Photosynthesis

Photosynthesis is the process by which green plants use light energy absorbed by chlorophyll to combine carbon dioxide and water into glucose, releasing oxygen as a by-product

Photosynthesis is what makes plants autotrophs: they manufacture their own food rather than eating it. This is also why plants sit at the base of every food chain. Almost all the energy flowing through living things on Earth started as sunlight captured by chlorophyll.

The two equations

The word equation:

carbon dioxide + water → glucose + oxygen

(in the presence of sunlight and chlorophyll)

The balanced symbol equation:

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

In plain English: six molecules of carbon dioxide combine with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The reaction is endothermic: it absorbs light energy from the sun and stores it as chemical energy in the glucose.

Compare this to aerobic respiration:

glucose + oxygen → carbon dioxide + water (releases energy)

Photosynthesis is the exact reverse of aerobic respiration. If you learn one, you know the other.

What the plant does with the glucose

Glucose has many uses inside the plant:

• Respiration: glucose is broken down to release energy for the plant's own life processes (transport, growth, reproduction).
• Starch storage: glucose is converted into the polysaccharide starch and stored in roots, tubers and seeds. Starch is insoluble, so it does not affect the cell's water balance.
• Cellulose: glucose is built into long chains of cellulose for new cell walls as the plant grows.
• Lipids: glucose can be converted into lipids for energy-dense storage in seeds (think of the oil in sunflower seeds or nuts).
• Amino acids and proteins: when glucose is combined with nitrogen from nitrate ions absorbed by the roots, the plant builds amino acids, which are then joined to form proteins.

Raw materials, products, and what each needs

A quick exam note: light is not a raw material. The exam mark scheme only counts CO₂ and water as raw materials, because they are the substances that combine in the reaction. Light is the energy source.