The Biosynthetic Phase: The Calvin Cycle (C3 Pathway)

The biosynthetic (dark) phase of photosynthesis, unlike the light reaction, takes place in the fluid stroma of the chloroplast rather than on its membranes, and does not itself require light directly -- although it depends entirely on the steady supply of ATP and NADPH generated by the light reaction, and several of its key enzymes are, in fact, activated by light indirectly, through light-dependent changes in stromal pH and ion concentration. This pathway was worked out in great biochemical detail by Melvin Calvin, together with Andrew Benson and James Bassham, through a long series of experiments in which the green alga Chlorella was allowed to photosynthesise for progressively shorter pulses of time in the presence of radioactively labelled ^14CO2, with the labelled intermediate compounds then identified using paper chromatography -- work for which Calvin was awarded the Nobel Prize, and after whom the pathway, the Calvin cycle, is named. Because the very first stable product this pathway forms is a three-carbon compound, the pathway is also universally known as the C3 pathway, and plants that rely on it exclusively (the great majority of all plant species) are called C3 plants.

The Calvin cycle proceeds through three distinct, sequential phases, all occurring repeatedly, in a true cycle, within the stroma.

The first phase, carboxylation, is the actual CO2-fixing step. A five-carbon sugar already present in the stroma, ribulose bisphosphate (RuBP), combines with one molecule of incoming CO2, in a reaction catalysed by the enzyme ribulose bisphosphate carboxylase-oxygenase, universally abbreviated RuBisCO -- the same enzyme mentioned earlier as the most abundant protein on Earth. This combination initially forms an unstable six-carbon intermediate, which is immediately and spontaneously split into two separate molecules of 3-phosphoglyceric acid (PGA), each containing three carbon atoms -- the first stable product of the pathway, and the source of its name.

The second phase, reduction, converts each molecule of PGA into a somewhat more energy-rich three-carbon sugar-phosphate called glyceraldehyde-3-phosphate (G3P). This conversion requires an input of both ATP and NADPH, drawn directly from the products of the light reaction -- the light reaction and the Calvin cycle are thus intimately, continuously linked: the reduction phase cannot proceed without a steady supply of exactly the two molecules the light reaction was built to produce. A small fraction of the G3P generated at this stage is exported from the chloroplast, or converted within it, to build the sugars, starch and sucrose that represent the net carbohydrate gain of photosynthesis.

The third phase, regeneration, uses the majority of the G3P produced -- everything not diverted to sugar synthesis -- to rebuild the original CO2 acceptor, RuBP, through a further series of enzyme-catalysed rearrangements that also consume additional ATP. Regenerating RuBP is essential to keep the cycle turning: without it, each individual "turn" of carboxylation would exhaust the supply of CO2 acceptor after a single use. …