Earth and Life Science







Photosynthesis

Green plants absorb light energy using chlorophyll in their leaves. They use it to react carbon dioxide with water to make a sugar called glucose. The glucose is used in respiration, or converted into starch and stored. Oxygen is produced as a by-product.
This process is called photosynthesis. Temperature, carbon dioxide concentration and light intensity are factors that can limit the rate of photosynthesis.

Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first step towards making food - not just for plants but ultimately every animal on the planet.
During photosynthesis:
  • Light energy is absorbed by chlorophyll, a green substance found in chloroplasts in some plant cells and algae 
  • Absorbed light energy is used to convert carbon dioxide (from the air) and water (from the soil) into a sugar called glucose
  • Oxygen is released as a by-product
This equation summarises what happens in photosynthesis:
Some glucose is used for respiration, while some is converted into insoluble starch for storage. The stored starch can later be turned back into glucose and used in respiration.
Three factors can limit the speed of photosynthesis: light intensity, carbon dioxide concentration and temperature.
  • rate of photosynthesis plotted against light intensity. the rate begins to slow as the light intensity continues to increase
    Light intensity
    Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.
  • rate of photosynthesis plotted against carbon dioxide concentration. the rate begins to slow as the carbon dioxide concentration continues to increase
    Carbon dioxide concentration
    Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.
  • rate of photosynthesis plotted against temperature. the rate begins to slow as the temperature continues to increase
    Temperature
    If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.
If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.
In practice, any one of these factors could limit the rate of photosynthesis.










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