AP Biology Full Course Review (All 8 Units)|Everything You Need for a 5 — 2026 AP Exam (with fixes)

This section introduces the AP Biology Exam, acknowledging its difficulty while offering reassurance. It highlights the video's goal to review every unit and topic in the course to help students succeed. The instructor, Mr. W, introduces himself and provides a link to a downloadable checklist at apbiosuccess.com/checklist for comprehensive study preparation.

Water is a polar molecule with unequal electron sharing, leading to partial charges. Hydrogen bonds are weak intermolecular bonds forming between the partial negative oxygen and partial positive hydrogen of water molecules, and are crucial in biological structures like DNA, RNA, and proteins. These bonds are responsible for water's unique properties: cohesion (water sticking to water), adhesion (water sticking to other substances, vital for plant transport), and surface tension.

Acidic solutions have a higher concentration of hydrogen ions (pH below 7), while basic solutions have more hydroxide ions (pH above 7). Although direct pH questions are rare, understanding this concept is fundamental for the AP Biology exam.

The essential elements for life are Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur (CHNOPS). Carbon forms the backbone of biological molecules. Monomers are small building blocks that combine to form polymers. Dehydration synthesis builds polymers by removing a water molecule, while hydrolysis breaks polymers by adding a water molecule. Functional groups (e.g., phosphate, methyl, hydroxyl, carboxyl, amino, sulfhydryl, acetyl) are critical structural components of molecules, influencing their properties and roles in biological processes like energy exchange, DNA regulation, and protein structure.

Carbohydrates are composed of monosaccharide monomers, forming disaccharides (like lactose) and polysaccharides (like starch, glycogen, cellulose) for energy storage and structural support. The video explains human lactose intolerance and the inability to digest cellulose, contrasting with ruminants that have symbiotic relationships to digest it. Lipids are nonpolar, hydrophobic molecules not built from repeating monomers, serving functions like energy storage (triglycerides), waterproofing (waxes), membrane formation (phospholipids), and signaling (steroid hormones). Phospholipid structure, with its hydrophobic tail and hydrophilic head, is essential for forming cell membranes.

Proteins are made of amino acid monomers, each with a central carbon, an amine group, a carboxyl group, a hydrogen atom, and a variable R-group. Protein structure has four levels: primary (linear sequence of amino acids), secondary (alpha helices and beta-pleated sheets formed by hydrogen bonds in the backbone), tertiary (3D shape from R-group interactions), and quaternary (multiple polypeptide chains interacting). Sickle cell disease serves as a key example, where a single amino acid substitution alters hemoglobin's structure and function, leading to painful crises and tissue damage, but also conferring malaria resistance in heterozygotes. Nucleic acids (DNA and RNA) are genetic information molecules, with nucleotides as monomers. DNA is a double helix for hereditary information, while RNA is versatile, involved in information transfer, regulation, and catalysis. Differences between DNA and RNA include sugar type (deoxyribose vs. ribose) and bases (thymine vs. uracil).

Cells are the fundamental units of life, possessing a membrane, DNA, and systems for maintenance and replication. Prokaryotic cells are smaller and simpler, lacking a nucleus and membrane-bound organelles, with circular DNA. Eukaryotic cells are larger, more complex, with a nucleus, linear chromosomes, mitochondria, and numerous membrane-bound organelles. Cell size is limited by the surface area-to-volume ratio: smaller cells have a higher ratio, allowing efficient diffusion of substances. Larger organisms often increase surface area in specific tissues (e.g., fish gills, elephant ears, intestinal villi) to facilitate exchange. The large size of whales is an adaptation to minimize heat loss in cold water due to their low surface area-to-volume ratio. Smaller mammals, conversely, have high metabolic rates to compensate for rapid heat loss.

Cellular compartmentalization in eukaryotes allows for distinct internal chemical environments and provides extensive internal surface area for membrane-bound reactions. Prokaryotes, though simpler, may have specialized internal regions for functions like photosynthesis. The endomembrane system, a dynamic network of interconnected membranes and compartments (including nuclear envelope, ER, Golgi, lysosomes, vacuoles), facilitates material flow and exchange within the cell. Mitochondria and chloroplasts originated from mutualistic endosymbiosis, supported by evidence like their own circular DNA, binary fission, bacterial-like ribosomes, and double membranes.

The Calvin cycle, occurring in the stroma, uses ATP and NADPH from the light reactions to convert CO2 into sugars. It has three phases: carbon fixation (CO2 combined with RuBP by rubisco to form 3-carbon molecules), energy investment and harvest (3-carbon molecules are reduced and phosphorylated using ATP and NADPH to form G3P, a sugar precursor), and regeneration (remaining G3P used to regenerate RuBP). The carbon atom accounting highlights the cyclical nature, demonstrating how three RuBPs combine with three CO2 to yield six 3-carbon G3Ps, with one G3P harvested and five used to regenerate RuBP, maintaining the carbon balance.

