AP Biology Quick Review

Life depends on water's unique properties, nearly all of which emerge from its polarity and its capacity to form hydrogen bonds with other water molecules and with other polar or charged substances.

Water is a polar molecule (bent shape, unequal electron sharing between O and H) that forms hydrogen bonds; these bonds explain water's high specific heat (resists temperature change, buffers organisms and climate), high heat of vaporization (evaporative cooling), cohesion (surface tension, capillary action) and adhesion (water climbs against gravity in plant xylem), and lower density as a solid than a liquid (ice floats, insulating aquatic ecosystems). Water is the ``universal solvent'' for polar and ionic solutes because of hydration shells. $pH = -$; buffers resist pH change by absorbing or releasing H$^+$.

Cohesion (water molecules bonding to EACH OTHER, producing surface tension) vs.\ adhesion (water molecules bonding to OTHER polar surfaces, producing capillary action in xylem).

Students attribute water's high specific heat to a strong covalent O--H bond breaking; the property comes from the energy needed to break HYDROGEN BONDS between separate water molecules, not the covalent bonds within one molecule.

Polar + H-bonds $arrow$ high specific heat, cohesion/adhesion, universal solvent, ice floats.

Carbohydrates and lipids are built (or, for lipids, aggregated) from smaller subunits whose structure directly determines their biological function in energy storage, structural support, and membranes.

Dehydration synthesis (condensation) links monomers into polymers and releases H$_2$O; hydrolysis breaks polymers into monomers by adding H$_2$O. Carbohydrates: monosaccharides (e.g., glucose) $arrow$ disaccharides $arrow$ polysaccharides; starch/glycogen (energy storage, $$-glycosidic bonds, branched/coiled) vs.\ cellulose/chitin (structural, $$-glycosidic bonds, indigestible by most animals). Lipids are hydrophobic and NOT true polymers: triglycerides (glycerol + 3 fatty acids, energy storage), phospholipids (polar head + nonpolar tails, form the bilayer), and steroids (e.g., cholesterol, four fused carbon rings, membrane fluidity/signaling).

Saturated fatty acids (no C=C double bonds, straight chains, pack tightly, solid at room temperature) vs.\ unsaturated fatty acids (C=C double bond(s) create kinks, pack loosely, liquid at room temperature, more fluid).

Students think starch and cellulose are chemically identical because both are glucose polymers; the bond orientation ($$ vs.\ $$ glycosidic linkages) makes cellulose indigestible to most animals while starch is readily hydrolyzed.

Dehydration synthesis (build, lose H$_2$O) vs.\ hydrolysis (break, add H$_2$O); starch/glycogen=$$ storage; cellulose/chitin=$$ structure; phospholipids=bilayer.

A protein's three-dimensional shape, determined by its amino acid sequence, dictates its specific function, while nucleic acids store and transmit the hereditary information that specifies that sequence.

Amino acids link via peptide bonds (dehydration synthesis) into polypeptides. Protein structure: primary (amino acid sequence) $arrow$ secondary ($$-helices/$$-pleated sheets via backbone hydrogen bonding) $arrow$ tertiary (overall 3D folding via R-group interactions: hydrophobic interactions, ionic bonds, hydrogen bonds, disulfide bridges) $arrow$ quaternary (multiple polypeptide subunits, e.g., hemoglobin). Denaturation (heat, pH extremes) unfolds a protein and destroys function. Nucleic acids: a nucleotide = nitrogenous base + pentose sugar + phosphate group; DNA (deoxyribose, bases A-T-G-C, double-stranded, antiparallel) vs.\ RNA (ribose, bases A-U-G-C, typically single-stranded).

Tertiary structure (folding of a SINGLE polypeptide chain via R-group interactions) vs.\ quaternary structure (assembly of MULTIPLE separate polypeptide subunits into one functional protein).

Students think denaturation breaks peptide bonds; denaturation disrupts the weaker bonds/interactions maintaining secondary/tertiary/quaternary shape (hydrogen, ionic, hydrophobic, disulfide) while the primary sequence (peptide bonds) remains intact.

Primary (sequence) $arrow$ secondary (H-bond folds) $arrow$ tertiary (R-group 3D fold) $arrow$ quaternary (multi-subunit); DNA=deoxyribose/ATGC/double; RNA=ribose/AUGC/single.

Eukaryotic cells achieve greater complexity than prokaryotic cells by compartmentalizing functions into membrane-bound organelles, each specialized for a distinct metabolic task.

Prokaryotes (bacteria/archaea) lack a nucleus and membrane-bound organelles, have circular DNA in a nucleoid region, and are smaller/simpler. Key eukaryotic organelles: nucleus (houses DNA, site of transcription), rough ER (ribosome-studded, synthesizes/modifies membrane and secretory proteins), smooth ER (lipid synthesis, detoxification, Ca$^2+$ storage), Golgi apparatus (modifies, sorts, and packages proteins/lipids for secretion, cis to trans), mitochondria (aerobic respiration/ATP production, double membrane, own DNA), chloroplasts (photosynthesis, double membrane, own DNA, plants/algae only), lysosomes (hydrolytic digestion, animal cells), and the cytoskeleton (microtubules, microfilaments, intermediate filaments --- shape, movement, transport).

Rough ER (studded with ribosomes; makes proteins destined for secretion or membranes) vs.\ smooth ER (no ribosomes; makes lipids, detoxifies drugs, stores calcium).

Students think the Golgi apparatus makes proteins; the Golgi only modifies, sorts, and packages proteins already synthesized by ribosomes/the rough ER, tagging them for their final destination.

Prokaryote = no nucleus/organelles; rough ER=protein (has ribosomes); smooth ER=lipids/detox; Golgi=modify+ship; mito=respiration; chloroplast=photosynthesis.

Mitochondria and chloroplasts likely originated as free-living prokaryotes engulfed by an ancestral eukaryotic cell, and a cell's maximum size is constrained by the physics of diffusion across its surface.

Endosymbiotic theory evidence: mitochondria and chloroplasts have their own circular DNA (like bacteria), their own ribosomes (similar in size to bacterial ribosomes), replicate independently by a process resembling binary fission, and are surrounded by a double membrane (inner = original bacterial membrane, outer = from the engulfing host's vesicle). As a cell grows, volume increases faster (scaling as $r^3$) than surface area (scaling as $r^2$), lowering the surface-area-to-volume ratio (SA:V) and limiting the rate of nutrient/waste exchange relative to metabolic demand --- this caps cell size and favors compartmentalization.

Surface area (scales with $r^2$, governs exchange rate with the environment) vs.\ volume (scales with $r^3$, governs metabolic demand) --- as cells get bigger, SA:V ratio DECREASES.

Students think a larger cell is always more efficient; a larger cell has a SMALLER surface-area-to-volume ratio, meaning less membrane is available per unit of cytoplasm needing resources --- why very large cells divide, fold membranes, or compartmentalize.

Mito/chloroplast = double membrane + own DNA + own ribosomes $arrow$ engulfed bacteria; SA:V $$ as size $$ $arrow$ caps cell size.

