Biology 20 · Photosynthesis
Light becomes sugar
Two stages in two places, connected by two carriers. The thylakoid membrane turns light into ATP and NADPH; the stroma spends both to build carbon into a molecule. Almost every question is about which happens where.
- 1. The words, first
- 2. The two stages, and why both are needed
- 3. How we know the oxygen comes from water
- 4. Pigments and limiting factors
- 5. What costs marks
The words, first
The idea: Location matters more here than in any other unit, because the two stages are separated in space on purpose.
| Word | What it means |
|---|---|
| Chloroplast | The organelle where photosynthesis happens. |
| Thylakoid | A flattened membrane sac inside the chloroplast; stacks of them are grana. Holds the pigments and the electron transport chain. |
| Stroma | The fluid surrounding the thylakoids, where the Calvin cycle runs. |
| Pigment | A molecule that absorbs some wavelengths and reflects others. Chlorophyll reflects green, which is why leaves look green. |
| Photosystem | A cluster of pigments plus a reaction centre that passes on an excited electron. |
| Photolysis | Splitting water to replace the electrons chlorophyll lost. The oxygen you breathe is the leftover. |
| NADPH | An electron and hydrogen carrier — the reducing power the Calvin cycle needs. |
| Chemiosmosis | Making ATP by letting a proton gradient collapse through ATP synthase. |
| Calvin cycle | The stroma reactions that fix CO₂ into sugar. Often miscalled the dark reactions. |
| Carbon fixation | Attaching inorganic CO₂ to an organic molecule — the cycle's first step. |
| RuBP / rubisco | The five-carbon acceptor and the enzyme that joins CO₂ to it. |
| Limiting factor | Whichever requirement is in shortest supply and therefore sets the rate. |
The two stages, and why both are needed
The idea: One stage captures energy but fixes no carbon. The other fixes carbon but captures no light. Neither works alone.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy in)
Light-dependent reactions — in the thylakoid membrane. Light excites electrons in chlorophyll; an electron transport chain pumps protons into the thylakoid space; the gradient drives ATP synthase; NADP⁺ is reduced to NADPH; water is split to replace the lost electrons, releasing O₂.
Calvin cycle — in the stroma. CO₂ is attached to RuBP by rubisco, and ATP and NADPH from stage one are spent reducing the result to sugar. RuBP is regenerated so the cycle can continue.
Why “dark reactions” is a bad name. The Calvin cycle needs no light directly, but it stops within minutes in the dark because its supply of ATP and NADPH stops.
How we know the oxygen comes from water
The idea: Both CO₂ and H₂O contain oxygen, so the equation alone cannot say which one is released. An isotope settles it.
Feed a plant water labelled with the heavy isotope ¹⁸O and the oxygen gas it releases is labelled. Feed it labelled CO₂ instead and the gas is not. The oxygen comes from water, through photolysis.
This is worth knowing not only as a fact but as a model of how a question like this gets answered: if two sources are possible, label one of them and see which end up where.
Pigments and limiting factors
The idea: A rate is set by whatever is in shortest supply, so increasing anything else changes nothing.
Absorption spectra. Chlorophyll a and b absorb strongly in the red and the blue and poorly in the green, which is reflected. Accessory pigments such as carotenoids absorb wavelengths chlorophyll misses and pass the energy on — and become visible in autumn when chlorophyll breaks down first.
Chromatography. Pigments separate on paper according to how strongly they are attracted to the paper versus the solvent. A pigment that dissolves well and clings weakly travels far. Colour is how you read the result; solubility is what does the separating.
Reading a rate graph. A curve that rises and then flattens means light has stopped being the limiting factor and something else — usually CO₂ concentration or temperature — has taken over. Commercial greenhouses raise CO₂ as well as light for exactly this reason.
Temperature behaves differently from light and CO₂: the rate rises, peaks, and then falls, because the enzymes denature.
What costs marks
The idea: Location, location, and one naming trap.
- Putting the Calvin cycle in the thylakoid. It is in the stroma.
- Saying the released oxygen comes from carbon dioxide. It comes from water.
- Treating the dark reactions as happening at night. They stop when the supplies stop.
- Saying chlorophyll absorbs green light. It reflects green — that is why you can see it.