Carbon Fixation in Photosynthesis: The Calvin Cycle, Photorespiration, and Hydrogen Peroxide Detoxification

The first step of carbon fixation occurs during the Calvin cycle when CO2 combines with RuBP. Photorespiration, in contrast, occurs when O2, instead of CO2, combines with RuBP. The product of this reaction is transported to a peroxisome, where H2O2 is generated and subsequently broken down.

The first step of carbon fixation occurs during the Calvin cycle in photosynthesis

The first step of carbon fixation occurs during the Calvin cycle in photosynthesis. Carbon fixation refers to the process of converting atmospheric carbon dioxide (CO2) into organic compounds like sugars. In the Calvin cycle, the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) facilitates the combining of CO2 with a five-carbon sugar molecule called ribulose-1,5-bisphosphate (RuBP).

During this step, one molecule of carbon dioxide combines with one molecule of RuBP to form an unstable six-carbon intermediate compound. This compound then quickly breaks down into two molecules of a three-carbon molecule called 3-phosphoglycerate. This reaction is catalyzed by the enzyme RuBisCO.

On the other hand, photorespiration is a wasteful pathway that can occur in plants under certain conditions, particularly when the concentration of carbon dioxide is low and oxygen levels are higher. Instead of CO2, oxygen (O2) combines with RuBP in a process called oxygenation. This results in the formation of a two-carbon compound called phosphoglycolate and a three-carbon compound called phosphoglycerate.

Phosphoglycolate is a non-useful byproduct of oxygenation and needs to be salvaged to keep carbon flow and energy production going for the plant. It is first transported out of the chloroplast and into the peroxisome, a cellular organelle. In the peroxisome, hydrogen peroxide (H2O2) is generated as a byproduct during the breakdown of phosphoglycolate. Hydrogen peroxide is toxic to cells, so it needs to be detoxified.

To address this, the peroxisome contains enzymes such as catalase that break down hydrogen peroxide into water (H2O) and oxygen (O2). The released water and oxygen can then be used by the plant or released into the atmosphere. The carbon skeleton from the salvaged phosphoglycolate can enter the chloroplast, be converted back into a useful molecule, and continue through the Calvin cycle for further carbon fixation.

Overall, the Calvin cycle is responsible for the primary fixation of carbon dioxide during photosynthesis. However, under certain conditions, photorespiration can occur as a wasteful pathway, leading to the production of toxic hydrogen peroxide. The detoxification of hydrogen peroxide occurs in the peroxisome, allowing the plant to salvage and recycle the carbon to maintain proper carbon flow and energy production.

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