The nucleus stores and protects DNA in chromosomes (chromatin when diffuse), with the nucleolus assembling ribosomes. Ribosomes (free or bound to ER) translate mRNA into proteins. Mitochondria convert food energy to ATP, characterized by folded inner membranes for ATP synthesis. The endoplasmic reticulum (ER) has rough (with ribosomes for protein synthesis for secretion/organelles) and smooth (lipid synthesis, detoxification) forms. The Golgi complex modifies, sorts, and packages ER products into vesicles. Lysosomes (in animal cells) contain hydrolytic enzymes for intracellular digestion and recycling. The cytoskeleton provides dynamic support and facilitates cell movement. Centrosomes in animal cells organize spindle fibers for chromosome separation during cell division. Plant cells feature a large central vacuole for water storage, macromolecule management, waste sequestration, and turgor pressure maintenance, and chloroplasts for photosynthesis, derived from endosymbiotic bacteria. Plant cell walls, made largely of cellulose, provide structural support and prevent overexpansion.

The cell membrane acts as a selectively permeable boundary. Phospholipids, with their hydrophobic tails and hydrophilic heads, form the fundamental bilayer. The fluid mosaic model describes the membrane as a dynamic mosaic of phospholipids, proteins, and cholesterol in constant motion. Proteins are integrated into the membrane in various ways: transmembrane (spanning the membrane with hydrophobic cores and hydrophilic ends), integral (partially embedded), and peripheral (attached to surface). Passive transport (diffusion, facilitated diffusion) does not require cellular energy, moving substances down their concentration gradient. Active transport uses cellular energy (often ATP) to move substances against their gradient. Bulk transport via endocytosis and exocytosis involves membrane vesicle formation and requires energy and cytoskeletal involvement. Membrane potential, an electrical charge across the membrane, is created by ion pumps and used in processes like ATP synthesis and nerve impulses.

Osmosis is the diffusion of water from a hypotonic (higher water, lower solute) to a hypertonic (lower water, higher solute) solution. In plant cells, this leads to turgor pressure in hypertonic environments, maintaining rigidity, or plasmolysis in hypotonic environments, causing wilting. Animal cells must be in isotonic environments to avoid bursting (in hypotonic) or shriveling (in hypertonic). Freshwater protists use contractile vacuoles for osmoregulation, expelling excess water. Leaf stomata, regulated by guard cells, control gas exchange and water loss. Guard cells pump potassium ions, affecting water potential and turgor, leading to opening or closing. Water potential, a quantitative measure, predicts water movement from higher to lower potential, incorporating solute and pressure components. The formula for water potential (Ψ) is Ψs + Ψp (solute potential plus pressure potential).

Enzymes, mostly proteins, catalyze biochemical reactions by lowering activation energy. They are highly specific, binding to substrates at their active sites due to complementary shape and charge. Enzyme activity is sensitive to environmental factors like pH and temperature, operating optimally within narrow ranges. Deviations can cause denaturation, altering the active site and reducing function, which can be reversible or irreversible. Substrate concentration affects activity; low concentration limits collisions, while saturation occurs when all active sites are occupied. Enzyme inhibitors can be competitive (blocking the active site) or non-competitive (binding to an allosteric site and altering the active site's shape). Metabolic pathways are linked series of enzyme-catalyzed reactions.

Autotrophs produce their own food: photoautotrophs (like plants) use light for photosynthesis, while chemoautotrophs (some bacteria/archaea) use chemical energy from inorganic substances. Heterotrophs obtain energy by consuming other organisms or organic compounds.

Exergonic reactions release energy and increase entropy (e.g., cellular respiration), while endergonic reactions require energy and decrease entropy (e.g., photosynthesis). ATP (adenosine triphosphate) is the cell's energy currency, storing energy by adding a phosphate to ADP (dehydration synthesis) and releasing energy by breaking off a phosphate (hydrolysis). Energy coupling links exergonic reactions to drive endergonic ones, making cellular work possible.

Photosynthesis uses light energy to convert carbon dioxide and water into carbohydrates and oxygen. It is an endergonic reaction, increasing organization and storing energy. Photosynthesis evolved approximately 3.5 billion years ago, oxygenating Earth's atmosphere and enabling aerobic metabolism and land colonization. It consists of two phases: light reactions (converting light energy into ATP and NADPH) and the Calvin cycle (using ATP and NADPH to fix carbon dioxide into sugars). Chlorophyll, and other pigments, absorb light energy, driving the process. The action spectrum shows blue and red light are most effective, while green light is reflected, making leaves appear green. Chloroplasts, with their double membranes, thylakoids (site of light reactions), and stroma (site of Calvin cycle), are the organelles for photosynthesis.

Light reactions in the thylakoid membranes convert light energy into chemical energy (ATP and NADPH). Photosystems (complexes of proteins and chlorophyll) absorb light, exciting electrons. The Z-scheme illustrates this process, showing how electrons are boosted in Photosystem II (splitting water to release oxygen and protons, creating an ATP-generating proton gradient through an electron transport chain and ATP synthase) and Photosystem I (further exciting electrons to reduce NADP+ to NADPH). ATP and NADPH, the products of light reactions, power the Calvin cycle.

