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Chapter 5 Photosynthesis
 
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1.

Photosynthesis is a chemical process in which plants make their own food. During photosynthesis, plants take in carbon dioxide from the air and water from the soil. Sunlight shines on the plant and is absorbed by chlorophyll in the leaves. The carbon dioxide and water react to form glucose and oxygen.
Glucose can be used as a food by plants. Some of the glucose from photosynthesis is converted to starch as an energy store.
2.

Conditions needed for photosynthesis

Four conditions are essential for photosynthesis:

  • light
  • chlorophyll
  • carbon dioxide
  • water

If one of these is missing, photosynthesis cannot occur.

Leaves are flat and thin with a large surface area. This means that leaves can absorb lots of light from the Sun. Because leaves are thin, the carbon dioxide can diffuse through the leaf and easily reach all the cells.
3.

Testing for starch in plant leaves

(i) Allow a leaf to photosynthesise with black tape covering part of it.
(ii) Dip the leaf in boiling water for 30 seconds to soften it.
(iii) Put the leaf in hot ethanol for 10 minutes to remove the green chlorophyll.
(iv) Wash the leaf in hot water and add brown iodine solution.
(v) The parts of the leaf which have been photosynthesising will contain starch and turn dark blue.
4.

Factors affecting photosynthesis
There are five main factors which limit the rate of photosynthesis.

  • The frequency (colour) of light - chlorophyll absorbs red, orange, blue, indigo and violet light most effectively and therefore photosynthesis occurs faster in light with these colours.
  • The brightness (intensity) of light - the brighter the light, the faster the rate of photosynthesis.
  • The supply of water - in a plentiful supply of water, the rate of photosynthesis increases.
    In drought conditions, the rate of photosynthesis is very slow.
  • The concentration of carbon dioxide -. the rate of photosynthesis increases if the concentration of carbon dioxide increases. This explains why commercial growers boost carbon dioxide levels in their greenhouses.
  • The temperature - as the temperature rises from 0°C, the rate of photosynthesis increases. Above 35 °C, photosynthesis decreases as enzymes necessary for the process are denatured.
5.

How do plants use glucose?
The diagram below shows the four main uses of glucose from photosynthesis.
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