AP Biology Units 1–2 — molecules and the cell

The first two units of the course, and the layer everything later stands on. Five questions on the chemistry of life, five on the cell.

From Unit 1: which feature of a water molecule explains why salt dissolves and oil does not; which monomer builds which macromolecule; what denaturation does and, more to the point, what it does not do, since it disrupts folding and leaves every peptide bond intact; why an enzyme's rate collapses when the pH moves, which is a question about charge on the residues lining the active site rather than about kinetic energy; and the difference between DNA and RNA that actually matters for what each is for.

From Unit 2: the arithmetic of surface area against volume, worked out rather than asserted — the ratio for a sphere is three over the radius, so a cell that doubles in radius halves its ability to exchange anything across its own surface. Then the secretory route, membrane permeability, and tonicity.

Two of the ten are built as experiments rather than as recall, which is the half of this subject a question bank usually skips. In one, a drug raises the pH inside the lysosome from about five to seven and waste starts accumulating — with the enzymes still present, still inside, and no longer working. In the other, vesicles of pure phospholipid admit oxygen and urea and exclude sodium and glucose, a student concludes that size alone decides the matter, and the question is not whether the student is right but which single further test would make the two candidate rules disagree. The answer is a small charged ion, because that is the only case where "size alone" and "size and charge" predict different things.

That second question is the shape of most of the free-response section. Designing the test that separates two explanations is the skill; knowing the organelles is the layer underneath it.

There are no images here, and the real paper leans on graphs heavily. Every experiment and every number in this set is invented.

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• Explain water's behaviour as a solvent from the polarity of the molecule
• Match each class of macromolecule to its monomer, and say what denaturation disrupts and what it leaves intact
• Account for an enzyme's pH optimum in terms of the active site rather than in terms of energy
• Compute the surface-area-to-volume ratio of a sphere and say why it constrains cell size
• Trace a secreted protein through the endomembrane system, and predict what fails when a named organelle fails
• Predict which substances cross a bare phospholipid bilayer, and apply tonicity to a cell in solution
• Design the test that distinguishes two explanations which fit the same set of results

In an invented experiment, vesicles made of pure phospholipid, with no proteins in them at all, are placed in solutions of four substances. Oxygen and urea enter the vesicles; sodium ions and glucose do not. A student concludes that size alone decides what crosses. Which further test would best distinguish that conclusion from the alternative, that size and charge both matter?