The plasma membrane is a selectively permeable fluid mosaic of phospholipids and proteins, and small or nonpolar molecules cross it passively down their concentration gradient without any energy input.

Fluid mosaic model: phospholipid bilayer (hydrophilic heads out, hydrophobic tails in) embedded with proteins (integral/transmembrane and peripheral), cholesterol (moderates fluidity), and glycoproteins/glycolipids (cell recognition). Simple diffusion: small nonpolar molecules (O$_2$, CO$_2$) cross directly through the bilayer, high $arrow$ low concentration, no energy or protein required. Facilitated diffusion: polar/charged molecules (glucose, ions) cross via channel proteins (fixed pore, often gated) or carrier proteins (change shape) --- still passive (no ATP), still moving down the gradient.

Simple diffusion (no protein needed, small nonpolar molecules cross the lipid bilayer directly) vs.\ facilitated diffusion (requires a channel or carrier protein for polar/charged/large molecules, but is still passive/no ATP).

Students assume any transport requiring a protein must be active transport; facilitated diffusion uses transport proteins but is still PASSIVE (moves down the gradient, no ATP spent) --- only movement AGAINST the gradient requires ATP.

Simple diffusion = small/nonpolar, no protein; facilitated diffusion = polar/ions, channel/carrier protein, still passive (no ATP).

Water moves osmotically toward regions of lower water potential, while active transport allows cells to move solutes AGAINST their concentration gradient by directly or indirectly spending ATP.

Water potential $ = _p + _s$, where $_p$ = pressure potential and $_s$ = solute potential ($_s = -iCRT$, more solute = more negative $_s$); water always moves from higher to lower $$. Tonicity: hypertonic solution (more solute outside) $arrow$ cell loses water, shrinks (crenation/plasmolysis); hypotonic (more solute inside) $arrow$ cell gains water, swells/lyses; isotonic $arrow$ no net water movement. Active transport moves solutes against their gradient using ATP directly (primary, e.g., the Na$^+$/K$^+$ pump: 3 Na$^+$ out, 2 K$^+$ in per ATP) or indirectly via an established gradient (secondary/cotransport). Endocytosis and exocytosis move bulk material using vesicles and ATP.

Hypertonic (relatively MORE solute, cell LOSES water, shrinks) vs.\ hypotonic (relatively LESS solute outside, cell GAINS water, swells) --- always described relative to the cell's interior.

Students calculate water movement using solute concentration alone; in a plant cell, pressure potential (turgor pressure from the cell wall) also contributes to total $$, so a walled cell can reach equilibrium in a hypotonic solution without bursting.

$=_p+_s$, water moves high$arrow$low $$; hypertonic=cell shrinks; hypotonic=cell swells; Na$^+$/K$^+$ pump=3 out/2 in per ATP.

Enzymes are protein (or RNA) catalysts that speed up reactions by lowering the activation energy required, without being consumed or altering the reaction's overall energy change.

The active site binds a specific substrate; induced fit (the active site changes shape slightly around the substrate) improves on the simple lock-and-key model. Enzymes lower activation energy (E$_a$) but do NOT change $ G$ (free energy change) of the reaction. Rate depends on temperature (increases to an optimum, then denatures), pH (each enzyme has an optimal pH), and substrate concentration (rate increases then plateaus at saturation, $V_max$). Competitive inhibitors bind the active site directly (overcome by adding more substrate); noncompetitive/allosteric inhibitors bind elsewhere, changing enzyme shape (NOT overcome by more substrate). Cofactors (inorganic ions) and coenzymes (organic, often vitamin-derived) assist catalysis.

Competitive inhibition (binds the ACTIVE SITE, overcome by more substrate, raises apparent $K_m$) vs.\ noncompetitive inhibition (binds an ALLOSTERIC site, cannot be overcome by more substrate, lowers $V_max$).

Students think enzymes change whether a reaction is spontaneous; enzymes only speed up the RATE by lowering activation energy --- they never change $ G$ or make a nonspontaneous reaction spontaneous.

Active site + induced fit; enzymes lower $E_a$ NOT $ G$; competitive=active site (beat with more substrate); noncompetitive=allosteric site (can't beat).

Cellular respiration incrementally oxidizes glucose across connected stages, harvesting energy in small steps and storing it in electron carriers and a small amount of ATP made by substrate-level phosphorylation.

Glycolysis (cytoplasm, does not require O$_2$): glucose (6C) $arrow$ 2 pyruvate (3C), net 2 ATP (substrate-level phosphorylation) + 2 NADH. Pyruvate oxidation (mitochondrial matrix): each pyruvate $arrow$ acetyl-CoA + CO$_2$ + NADH. Krebs/citric acid cycle (matrix, runs twice per glucose): acetyl-CoA is fully oxidized to CO$_2$, generating per turn 3 NADH, 1 FADH$_2$, and 1 ATP (GTP). NADH and FADH$_2$ carry electrons to the electron transport chain.

Substrate-level phosphorylation (a phosphate is transferred DIRECTLY from a substrate to ADP; occurs in glycolysis & Krebs) vs.\ oxidative phosphorylation (ATP synthase uses a chemiosmotic H$^+$ gradient built by the electron transport chain).

Students think glycolysis requires oxygen because it's ``step 1'' of aerobic respiration; glycolysis occurs in the cytoplasm and proceeds identically with or without O$_2$ --- oxygen is only required later, as the final electron acceptor in the ETC.

Glycolysis (cytoplasm, glucose$arrow$2 pyruvate, net 2 ATP+2 NADH, no O$_2$ needed) $arrow$ pyruvate oxidation $arrow$ Krebs (matrix, 2 turns, CO$_2$+NADH+FADH$_2$+ATP).

The electron transport chain uses the energy released as electrons move downhill through a series of carriers to pump H$^+$ into the intermembrane space, and the resulting gradient drives ATP synthase to make the bulk of the cell's ATP by chemiosmosis.

NADH/FADH$_2$ donate electrons to protein complexes in the inner mitochondrial membrane; energy released as electrons pass along the chain pumps H$^+$ from the matrix into the intermembrane space, creating an electrochemical gradient. O$_2$ is the final electron acceptor, forming H$_2$O --- without O$_2$, the chain backs up and stops. Chemiosmosis: H$^+$ flows back down its gradient through ATP synthase, powering phosphorylation of ADP to ATP (oxidative phosphorylation), yielding the majority of ATP from one glucose ($≈$30--38 ATP total). Without O$_2$, cells use fermentation (lactic acid or alcoholic) to regenerate NAD$^+$ so glycolysis can continue, producing only glycolysis's net 2 ATP.

Aerobic respiration (O$_2$ present, full ETC runs, $≈$30--38 ATP total) vs.\ fermentation (no O$_2$, ETC stops, only glycolysis's 2 ATP net, but NAD$^+$ is regenerated so glycolysis can keep running).

