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Photosynthesis

Photosynthesis

  • This is the process for plants that produces their ‘food’.
    • Also needs sunlight and chlorophyll.
  • Plants need to respire as well as photosynthesise. Photosynthesis is their way of getting glucose (which is stored as starch) for respiration.
  • Chloroplasts in plant cells are where the reactions for photosynthesis takes place.
  • Light is absorbed by a green substance inside chlorophyll. Chlorophyll transfers the light energy into the stored chemical energy glucose and without it, photosynthesis cannot occur.

Leaf adaptions

  • The leaves of plants have been adapted for their function in many ways. These are:
    • Leaves, the main organ of a plant, are broad so there’s a large surface area exposed to light.
    • Leaves contain lots of chlorophyll in chloroplasts to absorb light.
    • Leaves are full of holes called stomata on their underside. They open and close to allow gases such as oxygen and carbon dioxide to diffuse in and out. They also allow water vapour to escape, which is transpiration.
    • It is thin to allow a shot space for Carbon dioxide to diffuse into.
    • The epidermis is thin and transparent to allow more light to reach the palisade cells.
    • As the palisade layer is at the top of the leaf it gets more sunlight than the layers below and to take full advantage of this, it has more chloroplasts in each cell to ensure all light available is absorbed.
    • The cuticle protects the leaf with its wax but it doesn’t block out sunlight.

Limiting factors

  • Many factors affect the rate of photosynthesis: the concentration of carbon dioxide, light intensity, temperature and water availability.
  • The rate of reaction varies, dependent upon the conditions. These conditions are known as limiting factors.
  • Limiting factors limit the rate at which a reaction can take place.
  • Limiting factor: These stop a reaction from occurring any faster.
  • If there is a variable in low availability this will slow or reduce the rate of reaction. For example, if the amount of carbon dioxide is low, there is a limited amount of carbon dioxide to react with water. Therefore, the rate of photosynthesis is determined by how much carbon dioxide is available.
  • If this concentration is increase, eventually carbon dioxide will no longer be a limiting factor and something else may limit the rate of reaction e.g. the light intensity.
  • Which factor is limiting at a particular time depends on the environmental conditions:
    • at night it’s pretty obvious that light is the limiting factor.
    • in winter it’s often the temperature.
    • if it’s warm enough and bright enough, the concentration of carbon dioxide is usually limiting.
  • As enzymes catalyse most of the reactions in living organisms, if their internal temperature becomes too high the biological reactions stop as enzymes become denatured.
  • There is an optimum temperature for all biological reactions which is the maximum temperature at which a reaction involving enzymes can take place before they begin to denature.
  • Once they begin to denature the rate of reaction slows.