Cellular respiration breaks down glucose and oxygen into carbon dioxide, water, and ATP. It is an exergonic reaction, releasing energy and increasing disorder. In eukaryotes, it begins with glycolysis in the cytoplasm, followed by the link reaction, Krebs cycle (in the mitochondrial matrix), and oxidative phosphorylation (electron transport chain and chemiosmosis on the inner mitochondrial membrane).

Glycolysis, an anaerobic process in the cytoplasm, invests 2 ATP to cleave glucose into two 3-carbon pyruvate molecules, generating 4 ATP (net 2 ATP) and 2 NADH. The link reaction converts pyruvate into acetyl-CoA (2 carbons) in the mitochondrial matrix, releasing CO2 and producing NADH. The Krebs cycle, a cyclical series of reactions in the mitochondrial matrix, oxidizes acetyl-CoA, generating 1 ATP, 3 NADH, and 1 FADH2 per acetyl-CoA molecule, releasing CO2 as a byproduct (the cycle runs twice per glucose molecule).

NADH and FADH2 deliver electrons to the electron transport chain (ETC) embedded in the inner mitochondrial membrane. Electron flow through the ETC powers proton pumps, moving protons from the matrix to the intermembrane space, creating an electrochemical gradient (proton motive force). Oxygen acts as the final electron acceptor, forming water. Protons then flow back into the matrix through ATP synthase, driving ATP production via chemiosmosis, generating the majority of ATP. Cellular respiration can generate heat instead of ATP in brown fat cells due to uncoupling proteins allowing protons to bypass ATP synthase, releasing energy as heat. The ATP generation mechanism in mitochondria and chloroplasts is fundamentally similar, utilizing electron transport chains and chemiosmosis, suggesting a shared evolutionary origin.

Aerobic respiration requires oxygen, generating significant ATP (approx. 32) across glycolysis, link reaction, Krebs cycle, and ETC. Anaerobic respiration (fermentation) occurs without sufficient oxygen, solely relying on glycolysis followed by fermentation, producing only 2 ATP. Fermentation (e.g., alcohol or lactic acid fermentation) regenerates NAD+ from NADH, allowing glycolysis to continue. These are essential for short bursts of energy when oxygen is scarce, as seen in muscle tissue during intense exercise.

Cells communicate directly through junctions or by secreting signaling molecules (ligands). Ligands can be hormones (long-distance) or local regulators (short-distance). Quorum sensing in bacteria, where signaling molecules trigger collective behaviors like biofilm formation, demonstrates cell communication even in simple organisms. Cell signaling proceeds in three phases: reception (ligand binds to receptor), transduction (signal converted and amplified internally), and response (cellular action). Nonpolar steroid hormones diffuse through the membrane to bind with intracellular receptors, acting as transcription factors to activate genes. Water-soluble hormones bind to cell surface receptors, triggering quick, widespread cellular responses. Epinephrine (adrenaline) signaling through G-protein coupled receptors exemplifies this, leading to the fight-or-flight response, including glucose release from the liver.

Epinephrine, a polar water-soluble hormone, binds to a G-protein coupled receptor on the cell membrane. This binding causes a conformational change in the receptor, activating a nearby G-protein by replacing GDP with GTP. The activated G-protein then dissociates and activates adenylyl cyclase, which converts ATP into cyclic AMP (cAMP), the second messenger. cAMP then activates a phosphorylation cascade involving protein kinases, amplifying the signal. This cascade ultimately activates enzymes like glycogen phosphorylase, converting glycogen to glucose for the fight-or-flight response. The system includes rapid deactivation mechanisms (e.g., ligand diffusion, G-protein GTP hydrolysis, phosphatase activity) to ensure the cellular response is transient.

Homeostasis maintains stable internal conditions, often regulated by feedback mechanisms. Negative feedback counteracts change to return a system to a set point (e.g., body temperature, blood glucose regulation by insulin and glucagon). Insulin lowers high blood glucose by promoting cellular glucose uptake and storage as glycogen, while glucagon raises low blood glucose by stimulating glycogen breakdown. Disruption of these feedback loops, as in type 2 diabetes (insulin resistance) or type 1 diabetes (lack of insulin production), leads to health issues. Positive feedback amplifies a process, driving it to completion (e.g., childbirth via oxytocin, fruit ripening via ethylene production). These loops are vital for maintaining physiological balance and developmental processes.

Mitosis (eukaryotic cell division) duplicates chromosomes and transmits them to daughter cells, resulting in genetically identical clones. Key functions include growth, tissue repair in multicellular organisms, and asexual reproduction in unicellular eukaryotes. The cell cycle consists of interphase (G1: growth, S: DNA replication, G2: preparation for division) and M-phase (mitosis and cytokinesis). Mitosis itself involves prophase (chromosomes condense, nuclear envelope breaks down, spindle forms), metaphase (chromosomes align at the metaphase plate), anaphase (sister chromatids separate and move to opposite poles), and telophase (new nuclear envelopes form, chromosomes decondense). Cytokinesis divides the cytoplasm, completing cell division. The G0 phase represents a non-dividing state for highly specialized cells.

