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3.3Unit 3 — Life on Earth

Photosynthesis — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 2 parts Lesson 3 of 6

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Section 1 of 2The two stages of photosynthesis

  • Carbon Dioxide + Water → (Light Energy & Chlorophyll) → Glucose + Oxygen
1Stage 1: Light Reactions (Photolysis)5 points
:
  • Light energy from the Sun is trapped by the green pigment chlorophyll in chloroplasts.
  • Light energy is converted into chemical energy: generates ATP from ADP and Pi.
  • Water is split into hydrogen and oxygen (photolysis):
    • Water → Hydrogen + Oxygen
  • Oxygen diffuses out of leaf cells through stomata as a waste by-product.
  • Hydrogen and ATP are passed on to Stage 2.
2Stage 2: Carbon Fixation3 points
:
  • A cycle of enzyme-controlled biochemical reactions (does not require light directly).
  • Hydrogen and ATP (from Stage 1) are combined with Carbon Dioxide from the air to produce Glucose.
  • Because it is enzyme-controlled, it is highly sensitive to temperature changes.
3Fates of synthesized glucose3 points
:
  1. Used immediately as a substrate in cellular respiration to release ATP.
  2. Stored as the insoluble carbohydrate starch for later energy needs.
  3. Converted into the structural carbohydrate cellulose to build plant cell walls.

Section 2 of 2Limiting factors of photosynthesis

  • A limiting factor is an environmental condition in shortest supply that restricts the rate of photosynthesis.
  • The three key limiting factors:
    • Light intensity: Provides energy for the light reactions; as light increases, rate increases until another factor becomes limiting.
    • Carbon dioxide concentration: Provides essential carbon atoms for carbon fixation; rate levels off when light or temperature limits.
    • Temperature: Affects the rate of enzyme action in carbon fixation; rate increases with temperature up to the optimum (~25–30 °C), then drops sharply due to enzyme denaturation.

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