Photosynthesis: Making Food from Air and Water

Photosynthesis is the process in which green plants use light energy to build food from carbon dioxide and water. The word itself tells the story: photo = light, synthesis = putting together.

The word equation is:

Raw materials (what goes in)

• Carbon dioxide — from the air, entering through the stomata
• Water — from the soil, taken up by the roots and carried in the veins

Requirements (needed, but not used up as ingredients)

• Light energy — usually sunlight
• Chlorophyll — the green pigment that captures the light

Products (what comes out)

• Glucose — a sugar; the plant's food and energy store. Extra glucose is stored as starch in leaves, stems, roots and seeds
• Oxygen — released through the stomata as a by-product

Worked example. A learner plants a maize seedling with a mass of 5 g in a pot. Four months later the plant has a dry mass of 900 g, but the soil in the pot has lost only 3 g of mass. Where did the other 892 g come from?

1. Water from the watering can supplied hydrogen and oxygen.
2. Carbon dioxide from the air supplied the carbon — and carbon makes up most of the dry mass.
3. The soil supplied only a tiny mass of minerals, which is why it lost so little.

The plant built itself mainly out of a gas you cannot see.