Steps of Photosynthesis

The six-carbon sugar produced, C₆H₁₂O₆, is glucose. The direct product of photosynthesis is actually a three-carbon molecule used to make sugar rather than glucose itself. Because the process that builds glucose from the three-carbon molecule immediately follows photosynthesis, the entire reaction is most easily summarized by the equation. However, the equation can be further simplified by showing only the net consumption of water:

The sugar produced in this reaction is directly built not during photosynthesis but in the respiration process that follows photosynthesis. Photosynthesis consists of two stages: the light-dependent reactions and the light-independent reactions, which include the Calvin cycle (or Calvin-Benson cycle), the cycle that fixes carbon into glyceraldehyde 3-phosphate (G3P), the precursor to glucose.

A photosystem is a complex of proteins and pigments working together to absorb energy from light and transfer it to an electron acceptor, which is a molecule that accepts an electron and transfers it to another molecule. Each photosystem consists of a light-harvesting complex, which is a group of molecules that takes in light, surrounding a reaction-center complex, where the light is converted to chemical energy. Many pigments (including several types of chlorophyll, as well as others such as carotenoids) are distributed throughout the light-harvesting complex. They absorb energy from light as it enters the complex and pass the energy along to the reaction-center complex. When light strikes the chlorophyll, it excites an electron to a higher energy state. That electron then drops back to its initial state, which releases energy, exciting an electron in the next chlorophyll. Thus, the series of pigments creates a pathway to the reaction-center complex.

In the reaction-center complex of photosystem II, a pair of chlorophyll molecules known as P680 (so named because the molecules best absorb light at a wavelength of 680 nanometers) donate electrons to the primary electron acceptor. This leaves the P680 molecules positively charged (denoted P680⁺). An enzyme catalyzes the splitting of water into two electrons, two hydrogen ions (H⁺), and an oxygen atom. The electrons from this reaction are transferred to P680, returning P680 to its initial state. The oxygen atom combines with another oxygen atom to make O₂. The hydrogen ions are released into the thylakoid lumen, where they create a proton gradient that will be used to form ATP. This process is chemiosmosis, the movement of ions across a semipermeable membrane down their electrochemical gradient, which results in a charge that can be used as a source of energy. The electrons from the primary electron acceptor are passed in a series of redox reactions from photosystem II to photosystem I via an electron carrier called plastoquinone (Pq), a cytochrome complex (an enzyme in the thylakoid membrane that forms part of the electron transport chain) and a protein called plastocyanin (Pc).

The second stage of photosynthesis is the Calvin cycle, also called the Calvin-Benson cycle, is named for its discoverers, Melvin Calvin and Andrew Benson. The Calvin cycle is an anabolic process (a chemical reaction that synthesizes molecules in metabolism) that builds the molecules that make up glucose, the six-carbon sugar that is the product of reactions following the Calvin cycle. The cycle uses energy to build a large molecule from smaller ones. The cycle takes place in the stroma (the fluid inside chloroplasts) in three main steps.

The first step is known as carbon fixation because it takes in CO₂ from the atmosphere and “fixes” it into organic molecules that can be used by living things. In this step, the enzyme ribulose bisphosphate carboxylase-oxygenase (abbreviated rubisco) adds one carbon to a five-carbon sugar called ribulose bisphosphate (RuBP)during carbon fixation. This forms a six-carbon sugar that is energetically unstable, meaning it cannot hold its form. It immediately splits into two 3-carbon molecules called 3-phosphoglycerate (3-PGA).

In the second step, each molecule of 3-phosphoglycerate receives a phosphate group from ATP, forming 1,3-bisphosphoglycerate. Because this process changes ATP to ADP, it is endergonic (uses energy). Then NADPH donates a pair of electrons to 1,3-bisphosphoglycerate, which reduces the molecule. This causes it to lose a phosphate group. The product of this reaction is glyceraldehyde 3-phosphate (G3P), a three-carbon sugar formed in the Calvin cycle that is a precursor to glucose. Importantly, for every three molecules of CO₂ that enter the cycle, six molecules of G3P are formed. However, five of these G3P molecules will be recycled into RuBP in the next step. Only one molecule leaves the cycle, resulting in a net production of one G3P for every three CO₂consumed.

The final step is the regeneration of RuBP from the five molecules of G3P in the previous step. This final step involves a complex series of reactions that rearrange the carbon atoms into the five-carbon sugar, consuming an additional three ATP in the process. At the end of this step, RuBP is again ready to receive CO₂, and the cycle begins again. Because only one G3P is produced from a single turn of the cycle, it takes six turns of the cycle to produce a single molecule of glucose.