Photosynthesis

Plants use energy from light to make carbohydrates from carbon dioxide and water. This process is called photosynthesis. Glucose is a carbohydrate produced by photosynthesis; oxygen is also produced.

The word equation for photosynthesis is:

Photosynthesis takes place in structures in plant cells called chloroplasts. Chloroplasts contain chlorophyll, a green pigment that absorbs light.

Chlorophyll transfers energy from light into chemical energy. The plant uses this energy to synthesise carbohydrates from carbon dioxide and water.

Molecular formulae

A molecular formula shows which elements are present in a molecule and how many atoms of each element it contains. It uses a chemical symbol for each element and a small number called a subscript after the symbol to show the number of atoms. If a symbol has no subscript, there is one atom of that element.

• Carbon dioxide, $$\ce{CO2}$$: one carbon atom and two oxygen atoms.
• Water, $$\ce{H2O}$$: two hydrogen atoms and one oxygen atom.
• Oxygen, $$\ce{O2}$$: two oxygen atoms.
• Glucose, $$\ce{C6H12O6}$$: six carbon atoms, twelve hydrogen atoms and six oxygen atoms.

Endothermic reactions

An endothermic reaction takes in energy from its surroundings overall. This energy is transferred to the reacting substances.

Photosynthesis is an endothermic reaction. Light transfers energy from the environment to chloroplasts, where it is used to synthesise carbohydrates. Energy is stored as chemical energy in the carbohydrates produced.