Biology
Photosynthesis — quick study summary
Photosynthesis is how plants, algae and some bacteria convert light energy into chemical energy stored in glucose. It happens in chloroplasts and runs in two coordinated stages: the light-dependent reactions on the thylakoid membrane and the Calvin cycle in the stroma. The overall reaction takes carbon dioxide and water and produces glucose and oxygen.
Key points
- Overall equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (driven by light)
- Light-dependent reactions split water, produce ATP and NADPH, release O₂
- Calvin cycle uses ATP + NADPH to fix CO₂ into G3P, then glucose
- Chlorophyll a absorbs mainly red and blue light; reflects green
- Rate is limited by light intensity, CO₂ concentration, and temperature
In more depth
Light-dependent reactions
These take place in the thylakoid membranes of the chloroplast. Chlorophyll absorbs light, which excites electrons that pass along an electron transport chain. Water is split (photolysis) to replace those electrons, releasing oxygen as a by-product. Energy released along the chain is used to make ATP, and the electrons end up reducing NADP+ to NADPH. Both ATP and NADPH carry energy into the next stage.
The Calvin cycle
In the stroma, the enzyme RuBisCO fixes carbon dioxide onto a five-carbon molecule, RuBP. The products are reduced using ATP and NADPH to form glyceraldehyde-3-phosphate (G3P). Some G3P leaves the cycle to build glucose and other organic molecules; the rest is recycled, using more ATP, to regenerate RuBP so the cycle can keep fixing carbon.
Limiting factors
The rate of photosynthesis is set by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature. Raising a factor that is not limiting has no effect. Temperature acts through enzymes such as RuBisCO, so the rate rises up to an optimum and then falls as enzymes denature.
Common mistakes
- Saying plants get their food from the soil. Most of a plant's mass comes from carbon dioxide fixed during photosynthesis.
- Thinking the oxygen released comes from carbon dioxide. It comes from splitting water in the light-dependent reactions.
- Calling the Calvin cycle the 'dark reactions' and assuming it only runs at night. It runs whenever ATP and NADPH are available, which is mainly in the light.
Practice quiz
Click each question to reveal the answer.
1. Where do the light-dependent reactions of photosynthesis occur?
- Stroma
- Thylakoid membrane
- Cytoplasm
- Mitochondria
Answer: Thylakoid membrane
The thylakoid membrane houses photosystems I and II, where light energy splits water.
2. What gas is released as a by-product of photosynthesis?
Answer: Oxygen
Oxygen comes from the splitting of water in the light-dependent reactions.
3. What molecule is the immediate product of the Calvin cycle?
Answer: G3P (glyceraldehyde-3-phosphate)
Glucose is made from G3P; G3P is the direct Calvin-cycle output.
Last reviewed: May 2026