AP Biology · Unit 4 · Topic 4.3
A single molecule arriving at the surface can change millions inside. Count the steps, and the number stops being a slogan.
Learning objective 4.3.A Explain how signal transduction pathways amplify cellular responses.
Every activated protein switches on several copies of the next one. Set how many each step activates and how many steps there are, and watch the bottom row. Then try to guess which of the two sliders matters more.
Press the two buttons above one after the other and compare. Doubling what each step activates is a change to a base. Adding two steps is a change to an exponent. Exponents win, and it is not close.
Look at the top two rows of the diagram. One signaling molecule, one activated receptor. The amplification has not started yet.
This matters for exam answers. “The receptor amplifies the signal” is wrong: a receptor detects one molecule and changes shape once. What amplifies is the relay stage below it, and saying so is the difference between describing and explaining.
Hormones circulate at concentrations far too low to have a direct chemical effect on anything. Amplification is what lets a cell act decisively on a message that arrived as a handful of molecules.
Step through one protein's cycle. Watch the protein count as you go, because it is the readout that answers the question students most often get wrong.
The ten thousand molecules at the bottom of the cascade were already sitting in the cytoplasm before the signal arrived. Nothing was transcribed, nothing was translated, and that is why a cell can respond to a signal in seconds rather than hours.
A cascade with kinases and no phosphatases would switch on once and stay on for the life of the cell. Being able to stop responding is as much a part of signaling as being able to start, and the enzyme that removes the phosphate is what makes it possible.
Not every relay is a protein. A small molecule or ion released into the cytosol spreads through the whole cell far faster than a protein could move, which is why the fastest responses use one.
A pathway can fail by not running when it should, or by running when it should not. Both are broken. Only one of them is what most students picture.
The signal is present in normal amounts and the cell behaves as though it were absent. From the outside this can look like a hormone deficiency, and treating it by giving more hormone would do nothing at all — because the problem was never the amount of signal.
Switch the signal off with the “stuck” fault selected. The response is the full response, produced by a cell that is being told nothing by anybody. Topic 4.6 follows this all the way to a tumour.
Read the two number columns. A normal cell's response depends on the signal; both faulty cells' responses do not, in opposite directions. That dependence is the thing a signaling pathway exists to provide, and it is what both faults destroy.
Answer these in your own words before you open the answers.
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That is the whole of the quantitative side of this topic. If a question gives you a cascade and asks for a number, this is the equation; if it asks why the number is so large, the answer is that the exponent is the number of steps.