Students think ATP synthase directly uses electrons for energy; ATP synthase is powered by the H$^+$ GRADIENT (chemiosmosis), not by electrons themselves --- electrons only power the pumps that build the gradient.

ETC pumps H$^+$ using electron energy $arrow$ gradient $arrow$ ATP synthase = chemiosmosis; O$_2$=final acceptor$arrow$H$_2$O; no O$_2$=fermentation, only 2 ATP net.

In the thylakoid membrane, light energy captured by pigments is converted into chemical energy (ATP and NADPH) using the same chemiosmotic principle that powers cellular respiration, while also splitting water and releasing O$_2$.

Chlorophyll a/b and accessory pigments absorb light (mainly red/blue wavelengths, reflecting green) in Photosystem II (PS II) then Photosystem I (PS I), embedded in the thylakoid membrane. Photolysis of water at PS II splits H$_2$O $arrow$ O$_2$ + H$^+$ + electrons (the source of atmospheric O$_2$), replacing electrons lost from chlorophyll as they're excited by light. Excited electrons pass down an electron transport chain, pumping H$^+$ into the thylakoid lumen and building a gradient that drives ATP synthase (chemiosmosis). At PS I, re-excited electrons are ultimately used to reduce NADP$^+$ to NADPH. ATP and NADPH power the Calvin cycle.

Photosystem II (comes first in the pathway despite the ``II''; splits water, source of O$_2$) vs.\ Photosystem I (comes second; produces NADPH).

Students think O$_2$ released in photosynthesis comes from CO$_2$; the O$_2$ released comes from the SPLITTING OF WATER at Photosystem II, not from carbon fixation.

Light hits PS II$arrow$splits H$_2$O (O$_2$ released)$arrow$ETC builds H$^+$ gradient$arrow$ATP; PS I$arrow$NADPH; both feed the Calvin cycle.

The Calvin cycle uses the ATP and NADPH generated by the light reactions to fix atmospheric CO$_2$ into organic sugar, and some plants have evolved workarounds to minimize water loss and photorespiration in hot or dry conditions.

Calvin cycle (stroma, light-independent): the enzyme RuBisCO fixes CO$_2$ onto RuBP (5C); using ATP and NADPH, the cycle produces G3P, used to build glucose and regenerate RuBP. C3 plants (most plants) fix CO$_2$ directly via RuBisCO but lose efficiency in hot, dry weather when stomata close, causing O$_2$ buildup and wasteful photorespiration (RuBisCO fixes O$_2$ instead of CO$_2$). C4 plants (e.g., corn) spatially separate initial CO$_2$ fixation (mesophyll cells) from the Calvin cycle (bundle-sheath cells) to concentrate CO$_2$ and avoid photorespiration. CAM plants (e.g., cacti) temporally separate the steps, fixing CO$_2$ at night and running the Calvin cycle by day.

C4 plants (SPATIAL separation of CO$_2$ fixation and the Calvin cycle, different cell types) vs.\ CAM plants (TEMPORAL separation, night vs.\ day, same cells).

Students think photorespiration is a beneficial, separate energy-producing pathway; it is a wasteful process where RuBisCO binds O$_2$ instead of CO$_2$, consuming energy without producing sugar --- C4/CAM adaptations exist to minimize it.

Calvin cycle: RuBisCO fixes CO$_2$ onto RuBP, uses ATP+NADPH $arrow$ G3P; C3=direct (photorespiration risk); C4=spatial separation; CAM=temporal (night fixation).

Cells communicate by releasing signaling molecules that bind specific receptors on or in target cells, triggering an amplifying cascade that converts an extracellular signal into a specific cellular response.

Three-stage model: reception (a ligand binds a specific receptor --- cell-surface receptors for large/polar ligands, intracellular receptors for small/nonpolar ligands like steroid hormones that cross the membrane), transduction (the signal is relayed/amplified through a cascade, often via second messengers like cyclic AMP or Ca$^2+$, and protein kinase phosphorylation relays), and response (a change in cell behavior --- gene transcription, enzyme activation, cytoskeletal rearrangement). G-protein coupled receptors and receptor tyrosine kinases are major cell-surface receptor classes. Signaling can be local (paracrine, synaptic) or long-distance (endocrine, via the bloodstream).

Cell-surface receptors (bind large/polar/charged ligands that cannot cross the membrane, e.g., peptide hormones) vs.\ intracellular receptors (bind small/nonpolar/lipid-soluble ligands that diffuse through the membrane, e.g., steroid hormones).

Students think transduction is a single step; it is typically a multi-step, AMPLIFYING cascade (one receptor activation can trigger many downstream molecules), which is why a small amount of signal can produce a large cellular response.

Reception (ligand+receptor) $arrow$ transduction (cascade/2nd messengers, amplifies) $arrow$ response (gene expression/enzyme activity change).

Organisms maintain internal stability (homeostasis) primarily through negative feedback loops that counteract change, while positive feedback loops amplify a change to drive a process toward completion.

Negative feedback: the response REDUCES or reverses the initial stimulus, returning a variable toward a set point (e.g., insulin/glucagon regulating blood glucose, thermoregulation) --- the dominant homeostatic mechanism in biology. Positive feedback: the response AMPLIFIES the initial stimulus, driving the variable further from the starting point until an endpoint is reached (e.g., oxytocin during childbirth contractions, the blood clotting cascade). Homeostasis is a dynamic equilibrium maintained within a normal range, not a static, unchanging condition.

Negative feedback (COUNTERACTS a change, most common, maintains stability, e.g., sweating to cool down) vs.\ positive feedback (AMPLIFIES a change, drives toward a definitive endpoint, e.g., labor contractions).

Students assume ``negative'' feedback is bad and ``positive'' feedback is good; the labels refer to the direction of the response relative to the stimulus (reducing vs.\ amplifying it), not to whether the outcome is desirable.

Negative feedback = counteracts, restores set point (most common); positive feedback = amplifies, pushes to an endpoint (childbirth, clotting).

The cell cycle is a tightly ordered sequence of growth and division phases that ensures a parent cell's genetic material is accurately duplicated and equally distributed to two genetically identical daughter cells.

Interphase ($≈$90% of the cycle): G1 (cell growth) $arrow$ S (DNA replication --- chromosome number doubles, sister chromatids form) $arrow$ G2 (further growth, preparation for division). Mitosis (M phase): prophase (chromatin condenses, spindle forms, nuclear envelope breaks down), metaphase (chromosomes align at the metaphase plate), anaphase (sister chromatids separate, pulled to opposite poles), telophase (nuclear envelopes reform, chromosomes decondense). Cytokinesis divides the cytoplasm (cleavage furrow in animal cells; cell plate in plant cells), producing two genetically identical diploid daughter cells.

Chromosome (the whole structure, one or two chromatids) vs.\ sister chromatid (ONE of the two identical copies joined at the centromere after S phase, before anaphase separates them).

Students say DNA content doubles during mitosis; DNA content doubles during S PHASE of interphase --- mitosis only SEPARATES the already-duplicated sister chromatids into two nuclei, creating no new DNA.

