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Biology

Photosynthesis

Carbon fixation (The Calvin Cycle)

Carbon fixation stage in photosynthesis - CO2 is accepted by the 5C compound ribulose 1,5-biphosphate (RuBP) to form an unstable 6C compound. The 6C compound formed immediately splits into two molecules of a 3C compound called glycerate 3-phosphate (GP). Energy is used to convert GP into triose phosphate, a 3 carbon compound. It is at this point that the ATP and hydrogen produced in the light dependent stage are used. Triose phosphate doubles up to form glucose. The cycle is completed when the leftover GP molecules are met with a carbon acceptor and converted into RuBP, which is then joined with carbon dioxide to re-start the cycle.

Carbon fixation stage in photosynthesis

The carbon fixation stage occurs in the stroma and results in the production of glucose [glucose: a simple sugar made by the body from food, which is used by cells to make energy in respiration.

It is a result of an enzyme controlled sequence of reactions requiring ATP and hydrogen (NADPH2) from the light stage, and carbon dioxide. It involves the reduction of carbon dioxide, that is the addition of hydrogen, to form carbohydrate.

  • CO2 is accepted by the 5C compound ribulose 1,5-biphosphate (RuBP) to form an unstable 6C compound.
  • The 6C compound formed immediately splits into two molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds of a 3C compound called glycerate 3-phosphate (GP).
  • Energy is used to convert GP into triose phosphate, a 3 carbon compound. It is at this point that the ATP and hydrogen produced in the light dependent stage are used.
  • Triose phosphate doubles up to form glucose. Glucose may then be used in either respiration to provide energy, stored as starch [starch: a type of carbohydrate. Plants can turn the glucose produced in photosynthesis into starch for storage, and turn it back into glucose when it is needed for respiration., or used to synthesise cellulose [cellulose: A carbohydrate. It forms the cell wall in plant cells. for cell walls.
  • The cycle is completed when the leftover GP molecules are met with a carbon acceptor and converted into RuBP, which is then joined with carbon dioxide to re-start the cycle.

All the major biological molecules in plants are derived from the photosynthetic process:

  • Proteins
  • Fats
  • Carbohydrates
  • Nucleic acids

Audio

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Plants (mp3, 3.6 MB)

Learn how plants harness the sun's energy using photosynthesis, and the factors which affect this process.

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