Photosynthesis metabolism. Illustration of the reactions that take place in the thylakoid stacks (centre right) in a plant's chloroplast during photosynthesis. Light energy is used to oxidise water (H2O) and produce oxygen (O2). The energy from this is cycled through the ATP/ADP (adenosine tri/diphosphate) and NAD (nicotinamide adenine dinucleotide) molecules (centre). This drives the Calvin-Benson cycle (left) that fixes carbon from carbon dioxide (CO2), producing starch and carbohydrates (CH2O). For this artwork without labels, see image C023/8810.