Photorespiration

Definition

Photorespiration is a metabolic process in plants that occurs when the enzyme RuBisCO oxygenates ribulose-1,5-bisphosphate instead of carboxylating it, leading to the consumption of energy and release of CO2. This process typically takes place under high light intensity and low carbon dioxide concentrations, which can negatively impact plant productivity by reducing the efficiency of photosynthesis.

5 Must Know Facts For Your Next Test

1. Photorespiration can occur when the concentration of oxygen is higher than that of carbon dioxide in the leaf, especially during hot and dry conditions.
2. This process leads to a net loss of fixed carbon as it releases CO2 instead of incorporating it into organic molecules.
3. While generally considered a wasteful process, photorespiration can help protect plant cells from excess light by dissipating energy.
4. C4 and CAM plants have evolved mechanisms to minimize photorespiration by concentrating CO2 around RuBisCO, making them more efficient in hot environments.
5. Environmental factors like temperature, light intensity, and water availability significantly influence the rate of photorespiration in plants.

Review Questions

• How does photorespiration affect the efficiency of the Calvin cycle and overall plant productivity?
  • Photorespiration negatively impacts the efficiency of the Calvin cycle because it leads to the release of fixed carbon as CO2 rather than its incorporation into carbohydrates. This process consumes energy and reduces the amount of carbon available for photosynthesis, ultimately lowering plant productivity. Under conditions where photorespiration is prevalent, such as high temperatures and low CO2 levels, plants can experience decreased growth and yield.
• What strategies do C4 and CAM plants employ to reduce photorespiration compared to C3 plants?
  • C4 plants minimize photorespiration by utilizing a unique pathway that captures CO2 more efficiently before it enters the Calvin cycle. They initially convert CO2 into a four-carbon compound in mesophyll cells, which is then transported to bundle sheath cells where it is converted back to CO2 for use in the Calvin cycle. CAM plants take this a step further by fixing CO2 at night when temperatures are cooler and stomata are open, storing it as an organic acid until daylight when it can be used in photosynthesis. Both strategies reduce RuBisCO's exposure to oxygen, thus decreasing photorespiration.
• Analyze how climate change may influence photorespiration rates in plants and their adaptation strategies.
  • Climate change can lead to increased temperatures and altered precipitation patterns, which may elevate the occurrence of photorespiration in many plant species. Higher temperatures can increase oxygen concentrations while decreasing stomatal conductance due to drought stress, both of which favor photorespiration over photosynthesis. In response, some plants might adapt by evolving more efficient photosynthetic pathways like C4 or CAM photosynthesis or adjusting their stomatal behavior to optimize gas exchange under changing climatic conditions. These adaptations will be crucial for maintaining productivity and survival in an increasingly variable environment.

Related terms

RuBisCO: Ribulose-1,5-bisphosphate carboxylase/oxygenase, an enzyme that catalyzes the first step of carbon fixation in the Calvin cycle but can also catalyze a reaction with oxygen leading to photorespiration.

C4 Photosynthesis:

A photosynthetic pathway that enhances carbon fixation efficiency by utilizing a four-carbon compound to deliver CO2 to the Calvin cycle, reducing photorespiration.

Stomatal Conductance:

The rate at which CO2 enters and water vapor exits the leaf through stomata, influencing the balance between photosynthesis and photorespiration.