Carbon dioxide is a molecule made up of one carbon atom (C) and two oxygen atoms (O2).

For photosynthesis to occur, a constant supply of carbon dioxide is required by the cells that are photosynthesising.

1. CO₂ enters the leaf through the stomata (pores in the leaf surface)
2. The carbon dioxide enters the  spaces between the cells inside the leaf
3. The carbon dioxide enters the cells where it can be used for photosynthesis

Leaves are adapted to maximise the transfer of gases and for photosynthesis:

• Their large surface area provides the maximum surface for transfer and light absorption.
• Leaves are thin so that light can penetrate all the way through maximising the amount of photosynthesis possible.

Oxygen, a by-product of photosynthesis, is released by the cells and exits the leaf through the stomata. This is what the animal kingdom relies on for survival as oxygen is what animals need for respiration.