AP Biology Units 3–4 — energy and signalling

Energy and information: how a cell gets what it needs to do work, and how it is told when to do it.

The five energetics questions are built around distinctions that are easy to state and hard to apply. Competitive inhibition against non-competitive, told apart not by where the inhibitor binds but by what happens when you add more substrate — the maximum rate is reached in one case and never in the other. An uncoupler that puts holes in the inner mitochondrial membrane, where the electron transport chain keeps running and the ATP stops, because the gradient rather than the chain is what the synthase is paid by. The two halves of photosynthesis and which needs what. A blocked step between glycolysis and the citric acid cycle. And a comparison of respiration with photosynthesis that goes past "one is the reverse of the other" to the redox direction, with three wrong answers that are each a real misconception — including the belief that plants do not respire.

The five on communication cover why a cascade amplifies, what happens when a second messenger is blocked, feedback in both directions with an example of each, the mitosis and meiosis distinction, and a checkpoint question that is not a recall question.

That last one is the one to sit with. Cells treated with a drug stop in mitosis with condensed chromosomes and never reach anaphase. Two explanations fit: kinetochores never attached, so the spindle checkpoint holds the cell; or the enzyme that cuts the link between sister chromatids is inhibited, so the checkpoint is satisfied and the cell still cannot proceed. The observed result is predicted by both, which is exactly why it cannot decide between them. The spindle can.

No images here, and the real paper leans on graphs heavily. Every experiment and every number is invented.

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• Tell competitive from non-competitive inhibition by what raising substrate concentration does to the maximum rate
• Explain why an uncoupler stops ATP synthesis while the electron transport chain keeps running
• State what the light reactions and the Calvin cycle each require and produce, and where each takes place
• Contrast respiration with photosynthesis by the direction of the redox chemistry, and reject three common misconceptions about them
• Explain why a phosphorylation cascade amplifies a signal, and predict the effect of blocking a second messenger
• Distinguish negative from positive feedback with a real example of each, and say where meiosis generates variation that mitosis does not
• Choose the observation that separates two explanations which both predict the observed result

In an invented experiment, cells treated with a drug arrest in mitosis with condensed chromosomes and never enter anaphase. Two explanations are on the table. First: the drug stops microtubules attaching to kinetochores, so the spindle checkpoint is never satisfied. Second: the drug directly inhibits the enzyme that cleaves the link between sister chromatids. Which observation would best distinguish them?