Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCOoxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, but approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle. This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C3 plants.[1]Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.

The first event in the C2 cycle is the initiation of the cycle, which involves the binding of a molecule called carbon dioxide (CO2) to a five-carbon compound called ribulose-1,5-bisphosphate (RuBP). This process is known as carbon fixation and is catalyzed by an enzyme called RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).

Carbon Fixation:
In this event, CO2 from the atmosphere enters the chloroplasts of the plant cells and diffuses into the stroma, which is the fluid-filled region surrounding the thylakoid membranes. The CO2 molecules then bind to RuBP, a process facilitated by the enzyme RuBisCO. The resulting molecule is unstable and quickly breaks down into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA).

RuBisCO:
RuBisCO is a key enzyme in the process of carbon fixation as it helps in binding CO2 to RuBP. It is one of the most abundant enzymes on Earth and plays a crucial role in the global carbon cycle. However, RuBisCO has a relatively low affinity for CO2 and can also react with oxygen, leading to a process called photorespiration. Photorespiration can be detrimental to the overall efficiency of photosynthesis.

RuBP Regeneration:
After the carbon fixation event, the next step in the C2 cycle involves the regeneration of RuBP. The 3-PGA molecules produced during carbon fixation are converted into another three-carbon compound called phosphoglyceraldehyde (PGA). Some of the PGA molecules are then used to regenerate RuBP, while others are used to produce glucose and other organic compounds.

Energy Consumption:
The initiation of the C2 cycle requires energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These energy-rich molecules are produced during the light-dependent reactions of photosynthesis, which take place in the thylakoid membranes of the chloroplasts. The ATP and NADPH are utilized in the C2 cycle to drive the various chemical reactions involved in carbon fixation and RuBP regeneration.

Overall, the first event in the C2 cycle is the carbon fixation process, where CO2 is bound to RuBP with the help of the enzyme RuBisCO. This event sets the stage for the subsequent reactions in the cycle, leading to the production of organic compounds and the regeneration of RuBP.