Edexcel IGCSE Biology 复习笔记 2.5.1 Photosynthesis

Edexcel IGCSE Biology 复习笔记 2.5.1 Photosynthesis

Photosynthesis Theory


  • Photosynthesis is an endothermic reaction in which energy from sunlight is transferred to the chloroplasts in green plants
    • Green plants use this energy to make the carbohydrate glucose from the raw materials carbon dioxide and water
    • At the same time, oxygen is made and released as a waste product


  • Photosynthesis can be defined as the process by which plants manufacture carbohydrates from raw materials using energy from light
  • Plants are
    • Autotrophs – they can make complex molecules (glucose) from simple molecules (carbon dioxide and water)
    • Producers - they can make their own food and so are the first organism at the start of all food chains




The products of photosynthesis

  • Plants use the glucose they make as a source of energy in respiration
  • They can also use it to
    • Produce starch for storage
    • Synthesise lipids for an energy source in seeds
    • To form cellulose to make cell walls
    • Produce amino acids (used to make proteins) when combined with nitrogen and other mineral ions absorbed by roots






The fate of glucose produced in photosynthesis



Exam Tip

If asked for the raw materials required for photosynthesis, the answer is carbon dioxide and water.Although required for the reaction to take place, light energy is not a substance and therefore cannot be a raw material.

Photosynthesis Equation


  • Photosynthesis can be summarised in a word equation as shown below:


Word equation for photosynthesis





Where do the reactants come from and where do the products go?




  • This equation can also be shown as a balanced chemical equation
    • Six carbon dioxide molecules combine with six water molecules to make one glucose molecule and six oxygen molecules





The balanced chemical equation for photosynthesis


Exam Tip

The photosynthesis equation is the exact reverse of the aerobic respiration equation so if you have learned one you also know the other one! You will usually get more marks for providing the balanced chemical equation than the word equation.