The Process of Photosynthesis (SQA National 5 Biology): Revision Note

The light reactions

• Photosynthesis is the process used by plants to produce their own biological molecules; it can be summarised as:

• The reactions of photosynthesis can be divided into two stages:

  1. the light reactions: light energy from the sun is captured

  2. carbon fixation: carbon dioxide from the atmosphere is used to produce sugar

• During the light reactions, light energy from the sun is trapped by chlorophyll, a green pigment found inside chloroplasts

• The light energy:

  • is converted into chemical energy in the form of ATP

  • splits water, releasing hydrogen and oxygen

• Products of the light reactions are therefore:

  • ATP: passed to the next stage of photosynthesis

  • hydrogen: also passed to the next stage of photosynthesis

  • oxygen: diffuses out of the cell as a waste product

Carbon fixation

• The second stage of photosynthesis involves a series of enzyme-controlled reactions

  • Enzyme activity is affected by temperature, so the carbon fixation reactions are temperature-dependent

• These reactions require:

  • hydrogen and ATP from the light reactions

  • carbon dioxide from the atmosphere

• Hydrogen, ATP and carbon dioxide are used to produce sugar