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The light independent reactions
The Calvin Cycles take place in the Stroma and uses the reduced NADP and ATP from the light dependent reactions.
Carbon dioxide is reduced to a carbohydrate.
1. Carbon dioxide combines with a 5-carbon compound called RIBULOSE BISPHOSPHATE (RuBP). This reaction is catalysed by the enzyme ribulose
bisphosphate carboxylase (RuBISCO).
2. The 6 carbon compound formed is unstable and immediately breaks down into two 3-carbon molecules, glycerate 3-phosphate (GP).
3. This 3-carbon compound is reduced to form a 3-carbon sugar phosphate called glyceraldehyde 3-phosphate (GALP). The hydrogen for the
reduction comes from the reduced NADP from the light dependent reactions. ATP from the light dependent reactions also provides the energy for
4. 2 out of every 12 GALPs formed are involved in the creation of a 6 carbon sugar (hexose) which can be converted to other organic
compounds, e.g. amino acids or lipids.
5. 10 out of every 12 GALPs are involved in the recreation of RuBP. The ten GALP molecules rearrange to form six 5-carbon compounds; then
phosphorylation using ATP forms RuBP.
The Calvin Cycle needs to turn 6 times to make one glucose molecule.
Six turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependent reactions.
New biological substances are synthesised:
The Calvin cycle is the starting point for making all the organic substances a plant needs.
CARBOHYDRATES: simple sugars e.g. glucose are made by joining 2 triose phosphate molecules together and polysaccharides e.g.
starch and cellulose is made by joining hexose sugars together in different ways.
LIPIDS: these are made using glycerol, which is synthesised from triose phosphate, and fatty acids, which are synthesised from
AMINO ACIDS: some are made from glycerate 3-phosphate