AP Biology

What people get wrong

1. Reversing dehydration synthesis and hydrolysis: students say water is released during digestion. Fix: hydrolysis (breaking polymers, e.g., digestion) adds water; dehydration synthesis (building polymers) removes water.
2. Treating cellulose and starch as chemically identical because both are glucose polymers. Fix: identify the glycosidic bond type — alpha-1,4 (starch, digestible, helical) versus beta-1,4 (cellulose, indigestible by most animals, rigid fibers).
3. Assuming every protein has quaternary structure. Fix: quaternary structure only exists if the functional protein is made of more than one polypeptide chain (e.g., hemoglobin); a single-chain protein like myoglobin stops at tertiary structure.
4. Attributing a bulk property of water (like high specific heat) to one single hydrogen bond. Fix: explain these properties as emerging from the collective, constantly reforming network of many hydrogen bonds, not one bond's strength.

What people get wrong

1. Assuming water moves toward the hypotonic (low-solute) side because 'water follows water.' Instead, remember water moves toward the side with lower water potential, which is the side with MORE solute (hypertonic) when pressure potential is zero.
2. Treating pressure potential (Ψp) as always positive or zero. In real plant systems, especially xylem, Ψp can be negative due to tension; don't drop it from the equation just because it 'should' be positive.
3. Assuming facilitated diffusion requires ATP because it uses a protein channel or carrier. It is still passive transport — it moves solutes down their concentration gradient and needs no energy input, unlike active transport.
4. Thinking bigger cells are more efficient because they have more total surface area. Compare the ratio, not the raw numbers: SA:V shrinks as cells grow, which is why cells stay small or fold their membranes (microvilli, cristae) instead of simply enlarging.

What people get wrong

1. Thinking enzymes change whether a reaction is spontaneous. Fix: enzymes only lower activation energy and speed up the approach to equilibrium; ΔG and equilibrium position are unchanged.
2. Mixing up competitive and noncompetitive inhibitor graphs. Fix: memorize that competitive inhibition raises Km only (Vmax unchanged, overcome by more substrate); noncompetitive/allosteric inhibition lowers Vmax only (Km unchanged, cannot be overcome by more substrate).
3. Swapping the locations of the light reactions and the Calvin cycle. Fix: light reactions happen in the thylakoid membrane (photosystems, ETC, chemiosmosis); the Calvin cycle happens in the stroma (carbon fixation, reduction, RuBP regeneration).
4. Assuming fermentation replaces cellular respiration entirely when oxygen is absent. Fix: fermentation only regenerates NAD+ so glycolysis (which always occurs in the cytoplasm) can continue; it does not run the Krebs cycle or ETC.

What people get wrong

1. Confusing the G1 checkpoint with the G2/M checkpoint on exam questions. Fix: attach a specific question to each checkpoint (G1 asks 'is there enough growth signal/nutrients?', G2/M asks 'is DNA replication complete and undamaged?', M asks 'are all chromosomes attached to the spindle?') rather than memorizing them as an unordered list.
2. Treating mitosis and the full cell cycle as the same thing, and including S phase (DNA replication) inside 'mitosis.' Fix: remember mitosis (M phase) is only the division of the nucleus and cell, occurring after interphase (G1, S, G2) is already complete.
3. Assuming a phosphorylation cascade only transmits the signal rather than amplifying it. Fix: state explicitly that one active kinase phosphorylates many copies of the next kinase, so the signal grows in magnitude at each step, not just gets passed along unchanged.
4. Mixing up proto-oncogenes and tumor suppressor genes when explaining a cancer scenario. Fix: ask whether the mutation makes a gene more active (proto-oncogene to oncogene, gain of function, one allele suffices) or less active (tumor suppressor, loss of function, usually needs both alleles hit).

What people get wrong

1. Thinking crossing over happens between sister chromatids — it actually happens between non-sister chromatids of homologous chromosomes during prophase I; sister chromatids are genetically identical and swapping between them does nothing.
2. Applying the standard 9:3:3:1 dihybrid ratio to an epistasis problem without adjusting it — instead, identify which genotype classes are masked by the epistatic gene and combine those categories (e.g., 9:3:4).
3. Using the product rule ('and') when a question asks for 'either/or' outcomes, or vice versa — decide first whether the question wants both events to occur together (multiply) or one of several possible mutually exclusive outcomes (add).
4. Assuming nondisjunction can only occur in meiosis I — it can also occur in meiosis II, and the two cases produce gametes with different chromosome compositions (non-identical homologs vs. identical sister chromatids).

What people get wrong

1. Copying the coding (non-template) strand as if it were read by RNA polymerase. Always identify the template strand first (read 3' to 5'), then build the mRNA 5' to 3' as its antiparallel complement.
2. Assuming any nucleotide change automatically damages the protein. Always translate before and after the change and compare amino acids — the redundancy of the genetic code makes many substitutions silent.
3. Treating all insertions and deletions like point mutations. Only insertions or deletions that are NOT a multiple of three shift the reading frame; check the number of bases added or removed before calling something a frameshift.
4. Reversing the logic of the lac operon. In the absence of lactose the operon is OFF by default because the repressor is bound to the operator; lactose (via allolactose) inactivates the repressor to turn transcription ON.

What people get wrong

1. Taking the square root of an observed phenotype frequency that is NOT homozygous recessive — only q² (the fully recessive phenotype) can be square-rooted directly; the dominant phenotype frequency is p² + 2pq and cannot be square-rooted to get p.
2. Assuming a population is automatically in Hardy-Weinberg equilibrium without checking whether the listed conditions (random mating, no selection, no drift, no gene flow, no mutation, large population) are actually met in the scenario.
3. Treating genetic drift and natural selection as the same process — drift is random and has the largest effect in small populations (founder effect, bottleneck), while selection is non-random and driven by fitness differences; a question describing a small isolated population after a storm or colonization event is testing drift, not selection.
4. Reading a phylogenetic tree left-to-right as if the order of the tip labels indicates evolutionary relatedness — relatedness is determined only by the position of branch points (nodes), so two tips can look far apart on the page yet share a very recent common ancestor.

What people get wrong

1. Confusing GPP with NPP: students plug GPP straight into the 10% rule and forget to subtract the producers' own respiration first. Always compute NPP = GPP − R before doing any trophic transfer calculation.
2. Applying the 10% rule by subtraction instead of multiplication (e.g., subtracting 10% instead of keeping only 10%). Remember that only about 10% is *retained*, so you multiply by 0.10 at each step, not subtract 10%.
3. Mislabeling symbiotic relationships by only considering one species' benefit. Always assign a +, −, or 0 to *each* species before naming the interaction — this is how you correctly distinguish commensalism (+/0) from mutualism (+/+) or parasitism (+/−).
4. Treating exponential growth as the normal, expected pattern in nature. In reality most populations are limited by resources and follow logistic growth; exponential growth only appears briefly, e.g., after colonizing a new habitat or during a population crash's absence of predators.