The Process of Photosynthesis (Edexcel IGCSE Biology (Modular))

Photosynthesis Theory

• Photosynthesis is an endothermic reaction in which energy from sunlight is transferred to the chloroplasts in green plants

  • Energy from sunlight is absorbed by chlorophyll, a green pigment found inside chloroplasts

  • Green plants use this energy to make the carbohydrate glucose from the raw materials carbon dioxide and water

  • At the same time, oxygen is made and released as a waste product

• Photosynthesis can be defined as the process by which plants manufacture carbohydrates from raw materials using energy from light

• Plants are

  • Autotrophs – they can make complex molecules (glucose) from simple molecules (carbon dioxide and water)

  • Producers – they can make their own food and so are the first organism at the start of all food chains

The products of photosynthesis

• Plants use the glucose they make as a source of energy in respiration

• They can also use it to

  • Produce starch for storage

  • Synthesise lipids for an energy source in seeds

  • To form cellulose to make cell walls

  • Produce amino acids (used to make proteins) when combined with nitrogen and other mineral ions absorbed by roots

Once the carbon from carbon dioxide is taken into the plant and converted to glucose in photosynthesis, it can then be converted and used in many different types of biological molecule.

Photosynthesis Equations

• Photosynthesis can be summarised in a word equation as shown below:

Word equation for photosynthesis

Sources of the reactants and uses of the products of photosynthesis

• This equation can also be shown as a balanced chemical symbol equation

  • Six carbon dioxide molecules combine with six water molecules to make one glucose molecule and six oxygen molecules

The balanced symbol equation for photosynthesis