Photosynthesis
Photosynthesis is the process plants, algae and some bacteria use to turn light energy into chemical energy. It takes in carbon dioxide and water and gives out glucose and oxygen. Almost all the oxygen in the atmosphere, and most of the energy in food chains, comes from it.
The overall equation
Written as one balanced equation, with light absorbed by chlorophyll:
The equation hides the detail. The oxygen released comes from the water, not from the carbon dioxide. This was shown in 1941 by tracing the isotope oxygen-18.
The two stages
The light-dependent reactions happen in the thylakoid membranes of the chloroplast. Light splits water, releasing oxygen, and the energy is stored for a short time in ATP and NADPH.
The Calvin cycle happens in the stroma. The enzyme RuBisCO fixes carbon dioxide onto a five-carbon sugar, and ATP and NADPH from the first stage turn the product into sugar. Three turns of the cycle fix three CO2 molecules and release one three-carbon sugar.
Limiting factors
The rate is set by whichever factor is in shortest supply: light intensity, carbon dioxide concentration, or temperature. Above about 40 °C the enzymes start to denature and the rate falls quickly.
References
- Ruben, S. et al. (1941). Heavy oxygen (O18) as a tracer in the study of photosynthesis. Journal of the American Chemical Society 63, 877–879.
- Urry, L. et al. Campbell Biology, 12th edition, chapter 10.