Light-independent reaction

RuBP and GP graph

• tend to have more GP as produce 2 molecules of it for every 1 molecule of RuBP

• levels are constant-equal quantities of GP use ATP and NADPH from light dependent reaction to become RuBP

• RuBP combines with carbon dioxide to become GP

• in the dark RuBP still combining with carbon dioxide to generate GP. However without NADPH from light dependent reaction, GP cannot be reduced to triose phosphate

• and without ATP we do not have energy to convert triose phosphate back into RuBP

Where does it occur?

In the stroma of chloroplats

Reactants

• carbon dioxide

• ATP

• reduced NADP from light dependent reaction

Products

• glucose

• other organic molecules

Three main stages of the light independent reaction: (cyclical process so sometimes called the Calvin cycle)

1. fixation

2. reduction

3. regeneration

Carbon fixation

1. Carbon dioxide reacts with a 5-carbon (5C) compound ribulose biphosphate (RuBP) to form an unstable 6C compound

2. This 6C compound splits into two 3C, glycerate 3 phosphate (GP) molecules

3. This is catalysed by the enzyme ribulose biphosphate carboxylase (RUBISCO)

Reduction of GP

1. Gp is reduced into triose phosphate

2. this uses energy from the hydrolysis of ATP

3. this also requires protons and electrons from reduced NADP, which itself is oxidised to regenerate NADP

4. the NADP that is reformed returns to the light dependent reaction to be reduced again

Regeneration of RuBP

1. most triose phosphate is used to regenerate RuBP using ATP

2. the rest of the triose phosphate can be used to make other organic molecules

For each turn of the cycle, five carbons are used to regenerate RuBP (5C) and only one carbon, from carbon dioxide (1C), is available to make new organic compounds.

This means six full turns of the Calvin cycle are needed to make one molecule og glucose (6C)

Apart from being used to regenerate RuBP, triose phosphate can be used to make other useful molecules. For example triose phosphate can be used to make:

• simple sugars (e.g. glucose)

• larger carbohydrates (e.g. sucrose, starch and cellulose)

• amino acids

• lipids

• nucleotides