Difference between C3, C4 & CAM Plants

Difference between C₃, C₄ & CAM Plants

To convert light energy into chemical energy, plants incorporated CO₂ from atmosphere in the presence of light absorbed by chloroplast and water from soil. The reduction of carbon dioxide (called Dark Reaction) in the process of photosynthesis forms carbohydrates.

In hot & dry conditions, plants close their stomata to prevent water loss. Under these conditions, CO₂ will decrease and oxygen gas, produced by the light reactions will increase, causing an increase of photorespiration by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase and decrease in carbon fixation. Some plants have evolved mechanisms to increase the CO₂ concentration in the leaves under these conditions.

Based on the method of reduction of carbon dioxide, plants are classified into C₃, C₄ & CAM plants.

C₃ Plants

• C₃ plants do not use PEP-carboxylase in carbon fixation because the primary carboxylation reaction, catalyzed by RuBisCO, produces the 3-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle. Hence the initial product of carbon fixation is the three-carbon compound and the enzyme involved in the primary carboxylation is ribulose-1,5-bisphosphate carboxylase.
• Photo respiration is high in these plants.
• Examples of C₃ plants are Rice, wheat, barley and soyabean.
• Over 90% of plants use C₃ carbon fixation, compared to 3% that use C₄ carbon fixation.

C₄ Plants

• Plants that use the C₄ carbon fixation process chemically fix CO₂ in the mesophyll cell by adding it to the 3-carbon molecule called phosphoenolpyruvate (PEP) catalyzed by an enzyme called PEP carboxylase, creating the 4-carbon organic acid oxaloacetic acid. This enzyme has high affinity for carbon dioxide and capable of assimilating carbon dioxide even at a lower concentration.
• The physical separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases CO₂fixation and, thus the photosynthetic capacity of the leaf.
• C₄ Plants can produce more sugar than C₃plants in conditions of high light and temperature.
• Examples of C₄ plants are maize, sorghum, sugarcane, pearl millet and other minor millets.
• C₄ minor millets, even under good management do not outyield C_{3 rice.}
• The photosynthetic advantage of C₄ plants compared with C₃ plants at the level of carboxylation is reduced at the whole leaf level. It is due to combination of stomatal and mesophyll resistance at the leaf level and by mutual shading and periods of low light at a plant community level. As a result, no consistence advantage of the C₄ pathway is evident in crop growth rates and crop yields of C_{4 plants.}

Crassulacean acid metabolism (CAM)

• Xerophytes, such as cacti and most succulents, also use PEP carboxylase to capture CO₂ in a process called Crassulacean acid metabolism (CAM).
• CAM plants have a different leaf anatomy from C₃ plants, and fix the CO₂ at night when their stomata are open.
• CAM plants store the CO₂ mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO₂ inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO.
• CAM is used by 16,000 species of plants.

Difference between C₃, C₄ & CAM Plants

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