Photosynthesis: energy, carbon dioxide, water and oxygenSpec C1.3.1, C1.3.2, C1.3.3
In short
Photosynthesis transforms light energy into chemical energy when carbon compounds are produced. Carbon dioxide is converted to glucose using hydrogen obtained by splitting water: carbon dioxide + water → glucose + oxygen. The oxygen comes from the splitting of water and is a by-product released by plants, algae and cyanobacteria. This supplies most of the chemical energy in ecosystems.
Photosynthesis involves the transformation of light energy to chemical energy. Light energy absorbed by pigments is used to make carbon compounds, and the energy is then held in the chemical bonds of those compounds. This energy transformation supplies most of the chemical energy needed for life processes in ecosystems: producers use it directly, and consumers and decomposers obtain it by feeding.
Carbon dioxide, water and glucose
Carbon dioxide is converted to glucose using hydrogen obtained by splitting water. Carbon dioxide provides the carbon and oxygen atoms of glucose, but glucose also contains hydrogen, which must come from somewhere: it comes from water molecules, split using energy from light.
Oxygen as a by-product
When water is split, the hydrogen is used and the oxygen is released as a by-product. So the oxygen produced by photosynthesis comes from the splitting of water, not from carbon dioxide. Oxygen-producing photosynthesis happens in plants, algae and cyanobacteria.
Do not write that the oxygen released comes from carbon dioxide. It comes from water; the oxygen atoms of CO₂ end up in glucose and water.
Linking question: the consequences of photosynthesis for ecosystems include supplying chemical energy to food chains and removing carbon dioxide from the atmosphere.
Written and checked against the IB Biology SL specification · Updated October 2026