The light-independent stage #
- The light-independent stage of photosynthesis is also known as the Calvin Cycle.
- It occurs in the stroma of the chloroplast.
- The enzyme called rubisco plays a crucial role in the Calvin Cycle.
- Carbon dioxide enters the stroma and combines with a 5C compound called RuBP, facilitated by rubisco.
- This reaction is called carbon fixation and produces two 3C molecules called glycerate 3-phosphate (GP).
- ATP and hydrogen from reduced NADP are used to convert GP into triose phosphate (TP).
- Triose phosphate is the first carbohydrate produced in photosynthesis.
- Most of the triose phosphate is used to regenerate RuBP, allowing more carbon dioxide to be fixed.
- This process requires the use of ATP as an energy source.
- The remaining triose phosphate is used to produce glucose, lipids, amino acids, and other organic substances needed by the plant cell.
- Some glycerate 3-phosphate is directly used to produce specific amino acids.
- The Calvin Cycle is light-independent and takes place in the stroma, which provides a large surface area for light absorption through the grana.
Reactions of the Calvin Cycle #
- Carbon Fixation: The cycle begins with the enzyme called rubisco combining carbon dioxide (CO2) with a 5-carbon compound called ribulose bisphosphate (RuBP). This reaction is known as carbon fixation, resulting in the formation of an unstable 6-carbon compound.
- Formation of Glyceraldehyde-3-Phosphate (GP): The unstable 6-carbon compound formed in step 1 quickly breaks down into two 3-carbon molecules called glycerate 3-phosphate (GP), also known as phosphoglycerate (PGA). Each molecule of GP contains three carbons.
- Energy and Hydrogen Utilization: Energy from ATP and hydrogen from reduced NADP are utilized to convert GP into a carbohydrate known as triose phosphate (TP) or glyceraldehyde-3-phosphate (GALP). This step requires ATP as an energy source and NADPH as a reducing agent.
- Carbohydrate Production: Triose phosphate (TP) is the first carbohydrate produced in photosynthesis. Most of the TP is used to regenerate the original 5-carbon compound, ribulose bisphosphate (RuBP). This regeneration allows more carbon dioxide to be fixed and the cycle to continue.
- Regeneration of RuBP: The cycle requires the regeneration of ribulose bisphosphate (RuBP) for more carbon dioxide to be fixed. To achieve this, some of the TP molecules are used to recombine with ATP, utilizing energy in the form of phosphate groups, to form RuBP again.
- Utilization of TP: The remaining triose phosphate (TP) that is not used in the regeneration of RuBP is used to produce glucose, lipids, amino acids, and other organic substances needed by the plant cell. These include polysaccharides like starch for energy storage, cellulose for making cell walls, sucrose for transportation, amino acids for protein synthesis, lipids for energy storage, and nucleotides for DNA and RNA synthesis.
- Direct Amino Acid Production: Some glycerate 3-phosphate (GP) molecules are used directly in the production of specific amino acids.
- Light Independence: Unlike the light-dependent stage of photosynthesis, the Calvin Cycle is light-independent. It can occur even when light is not present.
- Increased Light Absorption: The stroma, where the Calvin Cycle takes place, provides a large surface area for light absorption through the grana, enhancing the efficiency of light energy utilization.
Practice questions #
Outline/ describe the main features/reactions of the Calvin Cycle.
- RuBP 5C;
- combines with carbon dioxide;
- rubisco;
- to form an unstable 6C compound;
- which forms 2 X GP (PGA);
- ATP;
- energy source;
- and reduced NADP;
- forms TP (GALP);
- TP used to form glucose / carbohydrates 1 lipids / amino acids;
- TP used in regeneration of RuBP;
- requires ATP;
- as source of phosphate;
- light independent;
- Grana increase surface area for light absorption
Photosynthesis questions and answers
