted tubules; medullary pyramids contain loops and collecting ducts; renal blood flow supports filtration and exchange.
Filtrate passes Bowman capsule -> proximal tubule -> loop of Henle -> distal tubule -> collecting duct.
Hydrostatic pressure filters water and small solutes from glomerular capillaries into Bowman space while retaining cells and most proteins.
Reabsorption moves substances from tubular fluid to blood; secretion moves substances from blood to tubular fluid.
The proximal tubule reabsorbs most filtered water and solutes, including glucose, amino acids, bicarbonate, and sodium, while secreting selected compounds.
Descending limb is water-permeable; ascending limb reabsorbs salts and is relatively water-impermeable, generating a medullary osmotic gradient.
ADH increases collecting-duct water permeability, allowing water to follow the medullary gradient and concentrate urine.
Aldosterone promotes sodium reabsorption and potassium secretion in distal nephron, tending to increase extracellular volume when water follows.
Low renal perfusion or related signals promote renin release, angiotensin II formation, vasoconstriction, and aldosterone-mediated sodium retention.
Kidneys reabsorb filtered bicarbonate, secrete H+, and generate new bicarbonate, providing slower but powerful pH regulation.
Ureters transport urine to bladder; bladder stores it; urethra carries it outward; smooth and skeletal sphincters coordinate micturition.
Testes produce sperm and androgens; ducts, accessory glands, and penis support sperm maturation, transport, and delivery.
Ovaries produce oocytes and hormones; uterine tubes transport gametes/embryo; uterus supports implantation and development; cervix and vagina form lower tract.
GnRH stimulates LH and FSH; gonadal hormones and inhibin provide feedback and regulate gametogenesis and reproductive tissues.
FSH supports Sertoli-cell functions and spermatogenesis; LH stimulates Leydig-cell testosterone production.
Follicular development, ovulation, and corpus-luteum formation are coordinated by FSH, LH, estrogen, and progesterone.
The endometrium undergoes menstrual, proliferative, and secretory changes under ovarian hormone control.
Fertilization restores diploidy and initiates development; after implantation, trophoblast-derived hCG maintains corpus-luteum progesterone early in pregnancy.
The placenta mediates maternal-fetal exchange and produces hormones while keeping maternal and fetal blood largely separate.
Labor involves uterine contractions reinforced by cervical stretch and oxytocin positive feedback, with additional hormonal and mechanical regulation.
Prolactin promotes milk production; oxytocin triggers myoepithelial contraction and milk ejection in response to suckling.
Genetic, gonadal, hormonal, and tissue-response factors coordinate sexual differentiation; puberty activates reproductive axes and secondary sexual characteristics.
Skeletal muscle is striated and voluntary; cardiac muscle is striated, branched, and involuntary; smooth muscle is nonstriated and involuntary.
Muscle is organized into fibers containing myofibrils composed of repeating sarcomeres.
Z lines bound a sarcomere; A band spans thick filaments; I band contains thin-only regions; H zone contains thick-only region; M line centers thick filaments.
Myosin heads cyclically bind actin, perform power strokes, detach upon ATP binding, and recock after ATP hydrolysis.
In skeletal and cardiac muscle, Ca2+ binds troponin, shifting tropomyosin and permitting actin-myosin interaction.
Somatic motor neurons release ACh at nicotinic receptors on skeletal muscle; one motor neuron and its fibers form a motor unit.
A muscle action potential enters T-tubules, triggers SR Ca2+ release, and initiates contraction; Ca2+ reuptake supports relaxation.
Slow oxidative fibers resist fatigue and contain many mitochondria/myoglobin; fast glycolytic fibers generate high power but fatigue faster.
Smooth muscle uses Ca2+-calmodulin and myosin light-chain kinase, lacks sarcomeres, and is regulated by autonomic, hormonal, and local signals.
Cardiac cells are electrically connected by gap junctions and mechanically linked at intercalated discs; Ca2+-induced Ca2+ release supports contraction.
Muscles generate movement and posture, aid venous return, move contents through organs, and generate heat through shivering.
Skeletal muscle is generally under somatic voluntary control, whereas cardiac and most smooth muscle are involuntary and autonomically/hormonally regulated.
The skeleton provides support, protection, leverage for movement, mineral storage, and marrow space for hematopoiesis.
Bone combines collagen-rich organic matrix with mineral hydroxyapatite and contains osteoblasts, osteoclasts, and osteocytes.
Compact bone provides dense cortical strength; spongy bone contains trabeculae and marrow spaces, reducing mass while distributing loads.
Joints connect bones; articular cartilage reduces friction and distributes load; synovial fluid lubricates movable joints.
Ligaments connect bone to bone; tendons connect muscle to bone.
Endoskeletons lie within the body and grow/remodel with the organism; exoskeletons form an external supportive covering and may require molting.
PTH tends to raise blood Ca2+; calcitonin can oppose resorption in some contexts; vitamin D promotes intestinal calcium absorption and supports mineral balance.
Bone changes through growth plates, remodeling, and coordinated repair after fracture.
Skin includes epidermis over dermis, with subcutaneous tissue beneath; keratinocytes dominate epidermis, while dermis contains vessels, nerves, glands, and connective tissue.
Keratinized epidermis and extracellular lipids limit water loss and entry of harmful substances.
Sweat secretion permits evaporative heat loss; effectiveness depends on evaporation and environmental humidity.
Cutaneous vasodilation increases heat transfer to environment; vasoconstriction conserves core heat.
Hair and arrector-pili responses can alter insulation in furry animals; subcutaneous fat reduces conductive heat loss.
Skin protects against abrasion, pathogens, chemicals, UV exposure, and mechanical stress through keratin, calluses, nails, immune surveillance, and repair.
Skin limits water loss and adjusts heat transfer; sweating changes water and electrolyte balance.
Autonomic and endocrine signals influence sweating, vascular tone, hair growth, and skin metabolism.
Homeostasis emerges from coordinated sensing, transport, exchange, regulation, and excretion across multiple organ systems.
Exercise increases muscle ATP demand, ventilation, cardiac output, blood-flow redistribution, heat production, and metabolic waste handling.
Blood loss lowers venous return and pressure, activating sympathetic responses, RAAS, ADH, thirst, and fluid conservation.
Water loss raises plasma osmolarity and may lower volume, stimulating thirst and ADH; renal water conservation concentrates urine.
Lungs regulate volatile acid through CO2 excretion rapidly; kidneys regulate bicarbonate and fixed-acid excretion more slowly.
Heat balance is controlled through skin blood flow, sweating/evaporation, shivering, metabolic changes, and behavior.
Digestive absorption supplies nutrients; circulation distributes them; liver and endocrine signals regulate storage and release; tissues consume them according to state.
Local innate responses recruit immune cells and alter vascular permeability; systemic neural/endocrine responses can change temperature, metabolism, and behavior.