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 glucose and sucrose, 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 only two cell drawings 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 behavior 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; glucose and sucrose 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?
Sample question
In an invented experiment, a drug is added to cultured cells that raises the pH inside the lysosome from about 5 to about 7. Within a few hours, undigested material begins to accumulate inside the cells. What does this result most directly show?
- The enzymes inside the lysosome need its acidic interior in order to work, so raising the pH halts digestion even though the enzymes are all still there.
- The drug destroyed the lysosomal membrane and released the enzymes into the cytosol.
- The cell stopped manufacturing lysosomal enzymes as soon as the drug was added.
- Lysosomes digest material by acid alone, so neutralizing the acid removes the only agent involved.
See the answer
The enzymes inside the lysosome need its acidic interior in order to work, so raising the pH halts digestion even though the enzymes are all still there.
Lysosomal hydrolases have a pH optimum near 5, and the compartment is held at that pH by a proton pump in its membrane precisely so that they can work there — and, incidentally, so that any that escape into the neutral cytosol are inactive. Raise the pH and the enzymes are still present, still correctly folded in the loose sense, and no longer catalyzing; the cell fills up. It is the enzyme-and-pH question one unit earlier, happening inside an organelle. Membrane rupture would produce damage well beyond an accumulation of waste and nothing here tested for it. Halting synthesis would take far longer than hours to show, and would not explain why the enzymes already inside stopped working. And acid alone hydrolyses protein far too slowly to keep a cell clear, which is why lysosomes contain enzymes at all rather than merely being acidic.
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