B7.2 - Photosynthesis

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What happens during photosynthesis?

The equation for photosynthesis shows only the reactants and the end products.

                                            light energy

       6CO2        +    6H2O    =========>    C6H12O6  +     6O2


carbon dioxide +    water     =========>     glucose    +    oxygen

  • the green chemical chlorophyll absorbs light
  • the energy is used to rearrange the atoms of carbon dioxide and water
  • the products are glucose (a sugar) and oxygen (a waste product)
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How do plants use the glucose?


  • convert some of the glucose into chemicals needed for growth of plant cells, such as cellulose, protein and chlorophyll
  • convert some into starch for storage
  • use some respiration to release energy

Because it is insoluble, starch has little effect on the osmotic balance of cells. A high concentration of glucose cause water to be drawn into cells. So, starch is a better storage molecule than glucose.

Energy released in respiration is used to synthesise polymers such as;

  • starch and cellulose from glucose
  • amino acids and then proteins from glucose and nitrates

Plants roots absorb nitrates by active transport, a process that requires energy.

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Why does the rate of photosynthesis vary?

The rate of photosynthesis may be limited by low;

  • temperatures
  • carbon dioxide concentration
  • light intensity

The amounts of carbon dioxide and oxygen exchanged over a 24-hour period vary. At compensation points, the rate pf photosynthesis and respiration balance, so there is no net movement of these gases between a plant and the surrounding atmosphere.

The rate of photosynthesis is limited by the factor that is in the shortest supply.

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