How does photosynthesis work?
Photosynthesis uses light energy to build sugar from carbon dioxide and water, releasing oxygen. The light reactions in the thylakoid membranes make ATP and NADPH, and the Calvin cycle in the stroma spends them to fix CO2 into sugar.
Two linked stages
The overall equation is 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. It happens in chloroplasts, in two stages that depend on each other.
- Light reactions (thylakoid membranes): chlorophyll in photosystem II absorbs light and passes excited electrons to an electron transport chain. To replace them, water is split, releasing O2 and protons. The chain pumps more protons into the thylakoid space, and as they flow back out through ATP synthase, ATP is made. Photosystem I re-energizes the electrons, which reduce NADP+ to NADPH.
- Calvin cycle (stroma): the enzyme rubisco attaches CO2 to a five-carbon sugar, RuBP. The products are reduced using ATP and NADPH into G3P, a three-carbon sugar. Most G3P is used to regenerate RuBP; the rest leaves the cycle to build glucose and other molecules.
A worked count
To export one G3P, the Calvin cycle has to turn three times, fixing 3 CO2 and using 9 ATP and 6 NADPH. A glucose is built from two G3P, so it costs 6 CO2, 18 ATP and 12 NADPH. That is a useful check on exam questions that ask how many turns of the cycle, or how much ATP, one glucose needs.
Mistakes to watch for
- Saying the oxygen released comes from CO2. It comes from water, split in the light reactions; isotope-labeling experiments showed this.
- Calling the Calvin cycle the "dark reactions" and concluding it runs at night. It needs a steady supply of ATP and NADPH from the light reactions, so in most plants it runs in daylight.
- Thinking plants photosynthesize instead of respiring. Plant cells also carry out cellular respiration in their mitochondria, day and night.
- Forgetting that the proton gradient and ATP synthase work the same way here as in mitochondria. The membrane and the energy source differ; the mechanism, chemiosmosis, is shared.
Watching it happen
It helps to see where each molecule goes: water in on one side of the thylakoid, oxygen out, protons piling up inside, and ATP and NADPH carried over to the stroma. Ask Isotoon "How does photosynthesis turn light into sugar?" for a short narrated video of that path. Related pages: biology, AP Biology and the guide on how to study for AP Biology.
Questions
- Why are plants green?
- Chlorophyll absorbs red and blue light strongly and reflects much of the green, so green is the light that reaches your eyes.
- What is photorespiration?
- Rubisco can bind O2 instead of CO2, which wastes energy and releases fixed carbon. It is worse in hot, dry conditions, which is why C4 and CAM plants have ways to concentrate CO2 around rubisco.