Light Independent Reactions (DP IB Biology)

Carbon Fixation

Location of the light-independent reactions

• The light-independent reactions of photosynthesis take place in the stroma of the chloroplasts

• The stroma is within the double membrane and is a thick protein rich environment containing the enzymes needed for the light-independent reactions

Light-independent reactions: Carbon fixation

• The light-independent reactions of photosynthesis are also known as the Calvin cycle

• There are three main steps within the Calvin cycle:

  1. Carbon fixation: the enzyme Rubisco catalyses the fixation of carbon dioxide by combination with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound

  2. Reduction: GP is reduced to triose phosphate (TP) in a reaction involving reduced NADP and ATP

  3. Regeneration: RuBP is regenerated from TP in reactions that use ATP

• Carbon dioxide is converted into carbohydrates, namely glucose, during the cycle in a series of anabolic reactions

  • Anabolic reactions require energy in order to build large complex molecules from smaller simpler ones

• The Calvin cycle relies on the products of the light-dependent reactions namely ATP and reduced NADP

• During the cycle endergonic reactions take place that involve the hydrolysis of ATP and oxidation of reduced NADP

  • An endergonic reaction requires energy to be absorbed before the reaction can proceed

Carbon fixation details

• Carbon dioxide is the source of carbon for all organisms that carry out photosynthesis

• Carbon fixation involves carbon dioxide (1C) being removed from the external environment and becoming part of the plant, and is then said to be “fixed”

  • It is transformed into a three-carbon compound (3C) called glycerate-3-phosphate (sometimes shortened to as GP)

• During the fixation step of the Calvin cycle carbon dioxide is combined with a five-carbon compound (5C) called ribulose bisphosphate (RuBP) to make an unstable six-carbon (6C) compound that splits into two molecules of glycerate-3-phosphate

• This reaction is catalysed by the enzyme Rubisco

  • This is the most abundant enzyme on Earth

  • It works relatively slowly, therefore high concentrations of it is needed in the stroma

  • It is not effective in low carbon dioxide concentrations

• Glycerate-3-phosphate is then used in the next step of the cycle

Synthesis of Triose Phosphate

• Energy from ATP and hydrogen from reduced NADP (from the light-dependent reactions) are used to reduce glycerate-3-phosphate to a phosphorylated three-carbon molecule called triose phosphate (sometimes shortened to TP)

• After the reduction step one sixth of the triose phosphate is converted into usable products for the plant:

  • Hexose phosphates which can be used to produce carbohydrates such as starch, sucrose or cellulose

  • Glycerol and fatty acids which join to form cell membranes

  • Production of amino acids for protein synthesis

• It is important that not all the triose phosphate is converted to alternative compounds for the plant, or the supplies of ribulose bisphosphate would run out

• The remaining triose phosphate is used to regenerate RuBP

Regeneration of RuBP

• One sixth of the triose phosphate that is generated will be used in the synthesis of organic compounds, such as glucose

• The remaining five sixths of triose phosphate are used to regenerate the five-carbon compound ribulose bisphosphate (RuBP)

  • Five molecules of triose phosphate are converted to three molecules of RuBP

• This process requires ATP (from the light-dependent reaction)

• Once RuBP been has regenerated it can go on to fix further carbon dioxide and the cycle can begin again

Light-independent stage of photosynthesis diagram

The Calvin cycle of the light-independent reactions showing the regeneration of RuBP

Synthesis of Carbon Compounds

• Carbon containing compounds, to include carbohydrates, proteins and lipids, all rely on the products of the Calvin cycle in combination with other mineral nutrients

• Whilst glucose is the key respiratory substrate which sustains the metabolism of green plants, other required substances can be produced from the further metabolism of TP, for example:

  • Triose phosphate can be sent through the glycolysis and link reaction pathways to produce acetyl coenzyme A which can then be conjoined to make fatty acids

  • Triose phosphate can be used to make glycerol

  • Glycerol can then be joined to fatty acids to make triglycerides

  • All 20 amino acids can be produced in plants using nitrate or ammonium ions