Photorespiration

Definition

Photorespiration is a metabolic pathway that occurs in plants when the enzyme RuBisCO oxygenates ribulose bisphosphate instead of carboxylating it, leading to a loss of fixed carbon and energy. This process typically occurs under high oxygen and low carbon dioxide concentrations, causing inefficiencies in photosynthesis, particularly when the Calvin cycle is active. It connects with other pathways like C4 and CAM, which have evolved to minimize photorespiration in certain plant species.

5 Must Know Facts For Your Next Test

1. Photorespiration can significantly reduce the efficiency of photosynthesis by up to 50% under certain environmental conditions.
2. The process involves the conversion of 2-phosphoglycolate into glycerate-3-phosphate, consuming ATP and NADPH, which are crucial for energy in the plant.
3. It is more likely to occur in C3 plants, especially in hot and dry conditions where stomata close, increasing oxygen concentration inside leaves.
4. C4 and CAM plants have adapted mechanisms to reduce the impact of photorespiration, allowing them to thrive in environments where C3 plants may struggle.
5. Photorespiration plays a role in protecting plants from photoinhibition by dissipating excess energy that could damage chloroplasts.

Review Questions

• How does photorespiration affect the efficiency of photosynthesis in C3 plants?
  • Photorespiration negatively impacts the efficiency of photosynthesis in C3 plants by converting fixed carbon back into carbon dioxide. This process reduces the net gain of energy and sugars since it consumes ATP and NADPH without producing useful compounds. Under conditions of high oxygen and low carbon dioxide, such as when stomata are closed to prevent water loss, photorespiration becomes more pronounced and can cut the photosynthetic output significantly.
• Compare and contrast the mechanisms that C4 and CAM plants utilize to minimize photorespiration.
  • C4 plants minimize photorespiration by utilizing a two-step process where carbon dioxide is initially fixed into a four-carbon compound in mesophyll cells before being transported to bundle-sheath cells for the Calvin cycle. This separation allows them to maintain higher concentrations of carbon dioxide around RuBisCO. In contrast, CAM plants open their stomata at night to capture carbon dioxide, storing it as organic acids for use during the day when stomata are closed. Both adaptations effectively reduce photorespiration but are suited for different environmental conditions.
• Evaluate the ecological significance of photorespiration and its evolutionary implications for plant adaptation.
  • Photorespiration has ecological significance as it acts as a protective mechanism for plants under stress conditions like drought or high light intensity. By dissipating excess energy, it prevents damage to chloroplasts. Evolutionarily, this inefficiency prompted the development of C4 and CAM pathways as adaptations to enhance survival in challenging environments. The emergence of these pathways illustrates how plants have evolved diverse strategies for carbon fixation, showcasing their ability to adapt to varying levels of atmospheric carbon dioxide and oxygen.

Related terms

RuBisCO: Ribulose-1,5-bisphosphate carboxylase/oxygenase, an enzyme crucial for carbon fixation in the Calvin cycle, which can catalyze both carboxylation and oxygenation reactions.

C4 Photosynthesis: A photosynthetic pathway that minimizes photorespiration by initially fixing carbon dioxide into a four-carbon compound in mesophyll cells before transferring it to bundle-sheath cells for the Calvin cycle.

CAM Photosynthesis: Crassulacean Acid Metabolism is a water-conserving photosynthetic pathway used by some plants that opens their stomata at night to capture carbon dioxide, reducing photorespiration during the day.