Plants make their own food. In the chloroplasts of leaf cells, light energy is used to turn two simple substances into glucose, with oxygen given off as a by-product.

carbon dioxide + water → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Learn the word equation (all boards). Learn the formulae too: AQA asks you to recognise CO₂, H₂O, O₂ and C₆H₁₂O₆, and the balanced version is worth knowing for every board. Light is written over the arrow: it is not a reactant.

1. Count carbon: 6 × 1 = 6 on the left; 6 in one glucose on the right
2. Count hydrogen: 6 × 2 = 12 on the left; 12 in glucose on the right
3. Count oxygen: 6 × 2 + 6 × 1 = 18 on the left; 6 + 6 × 2 = 18 on the right
4. Balanced: the numbers are in the ratio 6 : 6 : 1 : 6

Photosynthesis takes in energy from its surroundings, so it is endothermic. The energy arrives as light and is absorbed by chlorophyll, the green pigment in chloroplasts. That energy ends up stored in the glucose.

• Carbon dioxide diffuses into the leaf through the stomata.
• Water comes up from the roots in the xylem.
• Oxygen diffuses out through the stomata (some is used by the plant's own respiration).

Respiration is the reverse reaction and it is exothermic. If a question asks which reaction stores energy, it is photosynthesis.

A plant does five main things with its glucose:

• Respiration, in every living cell, day and night, to release energy.
• Starch for storage: starch is insoluble, so it does not upset the water balance of cells by osmosis.
• Cellulose to build and strengthen cell walls.
• Fats and oils (lipids) for storage, especially in seeds.
• Amino acids, made with nitrate ions from the soil, then joined into proteins.

Starch is the plant store; glycogen is the animal store. Swapping them is the most common slip in this topic.

Iodine test result	Means
orange-brown (no change)	no starch: the leaf has not been photosynthesising
blue-black	starch present: glucose was made and stored