Photosynthesis

Overview of Photosynthesis

  • Photosynthesis is the process by which green plants, algae and some bacteria convert light energy, typically from the sun, into chemical energy in the form of glucose.
  • It takes place in two stages: the light-dependent reaction and the light-independent reaction, also known as the Calvin cycle.
  • This process requires light, water and carbon dioxide, and produces glucose and oxygen.

Light-Dependent Reaction

  • The light-dependent reaction occurs in the thylakoid membrane of the chloroplast.
  • Light energy is captured by chlorophyll and used to ‘split’ water molecules in a process known as photolysis.
  • Photolysis of water produces electrons, protons (H+ ions) and oxygen. The oxygen is released as a by-product and the electrons are used in the light-independent reaction.
  • The captured light energy is used to create ATP from ADP and Pi, and to reduce NADP to NADPH. These energy-rich molecules are then used in the light-independent reaction.

Calvin Cycle (Light-Independent Reaction)

  • The Calvin Cycle occurs in the stroma of the chloroplast and does not directly require light, hence being light-independent.
  • It uses carbon dioxide along with the ATP and NADPH produced in the light-dependent reaction to produce glucose. -Carbon fixation involves the combination of carbon dioxide with a five-carbon compound called ribulose bisphosphate (RuBP) to form an unstable six-carbon compound. This immediately splits to form two molecules of glycerate-3-phosphate (GP).
  • The GP is then reduced to a useful three-carbon compound called glyceraldehyde 3-phosphate (G3P), using the energy from ATP and NADPH.
  • Some of this G3P is used to regenerate RuBP, and some is used to synthesise glucose and other organic molecules.

Factors Affecting Photosynthesis

  • The rate of photosynthesis can be affected by light intensity, carbon dioxide concentration, temperature and water availability.
  • Light intensity: As light intensity increases, so does the rate of photosynthesis until a threshold is met, beyond which further increases in light intensity will have no additional effect – this is called the light saturation point.
  • Carbon dioxide concentration: Similar to light intensity, increasing carbon dioxide levels will increase the rate of photosynthesis up to a saturation point.
  • Temperature: Photosynthesis is a series of enzyme-catalysed reactions. Therefore, the rate increases as temperature increases, up to an optimal temperature. Beyond this optimum, the rate decreases as the enzymes start to denature.
  • Water availability: Without sufficient water, photosynthesis cannot occur due to limitations in the photolysis process.