C4 Pathway: Steps, Differences and FAQ

What is the C4 Pathway?

The C4 pathway is one of the metabolic procedures in photosynthesis that allows high efficiency in photosynthesis in certain plants, mainly of tropical origin and those growing in hot, dry climates. The C4 pathway starts by mainly fixing carbon dioxide with phosphoenolpyruvate in the mesophyll cells. This step is catalyzed by the PEP carboxylase.

Discovery – Hatch and Slack Pathway

First explained by M. D. Hatch and C. R. Slack in 1966, this process allows plants to fix carbon dioxide better than the plants having the C3 pathway in certain environmental conditions. The first reaction produces a four-carbon compound that is oxaloacetic acid. It is then diffused into the bundle sheath cells for further fixation in the Calvin cycle.

Hatch And Slack Pathway In C4 Plants

C4 is the alternate route to the C3 cycle in carbon fixation. The process gets its name "C4 cycle" from the first compound formed, which is a four-carbon molecule: oxaloacetic acid. This pathway dominates most grasses, including maize and sugarcane; the characteristic anatomy of the leaves in this pathway is called Kranz anatomy.

Kranz Anatomy – Structural Adaptation in C4 Plants

In leaves of C4 plants, each vascular bundle or rib is enclosed with a bundle sheath composed of larger parenchymatous cells. The bundle sheath cells have larger chloroplasts, lacking inter granal lamellae, and starch grains, while the mesophyll cells have small chloroplasts with grana. The anatomy is thus especially suited to increase carbon fixation efficiency due to the arrangement, so it is called Kranz anatomy from the German for "wreath".

Steps of the C4 Cycle

The C4 cycle takes four steps as follows:

Step 1 – Carboxylation (in Mesophyll Cells)

Carbon dioxide is captured in the chloroplasts of mesophyll cells by the three-carbon compound, phosphoenolpyruvate, producing oxaloacetic acid. This is made possible through the action of an enzyme called PEP carboxylase.

Step 2 – Reduction (Formation of C4 Acids)

The newly formed oxaloacetic acid is reduced by two enzymes, namely transaminase and malate dehydrogenase, to form malate and aspartate, respectively. These C4 products diffuse out from the mesophyll cells into the bundle sheath cells.

Step 3 – Decarboxylation (CO₂ Release)

This will be further cleaved by the bundle sheath cell enzymes to release free carbon dioxide and three-carbon pyruvate. The released CO₂ here will diffuse in the Calvin cycle, get combined with RuBP and get reduced into the formation of 3-PGA.

Step 4 – Phosphorylation (Regeneration of PEP)

The formed pyruvate molecules diffuse back into the mesophyll cells and in the presence of ATP get phosphorylated to re-generate phosphoenolpyruvate, which is catalyzed by the enzyme pyruvate phosphokinase.

Importance of the C4 Pathway

The C4 pathway thus confers an increased efficiency in carbon fixation—especially when photorespiration is high. From the energy point of view, it yields high growth rates for C4 plants in certain biomes as a consequence of reduced losses due to RuBisCO's oxygenase reaction. There are broad implications for agriculture also, with scientists field-testing whether key commercial crops can be genetically altered to produce C4 plants.

Energy Requirement Comparison

The C3 and C4 pathways require different amounts of energy. The table below shows the comparison between the energy required by the two pathways.

C4 Pathway NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Stages of C4 Pathway

• Kranz Anatomy

Practice Questions for NEET

Q1. Phosphoenol pyruvate (PEP) is the primary CO2 acceptor in:

1. C3 plants

2. C4 plants

3. C2 plants

4. C3 and C4 plants

Correct answer: 2) C4 Plants

Explanation:

Phosphoenolpyruvate (PEP), a three-carbon chemical found in mesophyll cells, is the main CO2 acceptor.
Bicarbonate ions are the form in which the CO2 is received.
Bicarbonate is created when carbonic anhydrase hydrates CO2.
The PEP carboxylase or PEPcase enzyme is in charge of this fixing.
It is crucial to note that the RuBisCO enzyme is absent from mesophyll cells.
The mesophyll cells produce the C4 acid OAA.
Other 4-carbon compounds, such as aspartic acid or malic acid, are subsequently formed in the mesophyll cells and transferred to the bundle sheath cells.

Hence, the correct answer is option 2) C4 plants

Q2. Which of the following enzymes catalyses primary CO2 fixation in the C4 pathway?

1. Alsolase

2. PEP carboxylase

3. RuBP carboxylase

4. Isomerase

Correct answer: 2) PEP carboxylase

Explanation:

During the C4 pathway, CO2 combines with 3 the carbon compound phosphoenol pyruvate to form 4 the carbon compound oxaloacetic acid. This reaction is catalysed by PEP carboxylase. The formation of oxaloacetic acid occurs in the mesophyll cells and helps minimize photorespiration. Oxaloacetic acid is then converted into malate or aspartate, which is transported to bundle sheath cells for further processing. This adaptation allows C4 plants, such as maize and sugarcane, to maintain high photosynthetic efficiency in arid environments.

Hence, the correct answer is option 2) PEP carboxylase

Q3. Identify the incorrect statement in relation to C4 photosynthesis

1. Kranz anatomy is an essential feature for c4 plants

2. C4 plants have higher water use efficiency than C3 plants

3. Photorespiration can be minimized when C4 the pathway is in operation

4. Conversion of oxaloacetate to malate occurs in the bundle sheath cells

Correct answer: 4) Conversion of oxaloacetate to malate occurs in the bundle sheath cells

Explanation:

The mesophyll cells, not the bundle sheath cells, in C4 plants are where oxaloacetate is converted to malate. After that, malate is sent to the bundle sheath cells, which release CO₂ in preparation for the Calvin cycle. C4 plants have Kranz anatomy, which aids in the fixation of CO2. Compared to C3 plants, C4 plants use water more efficiently. Photorespiration is reduced via the C4 pathway.

Hence, the correct answer is option 4)Conversion of oxaloacetate to malate occurs in the bundle sheath cells.

Also Read:

• MCQs on C4 Pathway

• Difference between C3 and C4 Pathway

• Cyclic Photophosphorylation