Photorespiration, photorespiration in plants, photorespiration cycle -

Photorespiration

During photosynthesis, the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) interacts with oxygen rather than carbon dioxide, a process known as photorespiration. The oxidative photosynthetic carbon cycle or the C2 photosynthetic pathway are other names for this mechanism. One way to think of photorespiration is as a byproduct of the primary photosynthetic activity.

Rubisco’s main job is to catalyze the Calvin cycle’s carboxylation of ribulose-1,5-bisphosphate (RuBP) with carbon dioxide, which produces two molecules of 3-phosphoglycerate (3-PGA). Rubisco can, however, also react with oxygen in place of carbon dioxide in some circumstances, producing one molecule of 3-PGA and one molecule of 2-phosphoglycolate (2-PG). 2-PG is not a helpful metabolite for the plant, in contrast to 3-PGA.

The photorespiratory pathway, a set of events that occur in several cellular compartments such as peroxisomes and mitochondria, is required to recover the 2-PG molecule. Through this mechanism, 2-PG is first converted to glycolate and then to glyoxylate. Eventually, glyoxylate is moved to the chloroplasts, where a sequence of events transforms it back into 3-PGA.

For the plant, photorespiration has a number of effects. It takes energy and takes carbon out of the Calvin cycle, which lowers photosynthesis’s efficiency. In addition, carbon dioxide is released, negating the initial carbon fixation that occurs during photosynthesis. Furthermore, under situations like high temperatures and low carbon dioxide levels, photorespiration may be more common.

Certain plants, referred to as CAM and C4 plants, have developed defense mechanisms against excessive photorespiration. These modifications lessen the detrimental impacts of the process and increase the effectiveness of carbon fixation. All things considered, photorespiration is a sophisticated physiological mechanism that aids in plant metabolism and environmental adaptability.

Photorespiration in plants

In plants, photorespiration is a metabolic activity that takes place in the chloroplasts of plant cells during photosynthesis. Rather than the typical carboxylation process, it is characterized by the oxygenation of ribulose-1,5-bisphosphate (RuBP) by the enzyme Rubisco. In place of the two molecules of 3-PGA in the Calvin cycle, this mechanism results in the synthesis of 3-phosphoglycerate (3-PGA) and 2-phosphoglycolate (2-PG).

The primary steps of photorespiration include:

Oxygenation of RuBP: In the presence of high oxygen levels, Rubisco catalyzes the oxygenation of RuBP, resulting in the production of 3-PGA and 2-PG.
Formation of Glycolate: 2-PG is further metabolized to glycolate in the chloroplasts.
Peroxisomal Reactions: Glycolate is transported to peroxisomes, where it undergoes a series of reactions known as the glycolate pathway. This process results in the release of carbon dioxide and the formation of glyoxylate.
Mitochondrial Reactions: Glyoxylate is then transported to the mitochondria, where it is converted to glycine.
Return to Chloroplasts: Glycine is transported back to the chloroplasts, where it is used to regenerate serine, which is eventually converted back into 3-PGA.

Energy expenditure is one of the effects of photorespiration since ATP is needed for the recovery of 2-PG and the following processes in the glycolate pathway. In addition, carbon and nitrogen are lost as a result of photorespiration because carbon dioxide is released during the 2-PG recovery process.

Different methods have developed by plants to deal with photorespiration and lessen its effects. Certain plants, notably those classified as C4 or CAM (Crassulacean Acid Metabolism) plants, have evolved biochemical adaptations that reduce the possibility of oxygenation by concentrating carbon dioxide around Rubisco. Particularly in climates with high temperatures and low carbon dioxide concentrations, these modifications aid in increasing the efficiency of carbon fixation and photosynthesis.

Photorespiration cycle

Many metabolic processes, particularly in the cytoplasm, peroxisomes, mitochondria, and chloroplasts of plant cells, are involved in the photorespiration cycle. This cycle begins when ribulose-1,5-bisphosphate (RuBP) is oxygenated rather than carboxylated by the enzyme Rubisco, which is in charge of carbon fixation in the Calvin cycle. The key phases in the photorespiration cycle are summarized as follows:

Oxygenation of RuBP (Calvin Cycle):
- Rubisco catalyzes the oxygenation of RuBP instead of its usual carboxylation reaction.
- This leads to the formation of one molecule of 3-phosphoglycerate (3-PGA) and one molecule of 2-phosphoglycolate (2-PG).
Chloroplasts:
- 2-PG is converted to glycolate in the chloroplasts.
- The enzyme glycolate oxidase is involved in the conversion of 2-PG to glycolate, releasing hydrogen peroxide (H2O2).
Peroxisomes (Glycolate Pathway):
- Glycolate is transported to peroxisomes, where it undergoes a series of reactions known as the glycolate pathway.
- In these reactions, glycolate is converted into glyoxylate, releasing carbon dioxide and reducing NAD+ to NADH.
Mitochondria:
- Glyoxylate is transported to the mitochondria, where it is converted into glycine through a series of reactions.
- These reactions involve the conversion of glyoxylate to glycine with the consumption of additional NAD+.
Return to Chloroplasts:
- Glycine is transported back to the chloroplasts.
- In the chloroplasts, glycine is converted back into serine, releasing CO2 in the process.
Conversion of Serine to 3-PGA:
- Serine is converted back into 3-PGA through a series of reactions.
- This completes the regeneration of the RuBP needed for the Calvin cycle.

The total result of the cycle of photorespiration is the recovery of carbon that was first lost as 2-PG. Nevertheless, the plant must use energy and resources throughout this process. Since photorespiration does not directly contribute to the creation of carbohydrates, unlike the Calvin cycle, it is frequently regarded as a wasteful activity. High temperatures and low carbon dioxide concentrations are two examples of environmental factors that might make photorespiration in plants more likely. In order to reduce the effects of photorespiration and increase total photosynthetic efficiency, several plant species have developed adaptations such as C4 and CAM photosynthesis.

photorespiration conclision

During photosynthesis, plants go through a metabolic process called photorespiration, in which the enzyme Rubisco oxygenates ribulose-1,5-bisphosphate (RuBP). 2-phosphoglycolate (2-PG) and 3-phosphoglycerate (3-PGA) are the products of this procedure. Because photorespiration uses energy and decreases the effectiveness of carbon fixation, it may be harmful to plant development. Plants have developed a number of adaptations, including the C4 and CAM pathways, to improve photosynthetic efficiency and reduce the effects of photorespiration in a variety of environmental settings.