CAM plants and C4 cycle

Question

Q.No.3. What are obligate and facultative CAM plants? Explain with examples. What are C4 plants & C4 cycle?

Answer 1

Obligate and Facultative CAM Plants

Crassulacean Acid Metabolism (CAM) plants are specialized to fix carbon dioxide (CO₂) at night, reducing water loss in hot, arid environments. CAM plants open their stomata at night to capture CO₂, which they store as organic acids. During the day, when the stomata are closed, they use these acids to release CO₂ for photosynthesis.

1. Obligate CAM Plants: These plants always follow CAM photosynthesis, regardless of environmental conditions. They are adapted to consistently arid or stressful conditions where water conservation is essential. Example: Opuntia (prickly pear cactus), Agave, Pineapple (Ananas comosus).
2. Facultative CAM Plants: These plants can switch between CAM and C3 photosynthesis depending on environmental conditions. In favorable conditions (high moisture), they use C3 photosynthesis. Under stress (drought or high salinity), they switch to CAM for water conservation. Example: Mesembryanthemum crystallinum (common ice plant), which shifts to CAM under drought conditions.

2. C4 Plants and the C4 Cycle

C4 plants have an adaptation to hot, sunny environments, enabling efficient photosynthesis by minimizing photorespiration. These plants initially fix CO₂ into a four-carbon compound (hence "C4") using an enzyme called PEP carboxylase. This adaptation is particularly useful under conditions of high light intensity, high temperatures, and low atmospheric CO₂.

1. Examples of C4 Plants: Maize (corn), Sorghum, Sugarcane, Amaranthus.

C4 Cycle (Hatch-Slack Pathway)

The C4 cycle is a two-stage process that takes place in two types of cells: mesophyll cells and bundle sheath cells.

1. CO₂ Fixation in Mesophyll Cells:
2. CO₂ is initially fixed by PEP carboxylase, forming a four-carbon compound, oxaloacetate.
3. Oxaloacetate is then converted to malate or aspartate, which is transported to the bundle sheath cells.
4. CO₂ Release and Calvin Cycle in Bundle Sheath Cells:
5. In bundle sheath cells, malate or aspartate is decarboxylated to release CO₂.
6. This released CO₂ enters the Calvin cycle, where it is fixed again by Rubisco, reducing photorespiration.

Summary

1. Obligate CAM plants always use CAM photosynthesis (e.g., Opuntia).
2. Facultative CAM plants switch between CAM and C3 (e.g., Mesembryanthemum crystallinum).
3. C4 plants have a two-step CO₂ fixation process to improve efficiency (e.g., Maize, Sorghum), and the C4 cycle minimizes photorespiration by concentrating CO₂ in bundle sheath cells.