Question

1. Explain the difference between C-3 and C-4 plants in relation to primary carboxylation (carbon fixation)

2. Give three examples of each including the common name and binomial nomenclature.

Answer 1

Photosynthesis is a crucial physiological process used by plants to turn light, CO₂, and H₂O into sugars, using the primary photosynthetic enzyme Rubisco. There are two ways by which CO₂ is fixed one is C3 pathway and other one is C4 pathway. Based on the pathway of CO₂ fixation, plants are grouped as C3 plants and C4 plants. The difference between this two pathways is explained as following.

C3 is a common photosynthesis pathway used by the majority of plant species on Earth, in which CO₂ enters a plant through its stomata, where the enzyme Rubisco catalyzes the fixation of CO₂ to produce 3 carbon molecule 3-phosphoglycerate which further undergo chemical reactions to produce sugars. The reactions through which CO₂ is converted into sugar is collectively known as Calvin-Benson cycle. However, there are two limitations of the C3 pathway

1. In addition to CO₂, Rubisco also has the ability to fix O₂, which leads to the generation of two carbon compound glycolate that is toxic. This toxic compound is recycled through a pathway called photorespiration. This pathway consumes ATP and NADP with no generation of sugars. Hence photorespiration costs the plant energy that could be used in photosynthesis.
2. Another limitation is that to let the CO₂ in the leaf, the stomata must remain open. This leads water vapor to leave out of the leaf. Hence it is disadvantageous under water deficit condition and heat stress.

Whereas in the case of C4 plants, four-carbon compound oxaloacetate is produced from CO₂ and phosphoenolpyruvate (PEP) in mesophyll cells. This PEP now converted into malate and transported into the unique leaf anatomical structure 'bundle sheath.' The bundle sheath cells are not exposed to O_2. In bundle sheath cells, the malate is decarboxylated to produce CO₂. This unique structure delivers CO₂ direct to Rubisco, efficiently eliminating its contact with O₂ and the need for photorespiration and allows CO₂ to concentrate in 'bundle sheath' cells around Rubisco enzyme. This feature will enable plants to retain water via its ability to continue fixing CO₂ while stomata are closed.

Examples of C3 and C4 plants

C3 plants C4 plants

Rice (Oryza sativa) Maize (Zea mays)

Barley (Hordeum vulgare) Sugarcane (Saccharum officinarum)

Soybean (Glycine max ) Sorghum (Sorghum bicolor)