What is the C4 pathway

Table of Contents

Hatch-Slack Cycle

It was first discovered by scientists named Hatch and Slack (1965).

This is also a cyclic process of fixing CO2 similar to the C3 cycle. But it is found in some special plants.

These plants are called C4 plants. These types of plants are found in dry and tropical areas.

This type of plants evolved (C4 plants evolve) to avoid photorespiration and carbon productivity of such plants is very high (productivity is higher than C 3 plants).

These plants are called C4 plants and this pathway is called C4-cycle because the first stable intermediate in this process is a 4-carbon organic acid (C4-Carbon Containing Acid), which is called Oxalo Acetic Acid.

You will remember in the previous post that we talked about C3-cycle in the same way, where the first stable organic intermediate compound is 3 carbon, which is called Phosphoglycerate in C3 Cycle and such plants are called C3- plants. hence this cycle is called C3-cycle.

It is important to note here that all plants will always use the C3-cycle to make glucose (either C3 or C4 Plants).

Mechanism of C4 Cycle

In this process the fixation of CO2 takes place twice, which means the process of carboxylation takes place twice.

The first time the fixation of CO2 takes place in the mesophyll cell, and the second time in the bundlesheath cell during the C3 cycle.

This primary CO2 acceptor molecule is 3 carbon Phosphoenol pyruvate, (Phosphoenol – Pyruvate PEP) which is present in the mesophyll cell.
Phosphoenyl pyruvate bonds with CO2 to form the first intermediate compound, which has 4 carbons. Its name is oxaloacetic acid. This process is catalysed by the enzyme PEP-carboxylase.

After formation of C4-acid, it is transported to the bundle sheath cell, its plasmodesmata (Plasmodesmata connection between Mesophyll & bundle sheath cells) is used.

The C4-acid reaches the bundle sheath cell, is converted to malic acid, and is then decarboxylated.

In decarboxylation, CO2 gas is released, which enters the C3 cycle running in bundles. And as we understood in the previous post, glucose is formed.

It is important to note one thing here that the RuBisCO enzyme is present in the bundle-leaf cell and the phosphoenyl carboxylase enzyme is present in the mesophyll cell.

The entire process of C4 cycle consumes 30 ATP and 12 NADPH.

Fixing CO2 means that it takes 5 ATP and 2 NADPH to fix each CO2 molecule.

Discovery Of C4 Cycle

Its initial discovery was done by Kortschak and his colleagues (Kortschak et al – 1965), that is why it is also called HSK-pathway (Hatch-Slack-Kortschak pathway).

They observed that radioactive CO2 (Radioactive Carbon dioxide where carbon is labelled) was first assimilated in the four carbon compound Oxalo acetic acid (OAA) in sugarcane leaves.

Hatch and Slack found that this is a regular process of CO2 fixation that is found in many tropical plants. For example – Maize, Sugarcane, Sorghum, Salsola, Antriplex, Pennisetum, Panicum, Amaranthus.

In monocot and dicot plants like Pennisetum and Panicum, it has two essential enzymes (RuBisCO & PEP carboxylase) Phosphoenol carboxylase enzyme and RuBisCO, which complete this process.

Characteristics of C4 plants

C4 plants grow mostly in shallow climates. Photorespiration does not occur in them. Maize, Sorghum, Pineapple & Sugarcane, Salsola, Penniperature are some of the main C4 plants.

There are two types of chloroplasts found in C4 plants. One is those which are in the mesophyll cell, and they do not have granum (thylakoid or granum is absent in chloroplast), meaning they are called Agranum Chloroplast.

Secondly, those chloroplasts which contain granum, and that granum is called chloroplast and it is found in bundle sheath cells.

Dimorphism is found in this type of chloroplast, which allows the process of photorespiration (photorespiration is absent in C4 plants).

A special type of anatomy is found in such plants, which is called Kranz Anatomy, where large cells are found around the vascular bundles. Which are called Bundle Sheath Cells.

Many layers of bundle cold cells surround the vascular bundle. A large number of chloroplasts are found in them.

The cell wall of these cells is thick. Which is impermeable to oxygen.

Hence no gaseous exchange takes place and no intercellular space is found between them.

High amount of glucose is found in these. Because their productivity is high.

What is kranz Anatomy?

Kranz anatomy refers to a specialized anatomical arrangement of cells found in the leaves of C4 plants. It is characterized by a ring-like arrangement of bundle sheath cells surrounding the vascular tissue, typically the veins. This structural adaptation is crucial for the efficient functioning of the C4 photosynthetic pathway.

