AP® Biology Cheat Sheets: Units, Formulas, Flashcards & Quiz

Unit 2: Cell Structure and Function

Unit 2 focuses on how cells use internal organization and membranes to maintain life. Prokaryotes are smaller and simpler, lacking a nucleus and membrane-bound organelles. Eukaryotes compartmentalize processes in organelles, which increases efficiency because different chemical environments can exist in different parts of the cell. For example, lysosomes maintain a low pH for digestion, mitochondria maintain proton gradients for ATP production, and chloroplasts separate light reactions from the Calvin cycle.

Endosymbiotic theory is a high-yield explanation topic. The evidence is that mitochondria and chloroplasts have their own DNA, have 70S ribosomes, divide by binary fission, and are surrounded by double membranes. A weak answer says only that “they came from bacteria.” A stronger answer names multiple evidence points and explains that engulfed prokaryotes formed a mutualistic relationship with early eukaryotic cells.

The plasma membrane is built from a phospholipid bilayer with proteins, cholesterol, and carbohydrates. Phospholipids form bilayers because their hydrophilic heads face water and hydrophobic tails avoid water. Transport depends on size, charge, polarity, and concentration gradients. Small nonpolar molecules cross directly. Ions and large polar molecules usually need channels, carriers, or pumps. Passive transport moves down a gradient and requires no ATP. Active transport moves against a gradient and requires energy.

Tonicity questions are best solved by tracking water. Water moves toward higher solute concentration, or lower water potential. In a hypertonic environment, water exits the cell. In a hypotonic environment, water enters the cell. Plant cells handle hypotonic environments better because the cell wall resists swelling and creates turgor pressure. Surface area-to-volume ratio explains why cells remain small: as cell size increases, volume grows faster than surface area, making exchange less efficient.

Unit 3: Cellular Energetics

Unit 3 explains how cells transform energy. ATP is not long-term energy storage; it is the short-term energy currency that powers cellular work. ATP hydrolysis releases energy that can be coupled to endergonic reactions. Exergonic reactions release free energy and have \(\Delta G<0\). Endergonic reactions require energy input and have \(\Delta G>0\). Enzymes speed both types by lowering activation energy, but they do not make an unfavorable reaction favorable by themselves.

Photosynthesis converts light energy into chemical energy stored in glucose. In the light reactions, water is split, oxygen is released, electrons move through photosystems and an electron transport chain, and the cell produces ATP and NADPH. The Calvin cycle uses ATP and NADPH to reduce carbon dioxide into sugar. The important distinction is energy flow versus carbon flow. Light energy becomes ATP and NADPH, then glucose. Carbon starts in \(CO_2\), becomes G3P and glucose, and may later be released again during respiration.

Cellular respiration releases usable energy from glucose. Glycolysis occurs in the cytosol and produces a small amount of ATP and NADH. Pyruvate oxidation and the Krebs cycle occur in the mitochondrial matrix, producing carbon dioxide and reduced electron carriers. The electron transport chain and chemiosmosis occur at the inner mitochondrial membrane. Oxygen is the final electron acceptor, and ATP synthase uses the proton gradient to produce most ATP.

Fermentation is often misunderstood. It does not produce additional ATP beyond glycolysis. Its main purpose is to regenerate \(NAD^+\) so glycolysis can continue when oxygen is absent. Lactic acid fermentation occurs in animals and some bacteria, while alcohol fermentation occurs in yeast. If oxygen returns, aerobic respiration can resume and ATP yield increases dramatically.

Unit 4: Cell Communication and Cell Cycle

Unit 4 connects information flow to cellular behavior. Signal transduction begins when a ligand binds a receptor. That binding changes receptor shape, triggering an intracellular cascade. Cascades often use kinases, phosphorylation, and second messengers such as cAMP or \(Ca^{2+}\). Amplification is important because one signal molecule can produce a large cellular response. The final response may change enzyme activity, open an ion channel, alter gene expression, or trigger cell division or apoptosis.

Communication can be direct, local, long-distance, or synaptic. Gap junctions and plasmodesmata allow direct cell-to-cell movement of materials. Paracrine signaling affects nearby cells. Endocrine signaling sends hormones through the bloodstream. Synaptic signaling is used by neurons to communicate quickly with target cells. The exam often gives a novel pathway and asks you to predict what happens if a receptor, kinase, or second messenger is blocked.

Feedback loops maintain homeostasis. Negative feedback reverses change and stabilizes systems, such as blood glucose regulation or body temperature. Positive feedback amplifies change and drives a process to completion, such as blood clotting or childbirth contractions. The key is whether the response reduces the original stimulus or intensifies it.

