AP Biology · Unit 3 · Topic 3.4
Two stages, two products passed between them. Follow the electrons, the ATP and the NADPH from light to carbohydrate.
Learning objective 3.4.A and 3.4.B Describe the light-dependent reactions and the Calvin cycle, and explain how the two are linked.
The diagram follows a single electron from the moment light knocks it loose to the moment its energy ends up in NADPH. The counters underneath track a whole round, which takes four such electrons and two molecules of water.
The diagram is wider than your screen — scroll it sideways to follow the electron all the way to ATP synthase.
Count the atoms at step 3. Two water molecules carry two oxygen atoms between them, and one O₂ carries two. Nothing else in the light reactions contains oxygen that could be released. The plant is not making oxygen on purpose — it is discarding what is left after taking the electrons and the hydrogen it wanted.
Watch steps 4 to 7. The electron leaves photosystem II with a lot of energy and arrives at photosystem I with very little, because the energy it lost on the way was spent pumping H+. It could not reduce NADP⁺ in that state. The second photon puts the energy back.
The numbering is historical: photosystem I was discovered first. The electron meets II first.
This is the same mechanism you will meet again in Topic 3.5, in a different membrane and running on a different source of electrons. If you understand it here, you have most of the next topic already.
Pigments absorb some wavelengths and not others. The wavelengths a leaf does not absorb are the ones that reach your eye, which is the whole explanation of what color a leaf is.
Set the slider to 550 nm. All three curves are near the floor. That light is not absorbed, so it is reflected and transmitted — and a leaf held up to the light looks green for the same reason a leaf on a tree does.
Compare the three curves at 640 nm and at 475 nm. No single pigment is best everywhere, and the plant does not have to choose — the accessory pigments pass the energy they capture on to the chlorophyll a at the reaction center.
A classic question gives you a graph like this and asks you to predict the rate of photosynthesis under light of a stated color. The reasoning is one step: read the absorbance at that wavelength, and the rate follows it.
Under green light a plant photosynthesises poorly, because almost none of the light is absorbed. Under red or blue light it does well. A plant is not short of light in a green room; it is short of light it can use.
One turn of the cycle does not produce anything a plant can use. Three turns do. Step through the four stages and keep your eye on the carbon count, which has to balance at every stage or the diagram is lying to you.
Five of the six G3P made never leave. They are rearranged back into the acceptor molecules the cycle began with, using three more ATP. A chain would run out of acceptor after one pass; a cycle rebuilds it every time.
The course framework puts the names of the individual enzymes and intermediates of this cycle, and their structures, outside what you can be asked. ATP synthase is the one enzyme name you are expected to know, and it belongs to the light reactions.
What you are expected to do is count: how many CO₂ go in, how many ATP and NADPH are spent, what leaves, and what is rebuilt. That is what this panel counts.
The Calvin cycle needs no light of its own. Switch the light off anyway and see how far it gets. Then switch the light back on and take the CO₂ away instead.
The cycle runs on the ATP and NADPH already sitting in the stroma, and there is only a small stock of those — the same point Topic 3.3 made about ATP generally. When it is gone the cycle stops, in the dark, with plenty of CO₂ still available.
This is what happens to a plant on a hot dry day when it closes its stomata to conserve water. There is bright light and plenty of ATP and NADPH, and the cycle still cannot turn, because there is nothing to fix.
Answer these in your own words before you open the answers. Questions 5 and 7 are the ones that separate grades.
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| Light-dependent reactions | Calvin cycle | |
|---|---|---|
| Where | Thylakoid membranes | Stroma |
| Needs | Light, water, ADP + Pi, NADP+ | CO2, ATP, NADPH |
| Produces | ATP, NADPH, O2 | G3P, ADP + Pi, NADP+ |
| Per unit | 2 H2O → 1 O2 + 3 ATP + 2 NADPH | 3 CO2 + 9 ATP + 6 NADPH → 1 G3P |
| Stops without | Light, or water | CO2, or ATP and NADPH |
Read the last row across: each half stops without something the other half supplies, which is why neither runs for long alone.