Biology
Photosynthesis — quick study summary
AP BiologyGCSE BiologyIB Biology HL
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
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