The bundle sheath cells in kranz anatomy are distinct from the mesophyll cells, which are located between the upper and lower epidermis of the leaf. The mesophyll cells are responsible for the initial fixation of CO2 using the enzyme PEP carboxylase, forming a four-carbon compound.

The bundle sheath cells are positioned tightly around the vascular bundles, which contain xylem and phloem vessels. They serve as a protective layer around the veins and are responsible for the subsequent steps of the C4 photosynthetic pathway.

The CO2 released by the mesophyll cells diffuses into the bundle sheath cells, where it is further fixed by the enzyme RuBisCO to form a three-carbon compound.

The close proximity of the bundle sheath cells to the veins allows for efficient transport of these three-carbon compounds, called malate or aspartate, from the bundle sheath cells to the mesophyll cells.

The malate or aspartate is then decarboxylated in the mesophyll cells, releasing CO2 for the Calvin cycle, while the three-carbon compound is regenerated in the bundle sheath cells.

This spatial separation of the initial CO2 fixation in the mesophyll cells and the subsequent CO2 release and Calvin cycle in the bundle sheath cells minimizes photorespiration, increases the concentration of CO2 around RuBisCO, and enhances the efficiency of carbon fixation in C4 plants.

Kranz anatomy plays a crucial role in optimizing the C4 pathway and contributes to the higher productivity and water-use efficiency observed in C4 plants compared to C3 plants.

Advantage of Kranz Anatomy in C4 Plants

Kranz anatomy is highly advantageous for C4 plants because it facilitates the efficient functioning of their unique C4 photosynthetic pathway. Here are some reasons why Kranz anatomy is useful for C4 plants:

1. Spatial separation of carbon fixation and Calvin cycle: Kranz anatomy allows for the spatial separation of the initial CO2 fixation and subsequent CO2 release and Calvin cycle.

The mesophyll cells, where CO2 is initially fixed into a four-carbon compound, are located away from the bundle sheath cells surrounding the veins.

This separation prevents direct contact between RuBisCO, the enzyme responsible for CO2 fixation, and oxygen, reducing the occurrence of wasteful photorespiration. By concentrating CO2 in the bundle sheath cells, C4 plants can efficiently fix carbon and minimize energy loss.

2. CO2 concentration mechanism: The bundle sheath cells in Kranz anatomy play a crucial role in concentrating CO2 around RuBisCO. They have a high density of mitochondria and chloroplasts, enabling efficient metabolism and uptake of CO2.

This localized CO2 concentration enhances the efficiency of RuBisCO in fixing CO2, even at low atmospheric CO2 levels.

The concentrated CO2 also favors the reaction of RuBisCO with CO2 rather than oxygen, minimizing photorespiration and maximizing carbon fixation efficiency.

3. Effective transport of metabolites: The close proximity of bundle sheath cells to the veins ensures efficient transport of metabolites between the mesophyll and bundle sheath cells.

The three-carbon compounds (malate or aspartate) produced in the mesophyll cells during CO2 release can readily diffuse to the adjacent bundle sheath cells through plasmodesmata or specialized transporters.

This transport allows the three-carbon compounds to be decarboxylated in the mesophyll cells, releasing CO2 for the Calvin cycle, while the regenerated three-carbon compounds can be transferred back to the bundle sheath cells for further CO2 fixation.

This coordination between mesophyll and bundle sheath cells enables an efficient exchange of metabolites, enhancing the overall efficiency of carbon fixation in C4 plants.

4. Reduced water loss: The spatial arrangement of Kranz anatomy, with the bundle sheath cells surrounding the veins, allows for efficient water use.

C4 plants can partially close their stomata during hot and dry conditions while still maintaining CO2 uptake through the concentrated CO2 in the bundle sheath cells. This reduces water loss through transpiration, making C4 plants more adapted to arid and hot environments.

Overall, Kranz anatomy provides C4 plants with an optimized anatomical structure that supports efficient CO2 fixation, minimizes photorespiration, enhances carbon concentration, facilitates metabolite transport, and reduces water loss.

These advantages contribute to the higher productivity and water-use efficiency observed in C4 plants compared to C3 plants.


We have tried to cover all the points related to C4 cycle, and have also shared important questions at the end.

But still any kind of suggestion or update and if you see any mistake, then you must tell us.

We will try our best to update your suggestion or any mistake which happened somewhere in the post.

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