The carbon reactions of photosynthesis, also known as the Calvin-Benson cycle or photosynthetic carbon reduction, consist of a series of steps that take place in the chloroplast. Broadly speaking, the carbon reactions consist of three steps: carbon fixation, reduction, and regeneration of the initial substrate of the cycle. Because the carbon reactions together form a metabolic cycle, there is no end product of the pathway as such. Rather, the cycle ‘exports’ reduced carbon in the form of triose phosphates.

Carboxylation

In the first step of the carbon reactions, CO₂ reacts with a high-energy five carbon substrate called ribulose bisphosphate (RuBP), resulting the the formation of two molecules of the three carbon compound phosphoglycerate (PGA). This carboxylation reaction is catalyzed by Rubisco, a large and evolutionarily ancient enzyme made up of 8 large and 8 small subunits. Rubisco is one of the most abundant proteins on Earth, and makes up 40% of the soluble protein in an average leaf. Rather than attaching the CO2 onto the end of the five carbon substrate, Rubisco adds it to the second carbon in the chain, forming a very unstable intermediate that splits between the 2 and 3 carbons almost immediately. The resulting PGA molecules enter the reduction phase of the cycle.

Reduction

The first step of the reduction phase of the carbon reactions is where the majority of the energetic compounds from the light reactions are used. In the first reaction of reduction, PGA is phosphorylated to make bisphosphoglycerate, a reaction that uses ATP produced by the light reactions. In the second reaction of the reduction phase, bis-phosphoglycerate is reduced to glyceraldehyde-3-phosphate (G3P) by the donation of electrons from NADPH, also a product of the light reactions. G3P is a triose phosphate carbohydrate and is the true product of the carbon reactions in the sense that it is exported from the cycle and enters the starch or sucrose biosynthetic pathways.

Regeneration

The rest of the carbon reactions, 10 individual enzyme-mediated reactions, make up the regeneration phase of the cycle. The complexity of the regeneration phase is due to the difficult task it is given: to make a highly reactive five-carbon molecule starting with a three-carbon molecule. To accomplish this, the regeneration phase employs a network of interrelated reactions in which the pool of triose phosphates serve as reactants many different times.

Regulation

The carbon reactions are regulated by two key factors: the pH of the stroma and a redox regulatory feedback loop. Although the carbon reactions are sometimes referred to as the ‘dark reactions’ to illustrate their separation from the light reactions, this is a misnomer. The carbon reactions are highly dependent on light, not just for the products of the light reactions that act as substrates, but also for regulation of several key enzymes. Three enzymes in particular have a pH optimum around 8, including Rubisco (the other two are fructose 1,6-bisphosphatase and sedoheptulose bisphosphatase). This high pH requires the transport of protons out of the stroma and into the lumen, thus the maximum activity of these enzymes occurs in the light.

A second point of regulation of the carbon reactions is a redox regulatory feedback loop. If you recall, PS I reduces an iron-sulfur protein called ferredoxin (Fd), which can supply electrons to NADP+. In addition, it can reduce another small redox-active protein called thioredoxin. When reduced, it can reduce disulfide bridges that exist in several enzymes in the carbon reactions, changing them from an inactive state to an active state.

Efficiency

To calculate the overall efficiency of photosynthesis, we need to correlate the input energy from light with the output energy in the form of carbohydrates. On average, it takes 8 photons of light to fix one molecule of CO₂ via the reduction of 2 NADP+ molecules and the synthesis of 3 ATPs. Thus it will require an input of 48 photons to fix one molecule of a hexose carbohydrate (six CO₂ fixation events). If we assume each of these photons contains the minimum energy necessary to drive photosynthesis, about 680 nm light, that would be 175 kJ per mol of photons. Therefore, multiplying 48 mol × 175 kJ per mol gives 8400 kJ of energy for 1 mole of hexose carbohydrate. This represents the minimum energy required by the plant to produce a mole of hexose. When fully oxidized by the cell, this carbohydrate yields 2804 kJ of energy, giving an overall theoretical efficiency of 33% (2804 / 8400).