Photosynthesis
Sun, water and air: the basic chemistry of life
Summary
Photosynthesis is the reaction in which plants use sunlight to turn water (H₂O) and carbon dioxide (CO₂) into glucose and oxygen. Whichever of the three requirements is scarcest (light, CO₂, temperature) sets the rate: Blackman's law of limiting factors.
In depth
The overall equation looks simple: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. But behind it there are two big stages inside the chloroplast: the light reactions (on the thylakoid membranes, where water is split, oxygen released and ATP + NADPH produced) and the Calvin cycle (in the stroma, where CO₂ is fixed and glucose built). The rate of production is limited by three main factors: light intensity, CO₂ concentration and temperature. The scarcest of them is called the limiting factor; until it changes, increasing the others does not raise the rate. Plant enzymes work best at around 25°C; at temperatures that are too low or too high the rate falls or the enzymes are denatured.
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