5.6.4 - The light-independent stage

• The light - independent stage of photosynthesis takes place in the stroma of chloroplasts. It does not directly use light energy but uses the products of the light - dependent stage. If the plant is not illuminate, the light-dependent stage soon ceases, as ATP and hydrogen are not available to reduce the carbon dioxide and synthesise large, complex organic molecules.
• Carbon dioxide is the source for the production of all organic molecules found in all the carbon-based life forms on Earth.

  These organic molecules may be used as structures such as cell membranes or act as energy stores such as starch.
• Carbon dioxide in air enters the leaf through the stomata and diffuses through the spongy mesophyll layer to the palisade layer, into the palisade cells, through their thin cellulose cell walls, and then through the chloroplast envelope into the stroma.
• The fixation of carbon dioxide in the stroma maintains a concentration gradient that aids diffusion:

  • Carbon dioxide that is a by-product of respiration in plant cells may also be used for this stage of photosynthesis.
• The Calvin cycle is a series of reactions whereby carbon dioxide is converted to organic molecules. It is a metabolic pathway of the light-independent stage of photosynthesis, occurring (in eukaryotic cells) in the stroma of chloroplasts, where carbon dioxide is fixed, with the products of the light-dependent stage, to make organic compounds. It also occurs in many photoautotrophic bacteria.
• The Calvin cycle is also known as carbon dioxide fixation because carbon from CO2 is 'fixed' into an organic molecule.
• The Calvin cycle:

  1. CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast. It then combines with the carbon dioxide acceptor, 5-carbon ribulosebiphosphate (RuBP). This is catalysed by ribulose diphosphate carboxylase oxygenase (RuBisCO)

  This creates an unstable 6-carbon compound, as RuBP has accepted the carboxyl group (COO-) and become carboxylated. This 6 carbon compound immediately breaks down into 2 molecules of glycerine-3-phosphate, GP, a 3-carbon compound.
• The Calvin cycle:

  2) GP is reduced by H+ ions which come from reduced NADP, turning it into a different 3-carbon compound called triose phosphate (TP).

  Energy from ATP is made during the light-dependent stage - 2 molecules of ATP from every molecule of carbon dioxide fixed into GP.

  The reduced NADP is recycled to NADP for reuse in the light-dependent stage.
• The Calvin Cycle:

  3) 10 out of every 12 molecules of TP have their atoms rearranged to regenerate 6 molecules of ribulose biphosphate. This process requires phosphate groups.

  Chloroplasts contain low levels of RuBP as it is continously being converted to GP, but continually being regenerate. The remaining 2/12 molecules of TP are the product. Regenerating RuBP uses the rest of the ATP from the light-dependent stage.
• The products of the light-depedent stage, ATP and reduced NADP are continuously needed for the Calvin cycle to run.
• During the light-dependent stage, hydrogen ions are pumped from the stroma into the thylakoid spaces. This causes the concentration of free protons in the stroma to fall, and the pH to rise to around 8 - the optimum for enzyme RuBisCO, which is activated by extra ATP in the stroma.
• In daylight, the concentration of magnesium ions increases in the stroma. These ions attach to the active site of RuBisCO to act as cofactors. The Ferredoxin reduced by electrons from PSI activates enzymes involved in the reactions of the Calvin cycle.
• Some TP molecules are used to synthesise organic compounds. Pairs of TP molecules combine to form hexose sugars such as glucose, some of which may isomerise to fructose. These monosaccharides can combine to form disaccharides such as sucrose, and polysaccharides such as cellulose/starch.
• TP can be converted to glycerol and this may be combined with fatty acids to make lipids, amino acids and glycerol.
• The rest of the TP is recycled to regenerate the supply for RuBP. 5 molecules of the 3-carbon compound TP interact to form 3 molecules of the 5-carbon compound RuBP.