Photosynthesis

• Photosynthesis


  • process by which plants synthesise carbohydrates from water and carbon dioxide
  • in photosynthesis, chlorophyll in chloroplasts absorb energy from light and transfers it to chemical stores of energy in carbohydrates
  • photosynthesis releases oxygen
  • carbon dioxide + water -> glucose + oxygen
  • 6CO2 + 6H2O -> C6H12O6 + 6O2
• knowledge for experiments on photosynthesis - glucose


  • glucose is formed from carbon dioxide and water during photosynthesis, glucose is the simplest stable form of carbohydrates
• Knowledge for experiments on photosynthesis - starch


  • presence of starch in leaves suggests* that photosynthesis has taken place
  • When glucose is formed more quickly than it is used up, excess glucose is converted to starch for storage
  • presence of starch is tested using iodine solution
  • starch can be formed in roots or underground storage of organs in plants so presence of starch not = photosynthesis
• knowledge for experiments on photosynthesis

  • destarching must be carried out on plants before experiment so that starch present after the experiment must have been formed during experiment
  • put plants in the dark for two days. In darkness, photosynthesis stops and enzymes in the leaves covert starch to sucrose, which is transported to other parts of the plant
• Conditions needed for photosynthesis

  • Light

  • Carbon dioxide

  • Chlorophyll

  • Suitable temperature (affects enzymes needed for photosynthesis)

  • Water
• Glucose during photosynthesis

  • glucose is used immediately for cellular respiration to provide energy for cellular activities and to form cellulose cell walls
  • in daylight, rate of photosynthesis is very high so glucose is formed faster than it is used up, excess glucose is converted to starch
  • in darkness, photosynthesis stops and starch is converted by enzymes back to glucose
  • glucose is converted to sucrose, amino acids and fats
• Glucose to sucrose in plants

  • glucose is converted to sucrose which is transported to other parts of the plant or to storage organs via phloem
  • sucrose is converted to other forms of storage compounds at the storage organs and may be converted back to glucose for respiration
  • sucrose is a component of nectar in flowers which attracts insects for pollination
• Glucose to amino acids in plants

  • glucose is converted to amino acids in leaves
  • amino acids are used to form proteins in leaves which are used for synthesis of new protoplasm in leaves
  • excess amino acids are transported to other parts of the plant for synthesis of new protoplasm and for storage as proteins
• glucose to fats in plants


  • fats are used for storage, cellular respiration and synthesis of new protoplasm
• Why Is Photosynthesis Important?

  • It converts energy from the sun into chemical energy, which is needed by other organisms

  • It removes carbon dioxide from the atmosphere.

  • It produces oxygen needed for respiration by other organisms.

  • It contributes to the energy stored in fossil fuels