Mechanisms 1 — what happens next, and what fails

Fifteen questions spread across the four big ideas of the course, and not one of them asks what a term means.

They ask about mechanism. Block the spindle apparatus and say what halts and why. Mutate a promoter and say what stops. Name the immediate next step after a ligand binds a G-protein-coupled receptor. Trace a secreted protein from ribosome to membrane in order. If you can say what comes immediately before and after a step, you have the process; if you can only name it, you do not, and this is the format that finds out.

Four questions are small experiments described in words. Each names the independent variable, the dependent variable and the control, then gives a result and asks what it indicates. Their wrong options are the important part: they are not false biology but conclusions the data do not license — which is exactly how the wrong answers work on the real paper, and exactly the discipline the free-response section demands. A rate identical in a cell membrane and a synthetic lipid bilayer tells you something quite specific about how a molecule crosses, and several other true statements about membranes are not what it tells you.

Three questions turn on a confusion students actually have. Competitive against noncompetitive inhibition — distinguished here by whether adding more substrate restores the original rate, which is the only test that works. Innate against adaptive immunity, distinguished by memory. The light reactions against the Calvin cycle. Mitosis against meiosis. In each, the wrong options are real biology from the adjacent process, and the explanation says which process each one actually describes.

What this cannot do, said plainly. Section II is half the exam, and it is dominated by two things no multiple-choice bank can set: designing an experiment from scratch — stating a hypothesis, choosing variables, specifying a control, predicting a result — and writing a justification, where a correct claim with no mechanism behind it scores nothing. A graph or data table appears in most free-response questions, and these materials carry no images at all.

So use this for the layer underneath, which is where silent failure lives: a half-remembered mechanism produces an experimental-design answer that is wrong in a way better structure cannot rescue. Then go to College Board's released free-response questions, which come with real scored student answers and the readers' commentary, and write one under time.

One habit worth building while you work through these: for every mechanism, be able to say what happens if it fails. The free-response section asks that constantly, and it is the fastest test there is of whether you understand a process or have learned its name.

• State what halts, and why, when a named step in a process is blocked
• Put the steps of a process in order, including what comes immediately before and after any one of them
• Distinguish competitive from noncompetitive inhibition by whether added substrate restores the rate
• Tell apart adjacent processes students routinely confuse — mitosis and meiosis, transcription and translation, the light reactions and the Calvin cycle, innate and adaptive immunity
• Read an experiment described in words and identify the independent variable, the dependent variable and the control
• Refuse a conclusion the described data do not license, even when it is true biology
• Explain why a described result came out as it did, rather than naming the phenomenon

A researcher observes that adding an inhibitor decreases the maximum reaction rate, and that increasing the substrate concentration does not restore the original rate. What is the mechanism of this inhibition? — Whether more substrate can out-compete the inhibitor is the only test that separates the two kinds. It cannot here, so the inhibitor is not occupying the active site; it is binding elsewhere and changing the enzyme's shape, which lowers the maximum rate no matter how much substrate is present.