Light-Independent Reactions of Photosynthesis

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Returning, for reference’s sake, to the GCSE-level simplified equation of photosynthesis, we can see that the “end-product” of photosynthesis is a six carbon (C6) sugar: glucose. This is not strictly true, as in fact the triose phosphate (G3P) is first incorporated into the C12 sugar sucrose, a dimer of glucose and fructose. This is later metabolised to release a form of glucose, which is a substrate for many biochemical reactions. For the sake of consistency with most textbooks and for ease of stoichiometry (making the numbers of atoms add up), we shall skip this and continue to state glucose as the end product.

So far (in the light-dependent reactions) however, we have only produced ATP and NADPH. The subsequent reactions leading to the formation of glucose do not directly involve the harnessing of light energy (that has already been done to produce ATP and NADPH), and are therefore called light-independent. For most plants, the light-independent reactions can nevertheless only take place in the light, as they depend on the products of the light-dependent reactions as their substrates (the two processes can be spatially and/or temporally separated such as in the CAM or C4 carbon concentrating mechanisms of some higher plants).

So what are the light-independent reactions? They are in fact a series of reactions that make up a biochemical cycle called the Calvin-Benson-Bassham (CBB) cycle (also sometimes called the reductive pentose phosphate cycle):

Diagrammatic representation of the CBB cycle. CO₂ is fixed by RuBisCO via incorporation with ribulose bis-phosphate (RuBP, 5C), then phosphorylated and reduced to produce glyceraldehyde 3 phosphate (G3P, 3C), which can be converted through further reactions into sugars, including glucose (C₆H₁₂O₆).

Key points of interest in the CBB Cycle include the fixation of carbon by RuBisCO. This key enzyme is central to our research: read more about it here. Also note the stoichiometry (most easily done by counting carbons) of the reactions: 1 glucose molecule (6C) requires two “turns” of the CBB cycle, as each turn yields 1 molecule of G3P (3C). One turn of the cycle involves the fixation of 3 CO₂ molucules, thus the production of one molecule of glucose entails the fixation of 6CO₂. Additionally, the cycle involves the reduction of 9 ATP (18 per glucose) and 6 NADPH (12 per glucose) which are provided, as we have already seen, by the light-dependent reactions: this is where the 6H₂O on the reactants side, and the 6O₂ on the products side, of our simplified photosynthesis equation come in.