Cell cycle checkpoints regulate progression, ensuring conditions are met before advancing. Failure to meet conditions can lead to G0 or apoptosis (programmed cell death). Cyclins (proteins whose concentration fluctuates) and cyclin-dependent kinases (CDKs, constantly present) form maturation-promoting factor (MPF) complexes that drive cell cycle progression, particularly through the G2 checkpoint. Cancer is unregulated cell division caused by mutations in proto-oncogenes (promoting division, e.g., Ras G-protein) and tumor suppressor genes (inhibiting division, e.g., p53). A mutated Ras protein can become constitutively active, constantly promoting cell division. A mutated p53 fails to halt the cell cycle or induce apoptosis when DNA is damaged, allowing cells with mutations to proliferate, increasing cancer risk.

Meiosis is the process of reduction division in sexually reproducing eukaryotes that produces haploid gametes (sperm and egg cells) from diploid germ cells. This is crucial for transmitting genes from one generation to the next while maintaining chromosome number and generating genetic variation. Diploid cells (2 sets of chromosomes) contain homologous pairs, with one chromosome from each parent. Gametes are haploid (1 set of chromosomes). Homologous chromosomes are matching pairs that contain the same genes in the same order but may have different alleles. Meiosis involves DNA replication, followed by Meiosis I (separating homologous chromosomes) and Meiosis II (separating sister chromatids), resulting in four unique haploid gametes.

Meiosis generates genetic diversity through independent assortment and crossing over. Independent assortment (during metaphase I) refers to the random orientation and separation of homologous chromosome pairs, leading to many possible chromosome combinations in gametes (2^n, where n is the number of homologous pairs). Crossing over (during prophase I) involves the exchange of genetic material between homologous chromosomes at chiasmata, creating recombinant chromosomes with novel allele combinations. Fertilization further increases diversity by combining unique gametes from two parents. Collectively, these mechanisms ensure each offspring is genetically unique.

In mammals, sex is determined by sex chromosomes: XX for females and XY for males. The SRY gene on the Y chromosome triggers male development. It is the sperm (carrying X or Y) that determines the offspring's chromosomal sex. Birds have a ZW system, where females are ZW and males are ZZ, so the egg determines sex. In some reptiles, sex is determined by incubation temperature. Ants, bees, and wasps exhibit haplo-diploidy, where diploid individuals are female and haploid individuals (from unfertilized eggs) are male; this high relatedness among sisters (75%) may explain eusociality. Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis, leading to gametes with abnormal chromosome numbers (e.g., n+1 or n-1), which can result in conditions like Down syndrome (trisomy 21) or Turner syndrome (monosomy X).

Genes are the basic units of heredity, determining traits and coding for proteins. Mendel's principle of segregation states that individuals have two alleles for each gene (located on homologous chromosomes), but pass on only one allele to each gamete. Homozygous individuals have identical alleles, while heterozygous individuals have different alleles. Dominant alleles are always expressed in the phenotype and mask recessive alleles, which are only expressed in homozygous recessive individuals. Phenotype refers to observable characteristics, while genotype is the underlying genetic makeup. Monohybrid crosses (between two heterozygotes for a single trait) typically yield a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio in the offspring, calculable with Punnett squares. The P generation (parental) are typically true-breeding homozygotes, F1 (first filial) are their hybrid offspring, and F2 (second filial) are offspring from F1 interbreeding. Mendel's principle of independent assortment states that alleles for different traits (on non-homologous chromosomes) segregate independently during gamete formation. The rule of multiplication predicts the probability of multiple independent events by multiplying their individual probabilities.

Linked genes are located on the same chromosome and tend to be inherited together, violating Mendel's principle of independent assortment. However, crossing over during meiosis can separate linked genes, producing recombinant phenotypes. The frequency of recombination is proportional to the distance between genes on a chromosome, allowing for chromosome mapping. Sex-linked genes are on the X chromosome. Males (XY) express recessive sex-linked traits more often because they have only one X chromosome (e.g., hemophilia, color blindness). Females (XX) can be carriers and only express recessive sex-linked traits if homozygous recessive.

Non-nuclear inheritance involves genes on mitochondrial or chloroplast DNA, which are inherited maternally (only passed through the egg, not sperm). This pattern of inheritance does not follow Mendelian rules. Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygous phenotypes (e.g., pink flowers from red and white parents). Pleiotropy is when a single gene influences multiple phenotypic traits (e.g., sickle cell anemia and cystic fibrosis, where a single gene mutation leads to diverse symptoms across various organ systems). Environmental interaction with genotype means that the same genotype can produce different phenotypes under different environmental conditions (e.g., hydrangea flower color based on soil pH, Himalayan rabbit fur color based on temperature, human height/weight influenced by diet and genes).

DNA is a double-stranded helix composed of nucleotide monomers (deoxyribose, phosphate, nitrogenous base). The strands are held together by hydrogen bonds between complementary bases (A-T, G-C) and are antiparallel. DNA's structure allows for information storage (sequence of bases), replicability (each strand as a template), stability, and mutability (allowing evolution). RNA is primarily involved in information transfer during protein synthesis (mRNA, tRNA, rRNA) and gene regulation (small RNAs, microRNAs). In some viruses, RNA is the hereditary molecule. Prokaryotes store DNA in looped circular chromosomes; eukaryotes have multiple linear chromosomes wrapped around histones. Plasmids are small, extra-chromosomal DNA loops common in bacteria, involved in horizontal gene transfer and widely used in genetic engineering.

