Discuss photorespiration. What is the relationship between photorespiration and photosynthesis?

Photosynthesis: The Foundation

Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to create oxygen and energy in the form of glucose. It occurs in two main stages: the light-dependent reactions and the Calvin cycle. The light-dependent reactions capture light energy and convert it into chemical energy (ATP and NADPH), while the Calvin cycle uses this energy to fix carbon dioxide into sugars.

Photorespiration: An Unwanted Pathway

Photorespiration is a metabolic pathway that competes with photosynthesis. It occurs when RuBisCO, the enzyme responsible for carbon fixation in the Calvin cycle, binds to oxygen instead of carbon dioxide. This typically happens when CO₂ levels are low and O₂ levels are high, a common scenario in hot, dry environments where plants close their stomata to conserve water, limiting CO₂ intake.

The Steps of Photorespiration

1. Oxygenation of RuBP: RuBisCO catalyzes the reaction between RuBP and O₂, forming one molecule of 3-PGA (3-phosphoglycerate) and one molecule of phosphoglycolate.
2. Phosphoglycolate Processing: Phosphoglycolate is toxic and must be metabolized. This involves a complex series of reactions across three organelles: chloroplasts, peroxisomes, and mitochondria.
3. Glycolate Conversion: Phosphoglycolate is converted to glycolate in the chloroplast.
4. Glycolate Transport: Glycolate is transported to the peroxisome.
5. Peroxisomal Reactions: Glycolate is converted to glyoxylate and then to glycine. Hydrogen peroxide (H₂O₂) is produced and broken down.
6. Mitochondrial Reactions: Two molecules of glycine are converted to serine, CO₂, and NH₃. This is the only step where CO₂ is released during photorespiration.
7. Serine Conversion: Serine is transported back to the peroxisome and converted to hydroxypyruvate.
8. Hydroxypyruvate Conversion: Hydroxypyruvate is converted to glycerate.
9. Glycerate Return: Glycerate is transported back to the chloroplast and phosphorylated to 3-PGA, re-entering the Calvin cycle.

Relationship between Photosynthesis and Photorespiration

Photorespiration and photosynthesis are intrinsically linked, but often in a competitive manner. While photosynthesis builds sugars, photorespiration consumes ATP and releases CO₂, effectively reversing some of the work done by photosynthesis. The net result is a reduction in photosynthetic efficiency.

Feature	Photosynthesis	Photorespiration
Primary Function	Carbon fixation, sugar production	Metabolizes phosphoglycolate, CO₂ release
Enzyme Involved	RuBisCO (carboxylase activity)	RuBisCO (oxygenase activity)
CO₂	Consumed	Released
O₂	Produced	Consumed
ATP Consumption	Requires ATP	Consumes ATP
Organelles Involved	Chloroplast	Chloroplast, Peroxisome, Mitochondria

Adaptations to Minimize Photorespiration

Some plants have evolved mechanisms to minimize photorespiration. C4 plants and CAM plants are prime examples:

• C4 Plants: These plants spatially separate initial carbon fixation and the Calvin cycle. CO₂ is initially fixed in mesophyll cells by PEP carboxylase, which has a higher affinity for CO₂ than RuBisCO and doesn't bind to oxygen. The resulting four-carbon compound is then transported to bundle sheath cells where it releases CO₂ for the Calvin cycle. Examples include maize and sugarcane.
• CAM Plants: These plants temporally separate initial carbon fixation and the Calvin cycle. They open their stomata at night to take in CO₂ and fix it into organic acids, which are stored until daylight when the stomata close and CO₂ is released for the Calvin cycle. Examples include cacti and succulents.

Significance and Mitigation Strategies

Photorespiration is particularly problematic in tropical and subtropical regions where temperatures are high and CO₂ concentrations are low. It can reduce photosynthetic efficiency by as much as 25-50%. Researchers are exploring strategies to reduce photorespiration, including genetically engineering plants to improve RuBisCO specificity for CO₂ or to introduce C4 photosynthetic pathways into C3 plants.

Rice, a C3 plant, suffers significantly from photorespiration, especially in high-temperature environments. This contributes to yield losses. Research is ongoing to introduce C4 photosynthetic traits into rice to improve its photosynthetic efficiency and increase yields, particularly in regions facing climate change. National Mission for Sustainable Agriculture (NMSA) Under NMSA, various schemes like Soil Health Mission and Rainfed Area Development Programme promote practices that indirectly address issues related to plant stress and photosynthetic efficiency, which can mitigate the impact of photorespiration. 2010 Why does RuBisCO bind to oxygen sometimes? RuBisCO evolved in an atmosphere with much lower oxygen concentrations. It has a limited ability to discriminate between CO₂ and O₂, and under certain environmental conditions, oxygen becomes a more readily available substrate. Can photorespiration ever be beneficial? While generally detrimental, photorespiration can serve a protective role under conditions of extreme stress, such as high light intensity, by dissipating excess energy and preventing damage to the photosynthetic apparatus. RuBisCO Ribulose-1,5-bisphosphate carboxylase/oxygenase – the enzyme responsible for carbon fixation in the Calvin cycle, but which can also bind to oxygen, leading to photorespiration. Photorespiration can reduce photosynthetic efficiency by as much as 25-50% in C3 plants, especially in hot and dry environments. Knowledge cutoff - based on general scientific consensus C4 Photosynthesis A photosynthetic pathway where CO2 is initially fixed by PEP carboxylase, followed by its release near RuBisCO in bundle sheath cells, minimizing photorespiration. C4 plants are estimated to account for 5% of all terrestrial carbon fixation. Knowledge cutoff - based on general scientific consensus