The cell cycle is regulated by checkpoints. \(G_1\) checks growth signals and DNA damage, \(G_2\) checks replication, and the M checkpoint checks chromosome alignment. Cancer occurs when cell-cycle regulation fails, often due to activated oncogenes or disabled tumor suppressor genes. Apoptosis removes damaged or unnecessary cells. In FRQs, link checkpoint failure to uncontrolled division instead of just saying “mutation causes cancer.”

Unit 5: Heredity

Unit 5 is about how genetic information passes from parents to offspring and how variation is generated. Meiosis produces haploid gametes from diploid cells. The major sources of genetic variation are crossing over in prophase I, independent assortment in metaphase I, and random fertilization. Crossing over exchanges DNA between homologous chromosomes. Independent assortment creates different combinations of maternal and paternal chromosomes. Random fertilization combines gametes unpredictably.

Mendelian genetics depends on allele separation and probability. The law of segregation says allele pairs separate during gamete formation. A monohybrid cross of two heterozygotes often gives a \(3:1\) phenotype ratio, while a dihybrid cross can give a \(9:3:3:1\) ratio if genes assort independently. A test cross uses a homozygous recessive individual to reveal the genotype of an unknown dominant-phenotype individual.

Non-Mendelian patterns expand the basic model. In incomplete dominance, heterozygotes have an intermediate phenotype. In codominance, both alleles are fully expressed. Multiple alleles mean more than two forms exist in the population, though each diploid individual still carries only two. Polygenic traits involve many genes and often produce continuous variation. Epistasis occurs when one gene affects the expression of another.

Chi-square tests compare observed and expected results. The null hypothesis usually says that the observed differences are due to chance and that the expected inheritance pattern is supported. If the p-value is less than 0.05, reject the null. Linked genes are located close together on the same chromosome and recombine less often than genes far apart. Recombination frequency can be used to estimate map distance.

Unit 6: Gene Expression and Regulation

Unit 6 explains how genetic information is copied, expressed, and controlled. DNA replication is semiconservative because each new DNA molecule contains one original strand and one new strand. Helicase unwinds DNA, topoisomerase relieves twisting, primase lays RNA primers, DNA polymerase extends new DNA in the \(5' o3'\) direction, and ligase joins Okazaki fragments. The leading strand is built continuously, while the lagging strand is built discontinuously.

Gene expression includes transcription and translation. During transcription, RNA polymerase uses a DNA template to make mRNA. In eukaryotes, mRNA is processed with a 5′ cap, poly-A tail, and splicing that removes introns and keeps exons. During translation, ribosomes read mRNA codons, and tRNAs bring amino acids. The start codon AUG codes for methionine. Stop codons signal termination.

Regulation determines which genes are active. In prokaryotes, operons coordinate gene expression. The lac operon is inducible and turns on when lactose is available. The trp operon is repressible and turns off when tryptophan is abundant. In eukaryotes, regulation is more complex and includes transcription factors, enhancers, DNA methylation, histone acetylation, and alternative splicing. The key exam idea is that cells in the same organism usually have the same DNA, but they express different genes.

Mutations can change proteins. Silent mutations do not alter the amino acid sequence. Missense mutations change one amino acid. Nonsense mutations create a premature stop codon. Frameshift mutations insert or delete nucleotides in a way that shifts the reading frame. Viruses also appear in this unit: lytic cycles destroy host cells quickly, lysogenic cycles integrate viral DNA into the host genome, and retroviruses use reverse transcriptase to make DNA from RNA.

Unit 7: Natural Selection

Unit 7 is the largest conceptual unit because evolution connects genetics, ecology, and population change. Natural selection requires heritable variation and differential reproductive success. Individuals do not evolve during their lifetimes; populations evolve across generations as allele frequencies change. Fitness means reproductive success, not physical strength. A trait is adaptive only if it increases survival or reproduction in a particular environment.

Evidence for evolution includes fossils, biogeography, homologous structures, molecular homology, embryology, and direct observation such as antibiotic resistance. Homologous structures indicate shared ancestry, while analogous structures may result from convergent evolution. Molecular evidence is especially powerful because DNA and protein similarities can reveal evolutionary relationships even when external traits differ.

Evolutionary mechanisms include natural selection, genetic drift, gene flow, mutation, and nonrandom mating. Genetic drift is random and strongest in small populations. Bottlenecks and founder effects reduce genetic variation. Gene flow moves alleles between populations through migration. Mutation is the ultimate source of new alleles. Nonrandom mating changes genotype frequencies and can interact with sexual selection.

