Pep carboxylase

Definition

PEP carboxylase is an enzyme that catalyzes the conversion of phosphoenolpyruvate (PEP) and carbon dioxide into oxaloacetate, playing a crucial role in the C4 and CAM pathways of photosynthesis. This enzyme is essential for plants that have adapted to minimize photorespiration by effectively fixing carbon dioxide, which enhances their efficiency in carbon fixation under high light intensity and temperature conditions.

5 Must Know Facts For Your Next Test

1. PEP carboxylase has a higher affinity for carbon dioxide than Rubisco, making it more efficient in fixing CO2, especially in conditions where CO2 levels are low.
2. In C4 plants, PEP carboxylase operates in the mesophyll cells, while the Calvin cycle occurs in the bundle sheath cells, effectively separating the processes to enhance efficiency.
3. The activity of PEP carboxylase is influenced by various factors such as light intensity, temperature, and the concentration of substrates like PEP and CO2.
4. PEP carboxylase helps C4 and CAM plants maintain high rates of photosynthesis even in hot and arid environments, giving them a competitive advantage over C3 plants.
5. The enzyme is not found in all plants; it is mainly present in specific groups such as maize, sugarcane, and succulents that have adapted to specific environmental conditions.

Review Questions

• How does PEP carboxylase enhance the efficiency of carbon fixation in C4 plants compared to C3 plants?
  • PEP carboxylase enhances carbon fixation efficiency in C4 plants by first converting phosphoenolpyruvate (PEP) and CO2 into oxaloacetate, which is then transformed into malate or aspartate. This initial step allows C4 plants to concentrate CO2 in bundle sheath cells where the Calvin cycle takes place. By reducing the likelihood of photorespiration, which wastes energy and fixed carbon, C4 plants can maintain higher rates of photosynthesis under conditions that would be less favorable for C3 plants.
• Discuss how the role of PEP carboxylase differs between C4 and CAM pathways regarding carbon fixation and water use.
  • In C4 pathways, PEP carboxylase facilitates the fixation of carbon dioxide during the day, allowing for an efficient separation of light-dependent reactions and the Calvin cycle. In contrast, CAM pathways utilize PEP carboxylase at night to fix CO2, which is stored as malate until daylight when it can be utilized for photosynthesis. This adaptation minimizes water loss during the hotter parts of the day, showcasing how PEP carboxylase supports both efficient carbon fixation and water conservation strategies in different plant types.
• Evaluate how the presence or absence of PEP carboxylase affects plant adaptation to changing environmental conditions like drought or high temperatures.
  • Plants equipped with PEP carboxylase are better adapted to withstand drought and high-temperature environments because they can efficiently fix carbon even when CO2 concentrations are low or when stomata are partially closed to prevent water loss. This adaptation reduces reliance on Rubisco, which is less efficient under these stress conditions. In contrast, plants lacking this enzyme may struggle to maintain photosynthetic rates during adverse environmental changes, leading to reduced growth and survival rates. The ability to use PEP carboxylase allows these specialized plants to thrive where others may fail.

Related terms

C4 Pathway: A photosynthetic pathway used by certain plants to efficiently capture carbon dioxide and minimize photorespiration by converting CO2 into a four-carbon compound.

CAM Pathway: A water-conserving adaptation where plants fix carbon dioxide at night to reduce water loss during the day, utilizing PEP carboxylase for the initial carbon fixation.

Photorespiration:

A process that occurs when oxygen is fixed instead of carbon dioxide, leading to a loss of fixed carbon and energy; it can be detrimental to plants under certain conditions.