DNA replication is semiconservative: each new double helix consists of one original (conserved) strand and one newly synthesized strand. The process begins at an origin of replication, where helicase unwinds the DNA, creating a replication fork. DNA polymerase synthesizes new DNA by adding nucleotides to the 3' end of an existing strand. Primase lays down RNA primers, as DNA polymerase requires an existing strand to start. Single-strand binding proteins prevent rewinding. On the leading strand, replication is continuous, following the replication fork. On the lagging strand, replication is discontinuous, forming Okazaki fragments that are later joined by DNA ligase after DNA polymerase I removes RNA primers and replaces them with DNA.

The central dogma of molecular genetics is DNA makes RNA makes protein. Transcription is the synthesis of RNA from a DNA template. RNA polymerase binds to a promoter region on DNA, transcribing the template strand into an RNA sequence. RNA polymerase reads DNA 3' to 5' and synthesizes RNA 5' to 3'. Transcription ends at a terminator region. The template strand (also called non-coding, antisense, or minus strand) guides RNA synthesis. The coding strand (sense or positive strand) has the same sequence as the resulting mRNA (with U instead of T). In prokaryotes, transcription and translation can occur simultaneously due to the lack of a nucleus.

The genetic code is universal, specific, and redundant. Groups of three mRNA nucleotides (codons) specify one amino acid. Translation begins when the small ribosomal subunit binds to mRNA and travels to the start codon (AUG). A tRNA carrying methionine and the complementary anticodon (UAC) binds to AUG at the ribosome's P site. The large ribosomal subunit then joins. Elongation involves subsequent tRNAs entering the A site, peptide bonds forming between amino acids, and the ribosome translocating along the mRNA. Termination occurs when the ribosome encounters a stop codon, which is recognized by a release factor, causing the polypeptide to be released and the ribosomal complex to dissociate. The polypeptide then folds into its functional protein structure. Ribosomes are composed of rRNA and protein, with E, P, and A sites for tRNA binding.

An operon is a prokaryotic system of gene regulation consisting of structural genes, an operator (binding site for repressor), a promoter (binding site for RNA polymerase), and a regulatory gene (producing regulatory protein). The trp operon is a repressible operon that synthesizes tryptophan. When tryptophan is absent, the repressor cannot bind to the operator, allowing RNA polymerase to transcribe the genes. When tryptophan is present, it acts as a corepressor, binding to the repressor, enabling it to bind the operator and block transcription. The lac operon is an inducible operon that codes for enzymes to digest lactose. When lactose is absent, the repressor binds to the operator, blocking transcription. When lactose is present, it acts as an inducer, binding to the repressor, preventing it from binding the operator, thus allowing transcription. Both operons are negative feedback systems, saving energy by only producing enzymes when needed. Glucose is preferentially metabolized over lactose (diauxic growth).

Eukaryotic gene regulation is highly complex due to multicellularity, specialized tissues, and large genomes. Most eukaryotic DNA is non-coding, and genes contain introns. DNA expression is largely regulated by epigenetic modifications (methylation, histone acetylation), which dynamically alter DNA accessibility without changing nucleotide sequences, explaining cell differentiation and potentially being heritable. Transcription is controlled by regulatory DNA sequences (promoters, enhancers) and interacting proteins (activators, DNA bending proteins, mediator proteins, general transcription factors) that control RNA polymerase binding. Genes can be coordinated across different tissues through shared regulatory sequences (e.g., testosterone receptor activating different genes in different male lion tissues). Pre-mRNA processing in eukaryotes involves adding a 5' GTP cap and a 3' poly-A tail (protecting mRNA and aiding translation) and splicing out introns to join exons. Alternative splicing of exons allows for multiple protein isoforms from a single gene, increasing phenotypic variation. Small RNAs (like microRNAs) regulate gene expression post-transcriptionally through RNA silencing (degrading mRNA or inhibiting translation).

A mutation is a random change in DNA or chromosomes. Point mutations involve a single nucleotide change. Silent mutations alter DNA but not the amino acid due to genetic code redundancy. Nonsense mutations introduce a premature stop codon. Missense mutations change one amino acid to another, with impact varying by chemical similarity. Frameshift mutations (insertions or deletions not in multiples of three) alter the reading frame, causing extensive missense or nonsense. Sickle cell disease is a missense mutation causing a valine-for-glutamic acid substitution in hemoglobin, leading to sickled cells and health problems under low-oxygen conditions. Mutations can be positive (increasing fitness, e.g., loss of pelvic spine in sticklebacks in freshwater), negative (decreasing fitness), or neutral (no effect on phenotype due to non-coding region or silent mutation). Mutations are the raw material for evolution. Germline mutations occur in gamete-producing cells and are heritable; somatic mutations occur in body cells and affect only the individual.