G1 (grow) $arrow$ S (DNA replicates) $arrow$ G2 (grow) $arrow$ PMAT (prophase, metaphase, anaphase, telophase) $arrow$ cytokinesis.

Internal checkpoints, driven by cyclins and cyclin-dependent kinases, tightly regulate progression through the cell cycle, and cancer results when mutations disable this control, allowing uncontrolled division.

Checkpoints (G1, G2, and M/spindle checkpoints) verify conditions are correct (cell size, DNA integrity, chromosome attachment) before allowing progression. Cyclins bind and activate cyclin-dependent kinases (CDKs); fluctuating cyclin levels drive orderly phase transitions. Proto-oncogenes (normally promote division) can mutate into oncogenes (overactive, drive uncontrolled division --- a gain-of-function mutation). Tumor suppressor genes (e.g., p53, which halts the cycle or triggers apoptosis in damaged cells) normally restrain division; loss-of-function mutations in both copies remove this brake. Cancer cells often lose contact inhibition and can become immortal and metastasize.

Proto-oncogene $arrow$ oncogene mutation (GAIN of function, accelerator stuck on) vs.\ tumor suppressor gene mutation (LOSS of function, brakes fail) --- both push a cell toward cancer, by opposite mutation types.

Students think a single mutation is normally sufficient to cause cancer; cancer typically requires an ACCUMULATION of multiple mutations (in proto-oncogenes AND tumor suppressor genes) over time, which is why cancer risk increases with age.

Cyclin+CDK drive checkpoints; oncogene=gain-of-function accelerator; tumor suppressor (e.g., p53)=loss-of-function brake failure; cancer=loses contact inhibition.

Meiosis reduces a diploid cell's chromosome number by half to produce haploid gametes, and several independent mechanisms during the process generate the genetic variation that fuels evolution by natural selection.

Meiosis I (reductional division): homologous chromosomes pair up (synapsis, forming a tetrad) and separate --- this halves chromosome number (diploid $arrow$ haploid). Meiosis II (equational division, resembles mitosis): sister chromatids separate. Result: 4 genetically unique haploid cells from 1 diploid cell. Sources of variation: crossing over (homologous chromosomes exchange segments during prophase I, creating recombinant chromosomes), independent assortment (each homologous pair orients randomly at metaphase I, giving $2^n$ possible combinations), and random fertilization.

Meiosis I (separates HOMOLOGOUS chromosomes, reduces chromosome number, crossing over occurs here) vs.\ meiosis II (separates SISTER CHROMATIDS, mechanically like mitosis, no further reduction in number).

Students think crossing over and independent assortment are the same mechanism; crossing over exchanges DNA SEGMENTS between homologs (new allele combinations WITHIN a chromosome), while independent assortment shuffles WHOLE chromosomes relative to each other.

Meiosis I=homologs separate (reduces number, crossing over); Meiosis II=sister chromatids separate (like mitosis); variation = crossing over + independent assortment + random fertilization.

Mendel's laws of segregation and independent assortment describe how allele pairs separate and combine predictably during meiosis and fertilization, and the chi-square test lets biologists statistically evaluate whether observed offspring ratios match those predictions.

Law of segregation: the two alleles for a gene separate during gamete formation, so each gamete carries only one allele. Law of independent assortment: alleles for DIFFERENT genes segregate independently of one another. Monohybrid cross (Aa $×$ Aa) $arrow$ 3:1 phenotypic ratio; dihybrid cross (AaBb $×$ AaBb) $arrow$ 9:3:3:1 phenotypic ratio, assuming independent assortment. The product rule (probability of independent events BOTH occurring = multiply) and sum rule (probability of EITHER of mutually exclusive events = add) extend this math. Chi-square test compares observed vs.\ expected ratios: $^2 = /(o-e)^2e$; a calculated value exceeding the critical value (from a $^2$ table, at the appropriate degrees of freedom, typically $p=0.05$) means the observed data differs significantly from the expected ratio (reject the null hypothesis).

Product rule (multiply probabilities for events that must ALL happen together, ``and'') vs.\ sum rule (add probabilities for mutually exclusive alternative outcomes, ``or'').

Students conclude a hypothesis is ``proven wrong'' whenever $^2 > 0$; only a $^2$ value EXCEEDING the critical value at the relevant degrees of freedom indicates a statistically significant deviation --- small deviations are expected from chance alone.

Segregation=alleles split into gametes; independent assortment=different genes sort independently; monohybrid 3:1, dihybrid 9:3:3:1; $^2=/(o-e)^2e$, big $^2$ = reject expected ratio.

Many traits do not follow simple dominant/recessive Mendelian ratios because of more complex relationships between alleles, between genes, or between genes and chromosomes.

Incomplete dominance: heterozygote shows an intermediate/blended phenotype (e.g., red $×$ white $arrow$ pink flowers). Codominance: heterozygote shows BOTH parental phenotypes fully and simultaneously (e.g., AB blood type). Multiple alleles: more than two allele versions exist for one gene in the population (e.g., ABO blood type: I$^A$, I$^B$, i). Polygenic inheritance: a single trait (e.g., human height, skin color) is influenced by many genes, producing continuous phenotypic variation. Epistasis: one gene's alleles mask or modify another gene's phenotypic expression. Linked genes (close together on the same chromosome) are inherited together more often than independent assortment predicts, except when separated by crossing over. Sex-linked genes (on the X chromosome) show different inheritance patterns in males (hemizygous, one X) vs.\ females.

Incomplete dominance (heterozygote phenotype BLENDS into something intermediate) vs.\ codominance (heterozygote phenotype shows BOTH alleles fully expressed, not blended).

Students treat all multi-gene traits as polygenic; epistasis is about one GENE masking another gene's expression (e.g., coat color genes), while polygenic inheritance is about MANY genes each contributing a small additive effect to ONE continuous trait.

Incomplete dominance=blend; codominance=both show; multiple alleles=$>$2 versions in population; polygenic=many genes, one trait; epistasis=one gene masks another; linked genes=inherited together unless crossed over.

Chromosome behavior during meiosis explains classic inheritance patterns including sex determination and linkage, and even a fully determined genotype can produce different phenotypes depending on environmental conditions.

In humans, sex is determined by the 23rd chromosome pair: XX = female, XY = male (the Y chromosome carries the SRY gene triggering male development). Sex-linked traits (carried on the X chromosome, e.g., red-green colorblindness, hemophilia) appear more frequently in males because they need only ONE copy of the recessive allele (they have only one X). Nondisjunction (failure of homologous chromosomes or sister chromatids to separate properly during meiosis) produces gametes with abnormal chromosome numbers, leading to conditions like trisomy (e.g., Down syndrome = trisomy 21). Norm of reaction: the same genotype can produce a range of phenotypes depending on environmental conditions (e.g., temperature-dependent coat color in Himalayan rabbits, temperature-dependent sex determination in some reptiles).

Nondisjunction in Meiosis I (an ENTIRE homologous pair fails to separate, so all resulting gametes are abnormal) vs.\ nondisjunction in Meiosis II (sister chromatids fail to separate, only some resulting gametes are abnormal).

