B2.3 Photosynthesis

B2.3 Photosynthesis



Green plants and algae use light energy to make their own food. They obtain the raw materials they need to make this food from the air and the soil. The conditions in which plants are grown can be changed to promote growth.


  • Photo = light
  • Synthesis = making of (glucose)
  • Photosynthesis = making glucose using light

During photosynthesis:

  • light energy is absorbed by a green substance called chlorophyll, which is found in chloroplasts in some plant cells and algae.
  • This energy is used by converting carbon dioxide (from the air) and water (from the soil) into sugar (glucose).
  • Oxygen is released as a by-product.


Where does photosynthesis happen?

  • Leaves are the main site of photosynthesis.
  • Photosynthesis mainly in occurs in the mesophyll cells.
  • These cells contain lots of chloroplasts.
  • Chloroplasts contain chlorophyll.


A palisade mesophyll cell:


Factors that limit the rate of photosynthesis

  • Temperature

A low temperature will limit the rate as the molecules will move less and therefore the reaction happens slower

  • Carbon dioxide

A shortage of CO2 will limit the rate as fewer molecules will be available for the reaction.

  • Light intensity

A shortage of light means there is less energy to power the reaction.

Limiting factors explained:

  • Light, temperature and the availability of carbon dioxide interact and in practice any one of them may be the factor that limits photosynthesis.
  • If one of these factors is closest to its minimum value it will limit the rate.
  • Increasing this factor will increase the rate.
  • The rate will continue to increase until another factor becomes limiting.
  • Any further increase in the original factor will now not increase the rate.
  • With no limiting factors, increasing a factor above a certain level will not increase the rate. All chlorophyll molecules are being used.


Farming practices

  • Farmers artificially manipulate the environment in which they grow plants.
  • They grow plants in greenhouses or in polythene tunnels.
  • They can control the temperature in greenhouses using heaters and ventilation.
  • They can artificially increase the carbon dioxide levels.
  • They can control the light using fluorescent lamps.
  • By doing all of this, their plants grow faster and certain plants can be grown in this country out of their natural growth season. Eg tomatoes can be grown all year round.
  • Therefore, they increase their profits.


How do plants and algae use glucose?

  • The glucose produced in photosynthesis may be converted into insoluble starch for storage
  • Plant cells use some of the glucose produced during photosynthesis for respiration.
  • Some glucose in plants and algae is used:
  • to produce fat or oil for storage
  • to produce cellulose, which strengthens the cell wall
  • to produce proteins:
  • To produce proteins, plants also use nitrate ions that are absorbed from the soil.