Hardy-Weinberg equilibrium is a null model for no evolution. The five conditions are large population, no migration, no mutation, no natural selection, and random mating. The equations \(p+q=1\) and \(p^2+2pq+q^2=1\) connect allele frequencies to genotype frequencies. If observed genotype frequencies differ from Hardy-Weinberg expectations, at least one condition is not met and the population may be evolving.

Unit 8: Ecology

Unit 8 studies interactions among organisms and the environment. Population ecology begins with growth models. Exponential growth follows \(rac{dN}{dt}=rN\) and produces a J-shaped curve under unlimited resources. Logistic growth follows \(rac{dN}{dt}=rN\left(rac{K-N}{K} ight)\), where \(K\) is carrying capacity. Logistic growth is more realistic because populations are limited by resources, predation, disease, competition, and space.

Community ecology studies interactions among species. Mutualism benefits both species, commensalism benefits one and does not affect the other, predation and parasitism benefit one while harming the other, and competition harms both because each species uses resources the other needs. A niche includes an organism’s role, resources, and conditions. Competitive exclusion says two species cannot occupy the exact same niche indefinitely.

Energy flow and matter cycling are often tested together. Energy flows through ecosystems and is lost as heat at each trophic level. The 10% rule says only about 10% of energy transfers to the next trophic level. Matter cycles because atoms are reused. Carbon moves through photosynthesis, respiration, decomposition, and combustion. Nitrogen must be fixed before most organisms can use it. Water cycles through evaporation, condensation, precipitation, runoff, and infiltration.

Ecological disruptions include invasive species, climate change, habitat loss, pollution, and overharvesting. Keystone species have outsized effects on ecosystems, so removing them can trigger trophic cascades. Biodiversity increases ecosystem resilience because diverse systems often recover better from disturbance. On FRQs, explain the mechanism linking the disruption to population or ecosystem change.

AP® Biology FRQ Strategy

The free-response section tests both biology content and scientific reasoning. The two long questions usually require students to interpret experimental results, analyze data, describe procedures, evaluate evidence, or graph results. The four short questions assess investigation, conceptual analysis, model or visual analysis, and data analysis. A strong FRQ answer uses precise biological vocabulary, explains mechanisms, and connects evidence to the claim.

For experimental-design prompts, identify the independent variable, dependent variable, control group, controlled variables, and expected outcome. When a question asks for a prediction, give a clear biological reason, not just a direction. For graphing prompts, label axes with units, choose a scale that uses the grid well, plot points accurately, and draw the correct trend line or bar format. If error bars are provided, compare overlap only when the prompt asks about statistical confidence.

For data-analysis prompts, do not describe every number. Identify the trend, compare key values, and explain what the pattern supports biologically. For model-analysis prompts, describe the model first, then evaluate what it shows and what it leaves out. For claim-evidence-reasoning prompts, make a direct claim, cite data or observations, and explain the biological mechanism linking evidence to the claim.

• Use terms such as diffusion, gradient, selection, expression, regulation, allele frequency, and feedback accurately.
• When calculating, show substitution and units.
• When asked to justify, explain why your answer is biologically reasonable.
• When comparing groups, state which group is greater, lower, faster, slower, or unchanged.

How to Use This AP® Biology Cheat Sheet

Start by reviewing the eight uploaded cheat-sheet cards because they preserve the highest-yield topics in compact form. Then use the formula bank to practice calculations, the flashcards to test vocabulary, and the quiz to check application. Biology is easiest to remember when you organize it by process: molecules build cells, cells transform energy, cells communicate, genes control traits, evolution changes populations, and ecology connects organisms to environments.

1. Read one unit card. Identify every term that feels unfamiliar.
2. Make a mechanism sentence. For each term, explain what causes it, what happens, and why it matters.
3. Practice formulas. Work one example each for water potential, chi-square, Hardy-Weinberg, population growth, and SA:V.
4. Use flashcards. Say the answer before revealing it, then write one biological example.
5. Take the quiz. Missed questions show which unit to revisit.
6. Write FRQ explanations. Use claim, evidence, and reasoning rather than memorized definitions alone.
7. Estimate readiness. After timed practice, use the AP Biology score calculator.

A strong weekly review plan gives each unit a job. Day 1: chemistry and cells. Day 2: cellular energetics. Day 3: communication and cell cycle. Day 4: heredity and gene expression. Day 5: natural selection. Day 6: ecology and formulas. Day 7: mixed MCQ and FRQ practice. Rotate between content recall and data interpretation because the AP® Biology exam rewards students who can apply biology to new experiments and unfamiliar models.