Students think phenotype is determined solely by genotype; the norm of reaction shows the SAME genotype can yield different phenotypes across different environments, so genotype sets a range of possibilities rather than a fixed outcome.

XX=female, XY=male (SRY gene); sex-linked recessive traits hit males more (hemizygous); nondisjunction=trisomy/monosomy; norm of reaction=same genotype, environment shifts phenotype.

DNA replicates semiconservatively --- each new double helix retains one original (parental) strand and synthesizes one brand-new strand --- catalyzed by a coordinated team of enzymes working from specific origins.

Replication begins at origins of replication, where helicase unwinds/unzips the double helix and single-strand binding proteins keep the strands separated; topoisomerase relieves supercoiling ahead of the fork. DNA polymerase synthesizes new strands only in the 5'$arrow$3' direction, reading the template 3'$arrow$5', and requires an RNA primer (laid down by primase) to start. Because the template strands are antiparallel, the leading strand is synthesized continuously toward the fork, while the lagging strand is synthesized discontinuously in Okazaki fragments, later joined by DNA ligase. DNA polymerase also proofreads, giving replication very high fidelity.

Leading strand (synthesized continuously, in the same direction the fork opens) vs.\ lagging strand (synthesized discontinuously in Okazaki fragments, since DNA polymerase can only build 5'$arrow$3' away from the fork's opening direction).

Students think DNA polymerase can begin synthesis on a bare template strand; DNA polymerase can only EXTEND an existing strand, so an RNA primer is always required to provide a free 3' end to start from, on both the leading strand and each Okazaki fragment.

Semiconservative; helicase unwinds $arrow$ primase lays RNA primer $arrow$ DNA pol.\ builds 5'$arrow$3' $arrow$ leading=continuous, lagging=Okazaki fragments+ligase.

Transcription copies a gene's DNA sequence into a complementary, single-stranded mRNA molecule, which in eukaryotes must then be processed and edited before it can leave the nucleus.

RNA polymerase binds a promoter sequence (with help from transcription factors in eukaryotes), unwinds the DNA, and synthesizes mRNA 5'$arrow$3' by reading the template strand 3'$arrow$5' (using U instead of T); transcription ends at a terminator sequence. Eukaryotic pre-mRNA processing (absent in prokaryotes): a 5' cap and 3' poly-A tail are added, protecting the mRNA and aiding export/translation; RNA splicing removes noncoding introns and joins coding exons together (via the spliceosome); alternative splicing (different exon combinations) allows one gene to produce multiple distinct protein products.

Introns (noncoding, spliced OUT and not translated) vs.\ exons (coding, spliced together and retained in the mature mRNA).

Students think mRNA processing (capping, tailing, splicing) happens in prokaryotes; prokaryotes lack a nucleus and typically lack introns, so their mRNA is translated immediately as it's transcribed, with no processing step.

RNA pol.\ reads template 3'$arrow$5', builds mRNA 5'$arrow$3'; eukaryotes only: 5' cap + poly-A tail + splicing (introns out, exons joined).

Translation decodes the sequence of codons on an mRNA into a specific sequence of amino acids, using the genetic code, ribosomes, and transfer RNAs as the molecular ``adapters'' between nucleic acid and protein language.

The genetic code is read in nonoverlapping triplets (codons); it is degenerate/redundant (multiple codons can specify the same amino acid) but unambiguous (each codon specifies only one amino acid) and nearly universal across all life. AUG is the start codon (codes for methionine, sets the reading frame); UAA, UAG, UGA are stop codons (no amino acid, signal termination). tRNA molecules have an anticodon (complementary to a codon) at one end and carry the corresponding amino acid at the other; aminoacyl-tRNA synthetases attach the correct amino acid to each tRNA. Ribosomes have three sites: A (aminoacyl, incoming tRNA), P (peptidyl, growing chain), E (exit, tRNA leaves); the ribosome moves along the mRNA, catalyzing peptide bond formation as the polypeptide chain elongates.

Codon (a 3-base sequence ON the mRNA) vs.\ anticodon (the complementary 3-base sequence ON the tRNA that pairs with the codon).

Students think a change to any nucleotide changes the resulting protein; because the genetic code is degenerate, a nucleotide substitution can produce a synonymous/silent codon change that still codes for the SAME amino acid, leaving the protein unchanged.

mRNA codon (triplet) $≤ftrightarrow$ tRNA anticodon $arrow$ amino acid; AUG=start (Met); UAA/UAG/UGA=stop; ribosome sites A (in) $arrow$ P (bond) $arrow$ E (out).

Cells control which genes are expressed, and how much, through regulatory mechanisms acting before, during, and after transcription, allowing different cell types to specialize despite sharing identical DNA.

In prokaryotes, operons (e.g., the lac operon in E.\ coli) cluster related genes under one promoter/operator for coordinated control: a repressor can bind the operator to block transcription when lactose is absent --- an inducible operon, switched ON by the substrate's presence. In eukaryotes, regulation is layered: chromatin remodeling/epigenetics (histone acetylation loosens DNA for transcription; DNA methylation typically silences genes, without changing the DNA sequence), transcriptional control (transcription factors, enhancers, and promoters determine which genes RNA polymerase transcribes and how often), and post-transcriptional/translational/post-translational control (alternative splicing, mRNA degradation rate, protein modification). Differential gene expression --- not different DNA content --- explains why cells with identical genomes specialize.

Histone acetylation (loosens chromatin, generally ACTIVATES transcription) vs.\ DNA methylation (typically condenses chromatin, generally SILENCES transcription) --- both are epigenetic changes that don't alter the DNA sequence.

Students think all cells in the body have different DNA to explain their different functions; every somatic cell has the SAME genome --- specialization results from DIFFERENTIAL GENE EXPRESSION (which genes are turned on/off), not different genetic content.

lac operon=inducible, repressor blocks operator until lactose present; acetylation=loosens/activates; methylation=silences; same DNA, different expression = cell specialization.

A mutation is any permanent change to a DNA sequence, and its effect on the resulting protein --- and therefore on the organism --- depends heavily on WHERE in the gene it occurs and WHAT KIND of change it is.

Point mutations affect a single nucleotide: silent mutation (codon changes but still codes for the same amino acid due to code redundancy, no effect), missense mutation (codon specifies a DIFFERENT amino acid, may or may not affect function depending on the amino acid's properties and location), nonsense mutation (codon changes to a premature STOP codon, typically truncates and disables the protein). Frameshift mutations (insertions/deletions not divisible by 3) shift the entire downstream reading frame, usually drastically altering or destroying protein function --- generally more severe than point mutations. Germline mutations are heritable; somatic mutations are not heritable but can still affect the individual (e.g., contribute to cancer). Mutations are the ultimate source of new genetic variation for evolution.

Missense mutation (changes ONE amino acid, protein often partially functional) vs.\ nonsense mutation (creates a premature STOP codon, protein is truncated) vs.\ frameshift (shifts ALL downstream codons, usually catastrophic).

