# Photorespiration Photorespiration is a metabolic pathway that occurs in photosynthetic organisms. It is a respiratory process in green cells that is dependent on light, consumes oxygen, and releases carbon dioxide. Unlike cellular respiration, it does not produce any ATP. --- ## Mechanism of Photorespiration Photorespiration is initiated when the enzyme *Ribulose-1,5-bisphosphate carboxylase/oxygenase* (RuBisCO) acts as an oxygenase instead of a carboxylase. This typically happens under conditions of low internal $CO_2$ concentration (around 50 ppm) and high $O_2$ concentration. - **Initiation:** RuBisCO combines $O_2$ with Ribulose-1,5-bisphosphate (RuBP). - **Products:** This reaction produces one molecule of 3-phosphoglycerate (PGA), which can enter the Calvin cycle, and one molecule of a toxic 2-carbon compound called **phosphoglycolate**. - **Detoxification Pathway:** The plant must metabolize the toxic phosphoglycolate in a complex pathway involving three different organelles: the chloroplast, the peroxisome, and the mitochondrion. 1. **Chloroplast:** Phosphoglycolate is converted to glycolate. $$\text{RuBP} + \text{O}_2 \rightarrow \text{Phosphoglycolate} + \text{PGA}$$ $$\text{Phosphoglycolate} \rightarrow \text{Glycolate}$$ 2. **Peroxisome:** Glycolate is transported to the peroxisome and converted first to glyoxylate and then to the amino acid **glycine**. 3. **Mitochondrion:** Two molecules of glycine are transported into the mitochondrion, where they are converted into one molecule of the amino acid **serine**, releasing a molecule of $CO_2$ in the process. --- ## Disadvantages and Evolution - **Disadvantages:** - It is an energy-costly process that consumes ATP and reducing power. - It results in the net loss of previously fixed carbon as $CO_2$, reducing the efficiency of photosynthesis by as much as 25%. - Photorespiration is not essential for plant growth; inhibiting it does not harm the plant. - **Evolutionary Context:** - RuBisCO evolved in an ancient atmosphere with very little $O_2$ and high levels of $CO_2$. Its active site can bind to both molecules. - As photosynthesis released $O_2$ into the atmosphere over geological time, the oxygenase activity of RuBisCO became a significant problem, leading to the wasteful process of photorespiration. --- ## Factors Triggering Photorespiration - **RuBisCO's Dual Activity:** The enzyme's activity (carboxylase vs. oxygenase) depends on the relative concentrations of $CO_2$ and $O_2$. - **High Temperature and Dry Conditions:** On hot, dry days, plants close their stomata to conserve water. This prevents $CO_2$ from entering the leaf and $O_2$ from exiting. - **Shift in Gas Ratio:** As photosynthesis continues, the internal $CO_2$ level drops while the $O_2$ level rises, increasing the $O_2$ to $CO_2$ ratio and favoring the oxygenase activity of RuBisCO. --- ## C4 Photosynthesis: An Adaptation Some plants, especially those in tropical climates (e.g., maize, sugarcane), have evolved a mechanism to minimize photorespiration. This is known as **C4 photosynthesis** or the **Hatch-Slack pathway**. - **Spatial Separation:** C4 plants separate initial $CO_2$ fixation from the Calvin cycle into two different cell types: *mesophyll cells* and *bundle sheath cells*. - **CO₂ Pumping Mechanism:** 1. **Initial Fixation (Mesophyll Cells):** $CO_2$ is first fixed by the enzyme **PEP carboxylase**, which has a high affinity for $CO_2$ and no affinity for $O_2$. It combines $CO_2$ with a 3-carbon molecule, phosphoenolpyruvate (PEP), to form a 4-carbon molecule, **oxaloacetate**. 2. **Transport:** Oxaloacetate is converted to malate (another 4-carbon acid) and transported to the adjacent bundle sheath cells. 3. **CO₂ Release (Bundle Sheath Cells):** Inside the bundle sheath cells, malate releases the $CO_2$, creating a very high concentration of $CO_2$ around the RuBisCO enzyme. 4. **Calvin Cycle:** With high $CO_2$ levels, RuBisCO functions efficiently as a carboxylase, running the Calvin cycle and minimizing photorespiration. 5. **Regeneration:** The remaining 3-carbon molecule (pyruvate) is transported back to the mesophyll cell to regenerate PEP, a process that requires ATP. | Feature | C3 Photosynthesis | C4 Photosynthesis | |---------|-------------------|-------------------| | **Initial $CO_2$ Acceptor** | RuBP (5-carbon) | PEP (3-carbon) | | **Initial $CO_2$ Fixing Enzyme** | RuBisCO | PEP Carboxylase | | **First Stable Product** | 3-PGA (3-carbon) | Oxaloacetate (4-carbon) | | **Location of Calvin Cycle** | Mesophyll cells | Bundle sheath cells | | **Photorespiration Rate** | High, especially in hot/dry conditions | Negligible or very low | | **Optimal Temperature** | Lower (20-25°C) | Higher (30-40°C) | | **Example Plants** | Rice, Wheat, Soybeans | Maize, Sugarcane, Sorghum | --- ## Possible Questions and Answers - **Q:** What are the disadvantages of photorespiration? **A:** Photorespiration is a wasteful process that costs the plant energy (ATP) and results in the net loss of fixed carbon dioxide. It can reduce the efficiency of photosynthesis by up to 25%. - **Q:** How did photorespiration evolve? **A:** The enzyme RuBisCO evolved when Earth's atmosphere had very little oxygen. Its active site can bind both $CO_2$ and $O_2$. As oxygen levels in the atmosphere increased due to photosynthesis, the enzyme's ability to bind with $O_2$ became a liability, leading to the process of photorespiration. - **Q:** What is the effect of temperature on the activities of RuBisCO? **A:** High temperatures cause plants to close their stomata to conserve water. This leads to a decrease in the internal $CO_2$ concentration and an increase in the $O_2$ concentration inside the leaf. This high $O_2$:$CO_2$ ratio favors the oxygenase activity of RuBisCO, thus increasing the rate of photorespiration. --- ## References (Derived from FBISE textbook)