AP Biology · Unit 4 · Topic 4.2
Reception, transduction, response. Where the receptor sits depends on whether the signal can cross a membrane, which is a question you already know how to answer.
Learning objective 4.2.A Explain how signal transduction allows cells to respond to external signals.
Whether a receptor sits in the membrane or inside the cell is not something to memorise per hormone. It follows from one question you answered in Topic 2.4: can this molecule get through a phospholipid bilayer?
The middle of a membrane is a layer of hydrocarbon tails with no water in it. A molecule that is small, non-polar and uncharged dissolves into that layer and comes out the other side. A molecule that is large, or polar, or charged does not — so if the cell is going to know it is there at all, it has to be detected at the surface.
All three conditions have to be met. Being small is not enough: a neurotransmitter is small and charged, and it stays outside.
A receptor that is already inside the cell can carry its ligand to the nucleus and act on DNA directly. A receptor stuck in the membrane cannot go anywhere, so its message has to be relayed inward by other molecules — which is what the next two tabs are about.
So the difference in where the signal is detected produces a difference in what kind of response is available. Steroid responses are slower and last longer, because making new protein takes time.
Nothing passes through the receptor. The ligand does not travel into the cell and the receptor does not open a hole. What crosses the membrane is a shape change: the part of the protein sticking out changes when the ligand binds, and because it is one continuous molecule, the part sticking in changes too.
Three stages, in that order, every time. Step through them and notice how little has happened after the first one.
Read the first two columns across: identical. Read the last one: different every time. Whatever makes a liver cell respond differently from a heart cell, it is not in how the signal was received.
A receptor in the membrane cannot reach the enzyme it needs to switch on, or the gene it needs transcribed. The relay molecules are how the message travels that distance — and, as Topic 4.3 shows, how one arriving molecule ends up changing millions.
A pathway is a chain. Break a link and everything after it stops — but everything before it goes on happening exactly as it did. Two of these breaks look identical from outside the cell.
Knock out the receptor, then knock out the response protein. Watch the “response occurs?” readout: no in both cases. Watch “steps that still run”: not remotely the same.
This is why a question about a non-responding cell is worth reading carefully. “The cell does not respond” is an observation, and it is consistent with a fault anywhere along the chain.
Because each break leaves a different number of steps running, the pathway itself is the diagnostic. If you can detect an intermediate — measure whether the relay molecules were activated — you can say how far the signal traveled before it stopped, and therefore where the fault is.
Some inherited conditions are receptor faults: the hormone is present in normal amounts and the cell cannot detect it. Some cancers are the opposite — a relay protein stuck permanently in its active shape, so the pathway runs and the response happens with no signal arriving at all.
Both are faults in the same chain, at different points, in opposite directions. Topic 4.3 takes the second case further, and Topic 4.6 takes it as far as a tumour.
Answer these in your own words before you open the answers.
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| Stage | What happens | What it is for |
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If a question asks what a pathway does, the answer is in the third column. If it asks how, the answer is in the second.