Students think all mutations are harmful; silent mutations have no effect, and even some missense mutations are neutral (if the new amino acid has similar chemical properties or falls outside a critical region) --- mutations can be harmful, neutral, or rarely beneficial.

Silent=same amino acid (no effect); missense=different amino acid; nonsense=premature stop; frameshift (indel not $3$)=shifts everything downstream, usually severe.

Modern biotechnology techniques let scientists cut, copy, read, and edit DNA with precision, enabling everything from diagnostic testing to genetically modified organisms.

Restriction enzymes cut DNA at specific palindromic recognition sequences, often leaving ``sticky ends'' that can be rejoined (by DNA ligase) with DNA cut by the same enzyme from another source --- the basis of recombinant DNA technology. Gel electrophoresis separates DNA fragments by size: negatively charged DNA migrates toward the positive electrode through a gel matrix, with SMALLER fragments traveling FARTHER. Polymerase chain reaction (PCR) exponentially amplifies a target DNA sequence via repeated cycles of denaturation (heat separates strands), annealing (primers bind), and extension (heat-stable Taq polymerase synthesizes new strands). CRISPR-Cas9 uses a guide RNA to direct the Cas9 enzyme to cut DNA at a specific target sequence for gene editing.

Gel electrophoresis (SEPARATES/visualizes existing DNA fragments by size, does not create new DNA) vs.\ PCR (AMPLIFIES/copies a target DNA sequence exponentially, does not separate by size).

Students think larger DNA fragments move farther in gel electrophoresis; SMALLER fragments move FARTHER (and faster) through the gel matrix toward the positive electrode, because they experience less resistance.

Restriction enzymes cut (sticky ends) + ligase joins = recombinant DNA; gel electrophoresis: smaller=farther; PCR: denature$arrow$anneal$arrow$extend, exponential copies; CRISPR-Cas9=guide RNA + cut.

Evolution by natural selection occurs when heritable variation in a population leads to differential survival and reproduction, so individuals with traits better suited to their environment leave more offspring over generations.

Requirements for natural selection: heritable variation must exist in a trait, the trait must affect survival/reproduction (differential fitness), and the environment must impose a selective pressure. Fitness = an individual's relative reproductive success (passing on alleles to the next generation), NOT simply physical strength or survival alone. Three patterns of selection on a normally distributed trait: directional selection (favors one extreme, shifts the mean), stabilizing selection (favors the intermediate phenotype, reduces variation, e.g., human birth weight), disruptive selection (favors BOTH extremes over the intermediate, can increase variation/promote speciation).

Stabilizing selection (favors the AVERAGE/intermediate phenotype, narrows the distribution) vs.\ disruptive selection (favors BOTH extremes, can split the distribution into two peaks).

Students equate ``fittest'' with ``strongest'' or ``biggest''; fitness means REPRODUCTIVE SUCCESS (number of viable, fertile offspring produced), which can favor small size, camouflage, or cooperative behavior just as easily as strength.

Natural selection needs heritable variation + differential survival/reproduction + selective pressure; fitness=reproductive success; directional (shift), stabilizing (average wins), disruptive (extremes win).

The Hardy-Weinberg model provides a null hypothesis --- a population NOT evolving --- against which real allele and genotype frequencies can be compared to detect and measure evolutionary change.

Hardy-Weinberg equations: $p + q = 1$ (allele frequencies, $p$ = dominant allele frequency, $q$ = recessive allele frequency) and $p^2 + 2pq + q^2 = 1$ (genotype frequencies: $p^2$ = homozygous dominant, $2pq$ = heterozygous, $q^2$ = homozygous recessive). Five conditions required for equilibrium (i.e., NOT evolving): no mutation, random mating, no natural selection, extremely large population size (no genetic drift), and no gene flow (migration). Violating any one condition changes allele frequencies over time --- this IS evolution at the population-genetics level. Genetic drift (random changes in allele frequency, strongest in small populations) includes the bottleneck effect (a population is drastically reduced) and the founder effect (a small group establishes a new, genetically unrepresentative population).

Genetic drift (RANDOM changes in allele frequency, especially powerful in SMALL populations, not driven by fitness) vs.\ natural selection (NON-random, fitness-driven changes in allele frequency).

Students plug an observed dominant PHENOTYPE frequency directly in for $p$; the dominant phenotype frequency equals $p^2+2pq$ (everyone but the homozygous recessives), so you must start from $q^2$ (the recessive phenotype, only one possible genotype) to solve for $q$, then $p$.

$p+q=1$; $p^2+2pq+q^2=1$; 5 HW conditions (no mutation/selection/drift/migration, random mating) = not evolving; always solve from $q^2$ (recessive phenotype) first.

Multiple independent lines of evidence --- from fossils, biogeography, anatomy, embryology, and molecular biology --- all converge to support common descent and the fact of evolution.

The fossil record shows a chronological sequence of change and transitional forms across geological time (relative dating via rock strata; radiometric/absolute dating via isotope decay rates). Biogeography: the geographic distribution of species reflects evolutionary history and continental drift (e.g., island species like Darwin's finches). Comparative anatomy: homologous structures (similar structure, common ancestry, may differ in function, e.g., mammal forelimb bones) vs.\ analogous structures (similar function, NO common ancestry, convergent evolution, e.g., insect and bird wings) vs.\ vestigial structures (reduced, functionless remnants of an ancestrally functional structure, e.g., human appendix, whale pelvis bones). Comparative embryology shows shared developmental patterns among related species. Molecular evidence: the degree of similarity in DNA/protein sequences between species correlates with how recently they shared a common ancestor.

Homologous structures (SAME underlying structure/ancestry, possibly DIFFERENT function --- evidence of common ancestry) vs.\ analogous structures (DIFFERENT underlying structure/ancestry, SAME function --- evidence of convergent evolution, not common ancestry).

Students cite similar-looking structures (like a bird wing and a butterfly wing) as evidence of common ancestry; superficial functional similarity without shared underlying structure is convergent evolution (analogous), NOT evidence of a recent common ancestor.

Homologous=same structure/ancestry, diff function; analogous=diff ancestry, same function (convergent); molecular similarity $$ recency of common ancestor.

Phylogenetic trees and cladograms are hypotheses about evolutionary relationships, built primarily on shared derived characteristics that indicate how recently different lineages diverged from a common ancestor.

A cladogram represents hypothesized evolutionary relationships; branch points (nodes) represent common ancestors, and the sequence of branching indicates relative divergence times, NOT how ``advanced'' a lineage is. Shared derived characteristics (synapomorphies --- traits present in a group and its most recent common ancestor but not in more distant ancestors) are used to group taxa on a cladogram, as opposed to shared ancestral characteristics (older, more widespread, less useful for grouping). Outgroups (a taxon known to have diverged earliest) help root a tree and identify which traits are ancestral vs.\ derived. Molecular clocks estimate divergence times using the roughly constant rate of accumulation of neutral mutations.

