C4 And CAM pathway: Meaning, Process In C3 C4 And CAM Plants

Overview Of Photosynthesis

Photosynthesis is the biological process through which green plants and other autotrophs synthesize glucose through the conversion of light energy to chemical energy. It is a fundamental biological process/ through which photosynthesis in bacteria manufactures glucose using only carbon dioxide and water, with the gaseous release of molecular oxygen. The general chemical equation for photosynthesis can be summarized as follows:

6CO2+6H2O→C6H12O6+6O2

In this, plants use sunlight as the ultimate energy source in driving this important process, which has two major phases: a photochemical phase and a biosynthetic phase. The former is involved in the production of ATP and NADPH through light, while the latter is concerned with the biosynthesis of glucose through carbon fixation.

Importance Of Carbon Fixation

In the photosynthetic biosynthetic phase, carbon dioxide reacts with water to form carbohydrates, mainly sugar molecules. As this is the carbon-fixation reaction, diversity between the plants resulted in the fundamental division of plants into two major pathways: C3 and C4.

How The C3 Pathway (Calvin Cycle) Functions

The Calvin cycle, or the C3 pathway, is the most widespread carbon fixation pathway in plants. It is a 3-carbon acid, and the first stable product is the 3-phosphoglyceric acid (PGA). The Calvin cycle has three major steps: carboxylation, reduction, and regeneration.

• Carboxylation: In carboxylation, the enzyme RuBisCo catalyzes the reaction of carbon dioxide with RuBP to yield two molecules of PGA.

• Reduction: The ATP and NADPH produced in the light-dependent reactions are used in the phosphorylation and reduction of PGA to form glyceraldehyde-3-phosphate (G3P).

• Regeneration: A regeneration step in which some of the G3P molecules are used in generating more molecules of the acceptor molecule by combining with RuBP, whereas the rest of the G3P molecules are used to generate glucose and other carbohydrates.

Some of the common C3 plants are beans, rice, wheat, and potatoes, which grow in temperate regions.

Understanding The C4 Pathway (Hatch And Slack Pathway)

The C4 pathway is also known as the Hatch and Slack pathway. A photorespiratory escape mechanism is a rescue pathway developed by some plants on a normal mode of distribution and operation found in their tropical and arid regions. In the process, the first stable product of carbon fixation, is a 4-carbon compound, oxaloacetic acid (OAA).

Mechanism Of The C4 Pathway

The main C4 pathway employs a 3-carbon molecule- phosphoenolpyruvate (PEP) as the primary CO2 receptor molecule. This pathway is of the biphasic type and takes place through the subsequent steps.

• Carboxylation: The reaction of PEP with CO2 resulting from PEP carboxylase gives rise to oxaloacetic acid (OAA).

• Decarboxylation: OAA is further decarboxylated to produce malic acid or aspartic acid and then transferred to the bundle sheath cells from the mesophyll cells.

• Decarboxylation: In the bundle sheath cells, malic acid undergoes decarboxylation to release carbon dioxide, which is subsequently released in the Calvin cycle. Then, the 3-carbon molecule is diffused back to the mesophyll cells to regenerate PEP.

Examples of C4 plants are corn, sugarcane, and some bushes, favouring high light intensity and temperatures.

Main Contrasts Between The C3 And C4 Pathways

Both of these pathways involve the Calvin cycle while having some very important differences:

• First Stable Product: The first stable product in C3 plants is PGA while in C4 plants it is oxaloacetic acid (OAA).

• Site of Calvin Cycle: The Calvin cycle in C3 plants occurs only in mesophyll cells whereby the C4 plants initially fix the carbon in mesophyll cells and then carry the Calvin cycle in bundle sheath cells.

• Efficiency of Enzymes: C4 plants employ PEP carboxylase, an enzyme with greater efficiency in terms of its affinity for carbon dioxide than RuBisCO employed by C3 plants.

• Photorespiration: C4 plants possess adaptations for preventing photorespiration, thus they are highly productive in hot and arid climates.

• Ecological Adaptation: C3 plants typically exist within cooler and wetter climates, whereas C4 plants are located within hot and bright environmental conditions.

Significance Of C3 And C4 Pathways

Such an understanding of the differences between C3 and C4 pathways is pretty useful for plant physiology, ecology, and agriculture practices. Generally, C4 plants are more efficient in terms of carbon fixation and water use and can even represent a better crop to be cultivated in harsher climates. This knowledge could also drive the crop selection and breeding strategy toward improved yield and resilience against the alteration of climate conditions.