6.1) Photosynthesis

6.1) Photosynthesis

Photosynthesis: is the process by which plants manufacture carbohydrates from raw materials using energy from light.

  • light energy is absorbed by chlorophyll – a green substance found in chloroplasts in green 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

The necessity for chlorophyll, light and carbon dioxide for photosynthesis:

  • Chlorophyll is required because it help absorbs the “light” required.
  • CO2 is important because it is converted into the sugars such as glucose we need.
  • Light is important because it acts as the “fuel” or energy to drive the reaction

Stuff that should be kept constant

  • Temperature
  • Oxygen Levels


Factors affecting the rate of photosynthesis:

  • Light intensity
  • Carbon dioxide concentration
  • Temperature


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 rate of photosynthesis.

Carbon dioxide concentration:

Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.


If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.

The process of photosynthesis:

  • In land plants water is absorbed from the soil by the roots and carried in the water vessels of the veins.
  • Carbon dioxide is absorbed from the air through the stomata (pores in the leaf).
  • In the leaf cells, the CO2 and H2O are combined to make sugar.
  • The energy for this reaction comes from sunlight that has been absorbed by the green pigments chlorophyll.
  • Chlorophyll is able to absorb energy from light and use it to split water molecules into hydrogen and oxygen.
  • The oxygen escapes from the leaf and the hydrogen molecules are added to carbon dioxide molecules to form sugar.
  • In this way the light energy has been transferred into the chemical energy of carbohydrates as they are synthesised.


The plant’s use of photosynthetic products:

  • Glucose > starch > sucrose
  • Transported out of the cell into the food-carrying cells of the leaf veins.
  • These veins will distribute the sucrose to all parts of the plant that do not photosynthesis, eg. the growing buds, the ripening fruits, the roots and the underground storage organs.
  • The cells in these regions will use the sucrose in a variety of ways.
  • The sugar can be used to provide energy.
  • It is oxidised by respiration to CO2 and H2O, and the energy released is used to drive other chemical reactions such as the building up of proteins.
  • Sugar that is not needed for respiration is turned into starch and stored.
  • Different substances are built up (synthesised) from the sugar molecules and other molecules produced in photosynthesis, eg. cellulose for its cell wall, lipids for its cell membrane, proteins for its cytoplasm and pigments for its flower petals, etc.


Limiting factors: is something present in the environment in such short supply that it restricts life processes.


Limiting factors of photosynthesis:

  • Temperature
  • Light intensity
  • Carbon dioxide concentration


Although carbon dioxide concentration limits photosynthesis only directly, artificially high levels of carbon dioxide in greenhouses do effectively increase yields of crops.


Greenhouses also maintain a higher temperature and so reduce the effect of low temperature as a limiting factor, and they clearly optimise the light reaching the plants.


Parts of the world such as tropical countries often benefit from optimum temperatures and rainfall for crop production.


The stomata in a leaf may affect the rate of photosynthesis according to whether they are open or closed.


Compensation point: no intake or output of carbon dioxide or oxygen.


Effect of gas exchange of an aquatic plant kept in the light and in the dark:

Hydrogen carbonate indicator is used to show carbon dioxide concentration in solution. It is:

  • yellow in high concentrations of carbon dioxide
  • red in equilibrium with carbon dioxide in the atmosphere
  • purple in low concentrations of carbon dioxide

Place a leaf from a plant in a stoppered boiling tube containing some hydrogen carbonate indicator. You can then investigate the effect of light over a period of a few hours.

Plant cells respire in the light and the dark, releasing carbon dioxide. In the light, photosynthesis can also happen, and carbon dioxide is absorbed from the air. If the light is bright enough, the rate of absorption becomes greater than the rate of release.

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