Photosynthesis Key Terms and Equations

Every photosynthesis question starts from the same reaction, written in two ways.

• Word equation: carbon dioxide + water → glucose + oxygen, with light written above the arrow because light energy drives it and chlorophyll absorbs that light.
• Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The symbols you need to recognise are CO₂ (carbon dioxide), H₂O (water), C₆H₁₂O₆ (glucose) and O₂ (oxygen).

A balanced equation has the same number of each kind of atom on both sides, because atoms are only rearranged, never made or destroyed. Check it like this:

Atom	Left side	Right side
Carbon	6 × 1 in 6CO₂ = 6	6 in C₆H₁₂O₆ = 6
Hydrogen	6 × 2 in 6H₂O = 12	12 in C₆H₁₂O₆ = 12
Oxygen	6 × 2 + 6 × 1 = 18	6 + 6 × 2 = 18

Each glucose molecule needs six carbon dioxide molecules because glucose has six carbon atoms, and each carbon atom comes from one CO₂. A common slip is to write O instead of O₂ for oxygen gas: oxygen in the air is always made of pairs of atoms.

• Chlorophyll and chloroplast: chlorophyll is the green pigment; a chloroplast is the tiny structure in a cell that contains it.
• Glucose and starch: glucose is a small, soluble sugar; starch is a large, insoluble molecule made of many glucose units joined together, so it can be stored without being washed around the cell.
• Stoma and stomata: one pore is a stoma; two or more are stomata.
• Endothermic and exothermic: photosynthesis is endothermic (it takes in energy, from light); respiration is exothermic (it releases energy).

A plant does not keep its glucose as glucose for long. It uses it in five main ways:

1. Respiration, to release energy for its cells.
2. Starch, an insoluble store in leaves, seeds and organs such as potato tubers.
3. Fats and oils, compact energy stores, especially in seeds.
4. Cellulose, the fibres that strengthen cell walls.
5. Amino acids, to build proteins. For this, the plant also needs nitrate ions taken from the soil, because amino acids contain nitrogen and glucose does not.

The starch test is a simple way to show that a leaf has been photosynthesising: iodine solution turns from orange-brown to blue-black where starch is present.

Precise words earn marks because they show exactly what you mean. Compare these two answers to the question "Why does a plant in a dark cupboard stop growing?"

• Vague: "It has no sunlight, so it cannot make food."
• Precise: "Without light, chlorophyll cannot absorb energy, so photosynthesis stops and no glucose is made. The plant still respires, so it uses up its stored starch."

The second answer names the pigment, the process, the product and the store, and it remembers that respiration carries on in the dark.