Horizontal gene transfer (HGT) is the transfer of genetic material between organisms that are not parent and offspring, unlike vertical gene transfer. In bacteria, HGT includes: conjugation (transfer of plasmids via a pilus, important for antibiotic resistance), transformation (uptake of DNA fragments from the environment into the genome), and transduction (transfer of host DNA by bacteriophages due to errors in viral replication). Viral recombination, where different viral strains co-infect a host and exchange genetic material, leads to new viral strains (e.g., in influenza), capable of causing pandemics.

Recombinant DNA is artificially created DNA from multiple sources. Restriction enzymes cut DNA at specific recognition sites, creating sticky ends. These sticky ends can anneal with complementary sticky ends from other DNA fragments (e.g., human genes) cut with the same enzyme, and DNA ligase then seals the sugar-phosphate backbone, forming recombinant DNA. This technique is used to create recombinant plasmids containing human genes (e.g., for insulin production). For eukaryotic genes to be expressed in bacteria, introns must be removed, as bacteria lack splicing machinery. This can be achieved by using reverse transcriptase to synthesize cDNA from processed mRNA (which already lacks introns). Gel electrophoresis sorts DNA fragments by size, using an electrical current to move negatively charged DNA through a porous gel. PCR (polymerase chain reaction) is a cell-free technique to amplify specific DNA sequences using heat-resistant DNA polymerase, primers, and repeated heating/cooling cycles. DNA sequencing determines the precise order of nucleotides, used to study proteins, evolutionary relationships, cancer, and track viral variants.

Natural selection, a key mechanism of evolution, acts on heritable variation within a population where more individuals are born than can survive. Those with advantageous traits (adaptations) survive and reproduce more successfully, passing on their genes. Artificial selection (selective breeding) involves humans choosing desirable traits in organisms over generations (e.g., dog breeds, Brassica oleracea varieties). Sexual selection promotes traits that directly increase reproductive success, leading to sexual dimorphism. Intersexual selection involves mate choice (e.g., female turkeys choosing males with elaborate displays). Intrasexual selection involves competition among members of one sex for mates (e.g., elephant seal males fighting). Natural selection can lead to changes in phenotypic distribution: directional (favors one extreme), stabilizing (favors intermediates), or disruptive (favors both extremes). Adaptive melanism (darkening of body due to environmental darkening, e.g., rock pocket mouse, peppered moth) is a strong example of directional selection. Evolutionary fitness is measured by the number of surviving, reproducing offspring.

Population genetics studies allele distribution and change in populations over time, with allele frequency as a key measure. Evolution is defined as a change in allele frequencies in a gene pool. Dominant alleles are not necessarily more common. The Hardy-Weinberg principle describes a non-evolving population where allele and genotype frequencies remain constant across generations, provided five conditions are met: infinite population size, no natural selection, random mating, no gene flow, and no mutation. Violation of any of these conditions causes evolution. Genetic drift is random change in allele frequencies, significant in small populations, often caused by a population bottleneck (drastic reduction in population size) or the founder effect (a new population established by a few individuals). Gene flow (allele movement between populations) can alter allele frequencies and reduce differences between populations. Mutation (the ultimate source of variation) can also change allele frequencies if directional. Illustrated by sickle cell allele frequency in malaria-prone regions (heterozygote advantage), maintaining a harmful allele due to its protective effect against malaria.

Evidence for evolution is abundant. Homologous traits (shared underlying structure/embryological origin due to common ancestry, e.g., vertebrate forelimbs) result from adaptive radiation, where one common ancestor gives rise to diverse species adapted to different niches. Vestigial structures are homologous features with no current function but were functional in ancestors (e.g., whale pelvis, human tailbone). Analogous features have similar function but different underlying structure, arising from convergent evolution (e.g., bird and bat wings). Molecular homologies (similarities in DNA, RNA, protein sequences) indicate common ancestry, with more closely related species having fewer molecular differences (e.g., hemoglobin comparison). Pseudogenes are non-functional gene relics shared due to common ancestry (e.g., human GULO pseudogene). Universal biological features (DNA as genetic material, ATP, universal genetic code, ribosomes, shared metabolic pathways) indicate a single origin of life. Shared features among all eukaryotes (nucleus, mitochondria, endomembrane system, introns, linear chromosomes, sexual reproduction) point to a common eukaryotic ancestor. Embryological development shows similarities among early vertebrate embryos, indicating common ancestry, and vestigial embryonic features (e.g., gill slits, tails) further support descent with modification. Diverse species sharing common developmental genes (e.g., 'eyeless' gene for eye development) provide profound molecular evidence for ancient common ancestry. Biogeography (geographic distribution of species) supports evolution by showing patterns consistent with species evolving in one area and spreading (e.g., marsupial distribution).