Shared derived characters/synapomorphies (present in a subgroup and its immediate common ancestor, USED to build clades) vs.\ shared ancestral characters (older, more widespread traits, NOT useful for distinguishing subgroups).

Students read a cladogram left-to-right as a ranking of ``least evolved'' to ``most evolved''; branch order only reflects the SEQUENCE OF DIVERGENCE from common ancestors --- every living tip on the tree has been evolving for the same amount of time since the root.

Cladogram = hypothesis of relatedness; nodes = common ancestors; shared DERIVED traits (synapomorphies) group clades; branch length/order $≠$ ``more evolved.''

A new species forms when populations of a formerly interbreeding species become reproductively isolated from one another, accumulating genetic differences until they can no longer successfully interbreed.

The biological species concept defines species as populations capable of interbreeding and producing viable, fertile offspring in nature. Allopatric speciation: a physical/geographic barrier separates populations, which then diverge independently (the most common mode). Sympatric speciation: new species arise WITHOUT geographic separation, often via polyploidy (especially common in plants), habitat differentiation, or sexual selection. Reproductive isolating mechanisms prevent gene flow: prezygotic barriers act BEFORE fertilization (habitat, temporal, behavioral, mechanical, gametic isolation), while postzygotic barriers act AFTER fertilization (reduced hybrid viability, reduced hybrid fertility, hybrid breakdown).

Allopatric speciation (geographic SEPARATION drives divergence) vs.\ sympatric speciation (divergence occurs in the SAME location, no geographic barrier, e.g., via polyploidy).

Students label any barrier as ``postzygotic'' if a hybrid forms at all; barriers are classified by TIMING relative to fertilization --- if mating/fertilization is prevented from happening (e.g., different mating seasons), it's prezygotic, even if a zygote could theoretically have formed.

Allopatric=geographic barrier; sympatric=no barrier (e.g., polyploidy); prezygotic=blocks BEFORE fertilization; postzygotic=hybrid problems AFTER fertilization.

Evolution produces recognizable large-scale patterns over time --- convergence, divergence, and coevolution --- and proceeds at variable rates, sometimes punctuated by rapid change following mass extinction events.

Divergent evolution: related species become MORE different over time as they adapt to different pressures from a common ancestor. Convergent evolution: unrelated species independently evolve SIMILAR traits in response to similar environmental pressures (produces analogous structures, e.g., streamlined bodies in sharks and dolphins). Coevolution: two interacting species exert reciprocal selective pressure on each other (e.g., predator-prey arms races, flower-pollinator relationships). Gradualism (slow, continuous, incremental change) vs.\ punctuated equilibrium (long periods of stasis interrupted by relatively rapid bursts of change, often following speciation) describe different observed TEMPOS of evolution in the fossil record. Mass extinctions (e.g., the K-Pg extinction that ended the dinosaurs) dramatically reduce biodiversity but open new ecological niches, often triggering rapid adaptive radiation in survivors.

Convergent evolution (UNRELATED lineages independently evolve SIMILAR traits due to similar pressures) vs.\ divergent evolution (RELATED lineages, sharing a common ancestor, become increasingly DIFFERENT).

Students think punctuated equilibrium contradicts natural selection; it only describes a different observed TEMPO of change in the fossil record (rapid bursts + long stasis) --- the underlying mechanism (selection acting on variation) is unchanged.

Divergent=common ancestor, grows apart; convergent=unrelated, grows similar (analogous structures); gradualism=slow/steady; punctuated equilibrium=stasis + rapid bursts.

Behavior is a rapid, adaptive response to environmental stimuli, ranging from fixed innate patterns shaped entirely by genetics to flexible learned responses shaped by experience, and natural selection can favor behaviors that help relatives reproduce even at a personal cost.

Innate behaviors (fixed action patterns) are largely genetically programmed, stereotyped, and triggered by a specific sign stimulus, occurring the same way every time regardless of experience. Learned behaviors are modified by experience: habituation (decreased response to a repeated, harmless stimulus), classical/operant conditioning, and imprinting (rapid learning during a critical period early in life, e.g., geese imprinting on the first moving object seen). Migration and foraging behavior (e.g., optimal foraging theory --- balancing energy gained against energy/risk spent) are shaped by natural selection. Kin selection explains altruistic behavior via inclusive fitness: an allele for helping relatives can increase in frequency because relatives share a proportion of that allele (Hamilton's rule).

Fixed action pattern (INNATE, stereotyped, triggered reliably by a sign stimulus, unaffected by learning) vs.\ imprinting (a form of LEARNING, but restricted to a critical period early in development).

Students think altruistic behavior contradicts natural selection (since it costs the individual); kin selection shows helping close relatives can still increase the ALLELE's frequency in the population (inclusive fitness), even if it reduces the helper's own direct reproduction.

Fixed action pattern=innate, triggered by sign stimulus; imprinting=learned, critical period; kin selection $arrow$ inclusive fitness explains altruism toward relatives.

Populations grow according to predictable mathematical models, and whether growth is limited primarily by density-dependent or density-independent factors shapes how close a population stays to its environment's carrying capacity.

Exponential growth model: $/dNdt = rN$ (population grows unchecked at rate $r$, produces a J-shaped curve; occurs when resources are unlimited). Logistic growth model: $/dNdt = rN≤ft(/K-NK)$, where $K$ = carrying capacity (the maximum population an environment can sustainably support); growth slows as $N$ approaches $K$, producing an S-shaped curve. Density-dependent factors (limiting effect intensifies as density increases --- competition, predation, disease) regulate populations near $K$. Density-independent factors (affect a population regardless of density --- natural disasters, extreme weather) can cause sudden crashes unrelated to crowding. Life history strategies: r-selected species (many small offspring, little parental care, unstable environments) vs.\ K-selected species (few large offspring, high parental investment, stable environments near $K$).

Density-dependent limiting factors (intensity SCALES with population density, e.g., competition, disease) vs.\ density-independent limiting factors (impact is the SAME regardless of density, e.g., a wildfire or hurricane).

Students assume all populations follow a clean logistic curve; real populations often OVERSHOOT carrying capacity before crashing (due to lag time in the negative feedback) or oscillate around $K$ rather than smoothly leveling off.

Exponential: $dN/dt=rN$ (J-curve, unlimited); logistic: $dN/dt=rN(K-N)/K$ (S-curve, capped at $K$); density-dependent scales with crowding, density-independent doesn't.

Species within a community interact through a defined set of relationship types, each with a characteristic effect on the two interacting populations, and these interactions shape community structure over time.

Competition ($-/-$, both harmed, share a limiting resource): the competitive exclusion principle states two species cannot indefinitely occupy the exact same ecological niche in the same place --- resource partitioning allows similar species to coexist by using resources differently. Predation and herbivory ($+/-$) directly benefit one species at the other's expense. Symbiosis: mutualism ($+/+$, both benefit, e.g., mycorrhizae), commensalism ($+/0$, one benefits, other unaffected, e.g., barnacles on a whale), parasitism ($+/-$, parasite benefits, host harmed, usually without immediately killing the host). A niche is the full set of biotic/abiotic conditions an organism uses; a keystone species has a disproportionately large effect on community structure relative to its abundance. Ecological succession: primary succession (begins on bare rock/no soil) vs.\ secondary succession (begins in an area with existing soil after a disturbance) --- both progress toward a relatively stable climax community.

