C3 vs C4 Plants
Photosynthesis is the light driven reaction that converts carbon dioxide and water in to energy rich sugars. The process is coupled to the oxidation of water yielding oxygen as a biproduct. Depending on the photosynthetic mechanism there are 3 types of plants. C3 plants, C4 plants and CAM plants. The name C3 or C4 derives from the number of carbon atoms in the first stable photosynthetic product.
About 95% of the plants on earth are C3 plants. As the name indicates they carry out C3 photosynthetic mechanism. C3 photosynthesis is thought to have arisen nearly 3.5 billion years ago. These plants are mostly woody and round leaf plants. In these plants, carbon fixation is carried out in the mesophyll cells that are just beneath the epidermis. Carbon dioxide from the atmosphere that enters through the stomata is accepted by Ribulose bisphosphate and is fixed in to phosphoglycerate by Ribulose bisphosphate carboxylase enzyme (Rubisco). This process is known as carboxylation. Phosphoglycerate molecules enter the Calvin cycle in the mesophyll cells. C3 plants are known to be inefficient in terms of their photosynthetic mechanism. This is because of the occurrence of photorespiration in C3 plants. This effect is caused by the oxygenase activity of Rubisco. Oxygenation of Rubisco works in the opposite direction to carboxylation, effectively undoes photosynthesis by wasting large amounts of carbon originally fixed by the Calvin cycle at great expense, and results in loss of carbon dioxide from the cells that are fixing carbon dioxide. Interaction with oxygen and carbon dioxide occurs at the same site on Rubisco. These competing reactions normally run at a ratio of 3:1 (carbox: oxyg). Thus, it is clear that photorespiration is a light stimulated process that consumes oxygen and evolves carbon dioxide.
C4 plants are commonly seen in dry and high temperature areas. Approximately 1% of plant species have C4 biochemistry. Some examples for C4 plants are corn and sugarcane. As the name indicates these plants carry out C4 photosynthetic mechanism. C4 photosynthesis is thought to have arisen nearly 12 million years ago, long after the evolution of C3 mechanism. C4 plants may be better adapted now, as carbon dioxide levels are much lower now than 100 million years ago. C4 plants are much more efficient at capturing carbon dioxide. C4 photosynthesis is found in both monocot and dicot species. In contrast to C3 plants, the first stable product formed during photosynthesis is oxaloacetic, which is a C4 compound. The leaves of these plants show a special type of anatomy called “Kranz Anatomy”. There is a circle of bundle sheath cells with chloroplasts around vascular bundles by which C4 plants can be identified. In this pathway, carbon dioxide is fixed twice. In the mesophyll cell cytoplasm, CO2 is fixed with phospho enol pyruvate (PEP), which acts as a primary acceptor. The reaction is catalyzed by PEP carboxylase enzyme. Then PEP is converted to malate and then to pyruvate liberating CO2. This CO2 is then fixed with Ribulose bisphosphate, to form 2 phosphoglycerate to carry out Calvin cycle.
What is the difference between C3 Plants and C4 Plants ?
• C4 plants are more efficient than C3 plants
• C4 plant leaves show Kranz anatomy but C3 leaves do not.
• CO2 is fixed once in C3 plants and twice in C4 plants.
• RuBP accepts CO2 in C3 plants and PEP accepts CO2 in C4 plants.
• PEP carboxylase in C4 has a higher affinity for CO2 than RuBP carboxylase in C3 plants.
• Photorespiration present in C3 plants and it is absent in C4 plants.
• In C3 plants, photosynthesis does not take place efficiently under very high light concentrations or low CO2 concentrations but, in C4 plants, it does take place efficiently under very high light concentrations and low CO2 concentrations.