The biological species concept defines a species as a group of organisms that can interbreed naturally to produce viable, fertile offspring and are reproductively isolated from other such groups. Limitations exist for hybrids, extinct/asexual species, and prokaryotes. Reproductive isolating mechanisms prevent gene flow: prezygotic barriers prevent successful mating/fertilization (behavioral, temporal, mechanical, habitat, gametic isolation), while postzygotic barriers occur after fertilization, leading to inviable, sterile, or breakdown of hybrid offspring (e.g., mules). Allopatric speciation involves geographic isolation leading to genetic divergence and eventual reproductive isolation. Sympatric speciation occurs without a geographic barrier, often through polyploidy in plants or adaptation to microhabitats/sexual selection in animals (e.g., cichlids in Lake Victoria, specialized lice on birds). Adaptive radiation is the rapid diversification of one ancestral species into many descendant species, each filling a new ecological niche (e.g., Galapagos finches). Phylogenetic trees graphically represent these evolutionary histories. Phenotypic variation is critical for evolution as it provides the raw material for natural selection. Examples of advantageous phenotypic variation include varied phospholipid structures in mammals (saturated in core, unsaturated in extremities for cold adaptation), fetal hemoglobin with higher oxygen affinity than adult hemoglobin, and diverse chlorophyll types (A and B) increasing photosynthetic efficiency.

Explaining the origin of life involves several key steps: Earth becoming a habitable planet, abiotic synthesis of monomers (amino acids, nucleotides, fatty acids), abiotic synthesis of polymers from monomers, formation of protocells (encapsulated collections of polymers), and emergence of self-replicating cells. The Miller-Urey experiment (1950s) demonstrated that organic monomers (like amino acids) could form abiotically under simulated early Earth conditions (though specific atmospheric composition assumptions have since been refined). It is widely believed that RNA, not DNA, was the first hereditary molecule due to its dual capacity for genetic information storage (like in some viruses) and catalytic activity (ribozymes). This 'RNA world' hypothesis suggests that self-replicating RNA systems predated DNA-based life and the last universal common ancestor. The evolution from inorganic precursors to RNA monomers, then RNA polymers with catalytic properties, followed by encapsulation in lipid bilayers to form protocells, ultimately led to the emergence of the last universal common ancestor.

Organisms respond to environmental changes in diverse ways, often involving complex behaviors. Predator warnings (alarm signals) are altruistic behaviors, like those observed in Belding's ground squirrels (distinct calls for aerial vs. terrestrial predators). Such altruism can be explained by kin selection and inclusive fitness: individuals may risk themselves to protect close relatives who share their genes, thereby increasing the overall frequency of those genes in the population. Paul Sherman's study on Belding's ground squirrels showed that females, who typically remain closer to their natal burrow and relatives, are significantly more likely to give alarm calls than males, supporting kin selection. Eusociality (e.g., in bees, ants, some mole rats) is an extreme form of social organization where only a few individuals reproduce, and others are non-reproductive workers. Haplo-diploidy (sex determination where females are diploid and males are haploid), as seen in bees, results in high relatedness among sisters (75%), providing a genetic explanation for the altruism of worker bees, as they are more related to their sisters than their own offspring.

Metabolism is the sum of all chemical processes in an organism. Metabolic rate is the energy expended per unit time. Basal metabolic rate (BMR) is energy used at rest in a comfortable temperature. Metabolic rate can be measured by oxygen consumption, carbon dioxide production, or heat generation. Endotherms (mammals, birds) internally generate body heat (from cellular respiration) to maintain a constant body temperature, allowing activity regardless of external temperature, but at a high energy cost. Ectotherms (reptiles, amphibians) conform their body temperature to the environment, requiring much less food but limiting their activity in cold conditions. In ectotherms, metabolic rate generally increases with temperature. In endotherms, metabolic rate increases at cold temperatures (to generate heat) and high temperatures (for cooling mechanisms), with a minimum BMR within their thermoneutral zone.

Ecosystems comprise communities (living organisms) and abiotic factors (non-living environment). Food chains illustrate the linear transfer of energy and matter, while food webs show all interconnected feeding relationships. Trophic levels describe an organism's position in a food web: producers (autotrophs, e.g., plants), primary consumers (herbivores), secondary consumers (carnivores), tertiary consumers, and decomposers (breaking down dead organic matter). The pyramid of energy demonstrates the decrease in available energy at successively higher trophic levels. The '10% rule' states that only about 10% of chemical energy from one trophic level is transferred to the next, with the rest lost as heat (second law of thermodynamics), used in metabolism, or not assimilated. Other ecological pyramids (biomass, numbers) can show various patterns, sometimes inverted depending on the ecosystem.

Biogeochemical cycles trace the movement of elements/compounds between living (biotic) and non-living (abiotic) reservoirs (accumulation points) via fluxes (transfer mechanisms). The carbon cycle: CO2 from the atmosphere enters plants via photosynthesis (carbon fixation), moves through food webs via consumption, and returns to the atmosphere via respiration (plants, animals, decomposers) and combustion of fossil fuels (fossilized plant matter). The nitrogen cycle: Atmospheric N2 (78%) is fixed into ammonia by nitrogen-fixing bacteria (symbiotic in legumes, free-living in soil). Nitrifying bacteria convert ammonia to nitrites/nitrates (usable by non-nitrogen-fixing plants). Nitrogen moves through food webs via assimilation and consumption. Decomposers return nitrogen to ammonia (ammonification), and denitrifying bacteria convert nitrates back to N2 gas. The water cycle: Water evaporates from oceans/land (driven by solar energy), condenses into clouds, precipitates as rain/snow, flows across land as runoff, percolates into groundwater, and transpires from plants, eventually returning to oceans. The phosphorus cycle: Phosphate ions are released from weathered rocks, assimilated by plants, transferred via consumption, and returned to soil by waste/decomposition. In aquatic systems, phosphate enters via runoff, is assimilated by algae, moves through food webs, and is lost through long-term sedimentation. Fertilizers add significant human-induced fluxes to these cycles.