Primary succession (starts with NO soil, e.g., bare rock, pioneer species like lichens colonize first) vs.\ secondary succession (starts with EXISTING soil after a disturbance, proceeds faster).

Students think a keystone species is simply the most abundant species; a keystone species (e.g., a sea otter controlling urchin populations) has an outsized structural effect on the community relative to its LOW abundance --- removing it can collapse the community.

Competition=$-/-$; predation/herbivory=$+/-$; mutualism=$+/+$; commensalism=$+/0$; parasitism=$+/-$ (host survives); primary succession=no soil, secondary=existing soil.

Energy flows one-directionally through an ecosystem, from producers through successive levels of consumers, and a large fraction is lost as heat at each transfer, sharply limiting the length of food chains.

Primary producers (autotrophs, e.g., plants, algae, cyanobacteria) capture energy from the sun via photosynthesis (or chemical energy via chemosynthesis), forming the base of every food web. Energy flows: producers $arrow$ primary consumers (herbivores) $arrow$ secondary consumers $arrow$ tertiary consumers, with decomposers recycling nutrients from dead organic matter at every level. The 10% rule: on average, only about 10% of energy available at one trophic level is transferred to and stored as biomass in the next level (the rest is lost as metabolic heat via respiration, or unconsumed/undigested) --- why energy pyramids are largest at the producer base and food chains rarely exceed 4--5 levels. Gross primary productivity (GPP, total energy captured by producers) vs.\ net primary productivity (NPP = GPP $-$ producer respiration; energy actually available to consumers).

Gross primary productivity (TOTAL energy captured by producers via photosynthesis) vs.\ net primary productivity (energy remaining AFTER subtracting producers' own respiration --- what's actually available to the food web).

Students think energy CYCLES through an ecosystem the way nutrients/matter do; energy flows one-way and is progressively lost as heat at each trophic transfer (it does NOT recycle back to producers) --- only matter/nutrients are cycled via decomposition.

Producers $arrow$ primary $arrow$ secondary $arrow$ tertiary consumers; only $≈$10% energy transfers per level (rest lost as heat); energy flows one-way, matter cycles.

Unlike energy, matter is continuously recycled through ecosystems via biogeochemical cycles, moving between living organisms and abiotic reservoirs (atmosphere, water, soil, rock).

Carbon cycle: CO$_2$ is removed from the atmosphere by photosynthesis and returned by cellular respiration, combustion (including fossil fuels), and decomposition; oceans and fossil fuel deposits are major long-term carbon reservoirs. Nitrogen cycle: atmospheric N$_2$ (unusable by most organisms) is converted to usable forms by nitrogen fixation (nitrogen-fixing bacteria, e.g., Rhizobium in legume root nodules, or lightning); nitrification converts ammonium to nitrite then nitrate (usable by plants); denitrification (by bacteria) returns N$_2$ to the atmosphere. Water cycle: driven by evaporation/transpiration, condensation, and precipitation. Human activities (fossil fuel combustion, deforestation, synthetic fertilizer overuse) significantly accelerate/disrupt these natural cycling rates.

Nitrogen fixation (converts atmospheric N$_2$ into a USABLE form, ammonia/ammonium, mainly by bacteria) vs.\ denitrification (converts usable nitrate back into atmospheric N$_2$, REMOVING it from the usable pool).

Students think plants can directly use atmospheric N$_2$; plants cannot use N$_2$ gas directly --- they require nitrogen-fixing bacteria (or industrial fixation) to first convert it into ammonia/ammonium or nitrate they can absorb through their roots.

Carbon: photosynthesis (in) vs.\ respiration/combustion (out); nitrogen: fixation ($N_2arrow$usable) $arrow$ nitrification (ammonium$arrow$nitrate) $arrow$ denitrification (back to $N_2$).

Biodiversity underpins ecosystem stability and resilience, and human activity is currently the dominant driver of biodiversity loss and ecosystem disruption worldwide.

Biodiversity operates at multiple levels: genetic diversity (within a species/population), species diversity (richness = number of species; evenness = relative abundance across species), and ecosystem diversity. Greater biodiversity generally increases ecosystem resilience (ability to recover after disturbance) and stability. Major human-driven threats to biodiversity: habitat destruction/fragmentation (the single largest driver), invasive species (outcompete or prey on natives, often lacking natural predators in the new range), overharvesting, pollution, and climate change (rising CO$_2$ and temperature altering species ranges, phenology, and ocean chemistry/acidification). Ecosystem disruptions can trigger trophic cascades --- effects that ripple through multiple trophic levels (e.g., removing a top predator can indirectly cause a herbivore population boom and vegetation decline).

Species richness (the NUMBER of different species present) vs.\ species evenness (how EQUALLY abundant those species are relative to one another) --- two communities can share the same richness but very different evenness.

Students think an ecosystem responds to disturbance only at the level directly affected; removing or introducing a single species can trigger a TROPHIC CASCADE, producing indirect effects that ripple through multiple, non-adjacent trophic levels.

Biodiversity = genetic + species (richness+evenness) + ecosystem diversity; top threat = habitat loss; invasive species, climate change also major; trophic cascade = ripple effects across levels.

$p^2+2pq+q^2=1$; $=_p+_s$; $^2=(o-e)^2/e$; $dN/dt=rN$ or $rN(K-N)/K$; 10% energy rule.

When two terms sound alike, ask: passive or active? individual or population? random or fitness-driven? before or after fertilization?

Nucleus=DNA; ribosome=protein; rough ER=secreted protein; smooth ER=lipids; Golgi=ship; mito=respire; chloroplast=photosynthesize; lysosome=digest.

EVO, ENE, IST, SYI = the 4 lenses; every MCQ/FRQ also targets one of the 6 Science Practices above.

Variables $arrow$ trend with numbers $arrow$ mechanism $arrow$ controls $arrow$ stats tied back to the null hypothesis.

60 MCQ/90 min/50% + 6 FRQ (2 long, 4 short)/90 min/50% = 3 hours total, scored 1--5.

DNA (replicate) $arrow$ pre-mRNA (transcribe) $arrow$ mature mRNA (process/export) $arrow$ protein (translate) $arrow$ fold/modify.

Axes/units first $arrow$ read the trend numerically $arrow$ check error bars/$^2$ vs.\ critical value $arrow$ explain the biology.

If a claim sounds absolute (``always,'' ``never,'' ``exact opposite''), it is almost always the trap on the AP Bio exam.

Chemistry $arrow$ cells $arrow$ energetics $arrow$ communication/cycle $arrow$ heredity $arrow$ gene expression $arrow$ natural selection $arrow$ ecology --- the whole course in one pass.