Photosynthesis is how plants, algae, and some bacteria turn light energy into chemical energy stored in sugar. In plants it happens in chloroplasts, mostly in leaf cells.
The overall equation
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Check the balance: carbon 6 = 6, hydrogen 12 = 12, oxygen 6·2 + 6 = 18 on the left and 6 + 6·2 = 18 on the right.
Stage 1: Light-dependent reactions
- Location: the thylakoid membranes inside the chloroplast.
- Chlorophyll absorbs light, mainly red and blue wavelengths (green is reflected, which is why leaves look green).
- Water is split, releasing O₂ as a by-product.
- The energy is captured as ATP and NADPH.
Stage 2: The Calvin cycle (light-independent reactions)
- Location: the stroma, the fluid around the thylakoids.
- The enzyme RuBisCO fixes CO₂ onto a five-carbon sugar (RuBP).
- ATP and NADPH from stage 1 power the conversion into G3P, which the plant uses to build glucose and other molecules.
What limits the rate
- Light intensity: more light speeds things up until another factor limits it.
- CO₂ concentration: more CO₂ feeds the Calvin cycle.
- Temperature: enzymes work best in a range; too hot and they denature.
Photosynthesis vs. cellular respiration
Respiration runs roughly the reverse overall reaction, breaking glucose with O₂ to release energy as ATP. Plants do both: photosynthesis in light, respiration all the time.