Population size changes due to births, deaths, immigration, and emigration. Exponential growth occurs when populations have abundant resources, with a growth rate proportional to current population size (delta N/delta t = rN). This J-shaped curve is seen with invasive species or early epidemic phases. However, exponential growth is unsustainable due to environmental limits. Carrying capacity (K) is the maximum population size an environment can support, set by limiting factors. The logistic growth model (delta N/delta t = rN(K-N)/K) describes populations approaching K, as density-dependent limiting factors (competition, parasitism, predation, stress) increase, slowing growth. Extrinsic factors are external pressures, intrinsic factors are internal physiological responses. Density-independent factors (e.g., natural disasters) affect population size regardless of density. Populations may oscillate around K or catastrophically overshoot, leading to environmental degradation. Predator-prey cycles (e.g., lynx and hare) show oscillating population sizes, influenced by both predation and food availability.

Symbiosis describes close interactions between two species, with outcomes denoted by +/-/0 for benefit/harm/no effect. Competition (--) occurs when species vie for the same limited resource (e.g., predators for prey, trees for light). Mutualism (++) benefits both species (e.g., clownfish and anemones, algae in anemones). Predation (+-) benefits the predator, harms the prey (e.g., leopard and bushbuck). Herbivory (+-) benefits the herbivore, harms the plant (e.g., deer grazing). Commensalism (+0) benefits one species, with no effect on the other (e.g., cattle egrets and cattle, moss on trees). Parasitism (+-) involves a parasite living on/in a host, harming it (e.g., viruses, Giardia). Brood parasitism (e.g., cuckoos using other birds' nests) and parasitoidism (e.g., wasps laying eggs in caterpillars) are variations where the host is directly harmed or killed. Parasitism is a widespread ecological strategy.

An ecological niche defines how a species lives and uses resources. Gause's Competitive Exclusion Principle states that two species cannot coexist indefinitely in the same niche. Competition can drive evolution: species may evolve through resource partitioning (specializing in different aspects of the resource) and character displacement (e.g., finch beak sizes evolving to reduce overlap for food resources). This can lead to ecomorphs (species adapted morphologically to specific niches), sometimes via convergent evolution on different islands. The fundamental niche is the full range of resources a species could use, while the realized niche is the subset actually used due to competition (e.g., barnacle distribution in intertidal zones). Evolutionary arms races are positive feedback loops of adaptations and counter-adaptations between interacting species (e.g., predator-prey speed, camouflage, defenses), leading to extreme specializations.

Keystone species are critical for structuring entire biological communities; their removal can drastically alter biodiversity. They are often (but not always) predators that keep herbivore populations in check. For example, sea stars in the intertidal zone prey on mussels, preventing mussels from dominating and allowing other invertebrates to thrive, thus increasing biodiversity. Trophic cascades are system-wide effects that occur when the addition or removal of a single species impacts multiple trophic levels. The reintroduction of wolves to Yellowstone National Park (a keystone predator) led to a cascade: reduced elk populations, allowing aspen and willow regrowth, increasing beaver populations, creating aquatic habitats, and benefiting other wildlife. Not all keystone species are top predators; ecosystem engineers like beavers create habitat, increasing biodiversity.

Biodiversity encompasses ecosystem, species, and genetic diversity. High biodiversity increases ecosystem resilience, allowing systems to cope with change and adapt. Biodiversity offers intrinsic value, as well as present/potential benefits to humanity (e.g., medicinal compounds like paclitaxel from the Pacific yew) and essential ecosystem services (e.g., oxygen production, carbon sequestration, pest control). The Simpson diversity index quantifies diversity based on species richness (number of species) and species evenness (relative abundance of each species). Human activities cause significant ecosystem disruption and biodiversity loss. This includes habitat alteration/destruction (e.g., urbanization, agriculture), overexploitation (overhunting/overharvesting), habitat fragmentation (creating small, isolated populations prone to genetic drift and inbreeding), introduction of invasive species (outcompeting native species and disrupting food webs), and deforestation (especially of highly diverse rainforests). A major concern is increased atmospheric CO2 from fossil fuel combustion since the Industrial Revolution, leading to global climate change. CO2 is a greenhouse gas, trapping heat and causing rising global temperatures, leading to increased forest fires, glacier retreat, altered precipitation patterns (floods/droughts), and rising sea levels, all of which disrupt ecosystems and accelerate biodiversity loss. Solutions involve transitioning to renewable energy sources.