#103 The light-independent reactions (Calvin cycle)

The light-independent reactions take place in the stroma of the chloroplast, where the enzyme ribulose bisphosphate carboxylase, usually known as rubisco, is found.

1. Carbon fixation

CO2 diffuses into the stroma from the air spaces within the leaf. It enters the active site of rubisco, which combines it with a 5-carbon compound called ribulose bisphosphate, RuBP. The reaction is called carbon fixation.

The products of this reaction are two 3-carbon molecules, glycerate 3-phosphate, GP.

2. Reduction 

Energy from ATP and hydrogen from reduced NADP are then used to convert the GP into triose phosphate, TP. Triose phosphate is the first carbohydrate produced in photosynthesis.

3. RuBP regeneration

Most of the triose phosphate is used to produce ribulose bisphosphate (RuBP), so that more carbon dioxide can be fixed.

The rest is used to make glucose or whatever other organic substances the plant cell requires. These include:

• polysaccharides such as starch for energy storage and cellulose for making cell walls,
• sucrose for transport, 
• amino acids for making proteins,
• lipids for energy storage 
• nucleotides for making DNA and RNA.

Syllabus:  13.1  Photosynthesis as an energy transfer process Light energy absorbed by chloroplast pigments in the light dependent stage of photosynthesis is used to drive reactions of the light independent stage that produce complex organic compounds. Chromatography is used to identify chloroplast pigments and was  also used to identify the intermediates in the Calvin cycle. a)   explain that  energy transferred as ATP and reduced NADP from the light dependent stage is used during the light independent stage (Calvin cycle) of photosynthesis to produce complex organic molecules b)   state the sites of the light dependent and the light independent stages in the chloroplast c)   describe the role of chloroplast pigments (chlorophyll a, chlorophyll b, carotene and xanthophyll) in light absorption in the grana d)   interpret absorption and action spectra of chloroplast pigments  e)   use  chromatography to separate and identify chloroplast pigments and carry out an investigation to compare the chloroplast pigments in different plants  (reference should  be made to Rf  values  in identification) f) describe the light dependent stage as the photoactivation of chlorophyll resulting in the photolysis of water and the transfer of energy to ATP and reduced NADP (cyclic and non-cyclic photophosphorylation should  be described in outline  only) g)   outline  the three main stages of the Calvin cycle: • fixation of carbon  dioxide by combination with ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of GP (PGA), a 3C compound •   the reduction of GP to triose  phosphate (TP) involving ATP and reduced NADP •   the regeneration of ribulose  bisphosphate (RuBP) using ATP h)   describe, in outline,  the conversion of Calvin cycle intermediates to carbohydrates, lipids and amino  acids  and their uses in the plant cell