C4 Pathway in Plants

Introduction

The C4 pathway in plants  is also known as the Hatch and Slack pathway; is a primary stage of the Calvin cycle (C3) in some plants. It is named after M.D. Hatch and C.R Slack, who first discovered this phenomenon in specific plants in 1960. The C4 pathway in plants is the step they have adapted, to lessen the impact of photorespiration.

Photorespiration is a phenomenon that is found in all plants. Most plants, however, are C3 plants. This means that they do not have the essential features to combat the wasteful photorespiration pathway. But, there are some plants known as C4 plants and CAM Plants that can minimise the effects of photorespiration by adapting a unique way out of it. This enables them to fix atmospheric carbon dioxide more efficiently.  

Before we move to the process of the C4 pathway in plants, let’s go over a few important concepts intrinsic to plants.  

C3, C4, and CAM plants

C3 Plants

C3 plants photosynthesize by using the Calvin cycle only. All plants are C3 plants. 

C4 Plants

The photosynthesis in C4 plants happens by first creating a 4 carbon compound which later breaks into 3 carbon compounds for the Calvin cycle (C3) to take place. Sugarcane is an example of a C4 Plant. 

CAM Plants (crassulacean acid metabolism)

These store sunlight during the day and fix the carbon dioxide at night. This effectively separates the time of carbon dioxide fixation and the Calvin cycle. So, it avoids the process of photorespiration. Cactus is a prime example of CAM plants.

Photosynthesis

In photosynthesis ,plants use water, carbon dioxide, sunlight and chlorophyll to create energy in the form of sugar (glucose). It is done through cellular respiration that helps the plants to grow and fuel their activities. In this process, plants release the excess oxygen into the air as a byproduct of the photosynthesis process. 

The chemical equation of Photosynthesis is as follows:   (Sunlight) 6CO2 +6H2O———————->C6H12O6+6O2

During photosynthesis, plants use sunlight as the source of energy to prepare chemical energy as food to sustain themselves. The carbon dioxide that is changed to glucose in the process of photosynthesis also makes starch and cellulose in plants. 

Photorespiration occurs at times, resulting in plants giving out carbon dioxide in the air instead of oxygen. 

Photorespiration

Simply put, photorespiration is a roadblock in the process of photosynthesis. It occurs when RuBisCO, (Ribulose-1, bisphosphate carboxylase oxygenase) a key enzyme in the photosynthesis process picks up oxygen instead of carbon dioxide as a substrate (the substance that an enzyme acts on).  

It leads to plants wasting essential energy required for the process of photosynthesis and decreases the overall efficiency of the plants to convert light energy into chemical energy. Photorespiration is known as the opposite of photosynthesis because it doesn’t produce either ATP or NADPH. 

ATP (adenosine triphosphate) is the main molecule that stores and transfers energy in plant cells. Plants store this energy in these ATP molecules for their growth and sustenance. 

Similarly, NADPH (nicotinamide adenine dinucleotide phosphate hydrogen) is a product of the initial stage of the photosynthesis process and acts as a fuel to facilitate the reactions that take place in the second part of the photosynthesis process. 

They are both vital to the whole process of photosynthesis. 

Characteristics of C4 Plants

C4 plants are found in hot, dry weather and grow in tropical environments. They have a phenomenon called the Kranz Anatomy which is unique to them. Kranz means rings or wreath-like structures. It is a very important feature of C4 plants. 

In these C4 plants, photosynthesis happens even when the stomata are closed. Some popular examples are sugarcane, switchgrass, millet, corn. C4 plants work best in intense heat situations. 

Process of C4 Pathway In Plants

The gasses in plants move in and out through small pores called stomata located on the underside of the leaves. When the environmental conditions are hot, the stomata close to reduce the loss of water vapors. 

Invariably, this leads to a diminished supply of carbon dioxide for the plants. Plants have found innovative ways of fixing carbon dioxide before entering the Calvin cycle phase. The pathways to fix this carbon dioxide are known as the C4 and CAM pathways. 

Let’s look at the C4 pathway in plants and how they effectively implement it to fix the low concentrations of carbon dioxide. 

The C4 plants have evolved to make use of this pathway to optimise the storing of CO2. The first step of the C4 pathway starts with PEP (phosphoenolpyruvate). This is a three-carbon molecule and the primary carbon dioxide acceptor for the C4 pathway. 

Through an intricate process, the plants fix carbon dioxide into oxaloacetate (OAA) which is a four-carbon compound. This takes place in the mesophyll cells of the plants by the non-rubisco enzyme known as PEP. 

The oxaloacetate is then converted to malate which is transported to bundle sheath cells in the plants. When inside the bundle sheaths, this malate breaks down releasing the molecule of carbon dioxide. The carbon dioxide is then made into sugars by RubisCO with the Calvin cycle in the normal photosynthesis process. The use of the C4 pathway is generally seen in plants growing in hot habitats.

Key points of the C4 Pathway at a glance

1) First, the atmospheric CO2 is converted into a three-carbon compound Phosphoenolpyruvate carboxylase (PEP). This is a non-rubisco enzyme and doesn’t bind oxygen. That is then fixed into oxaloacetate. 

2) The oxaloacetate (OAA) is converted  into malate and transported to the chlorophyll of the bundle sheath cells from the mesophyll cells.

3) Once inside the chloroplasts of the bundle sheath cells, the malate breaks up to produce carbon dioxide, pyruvate, and NADPH. Then the carbon dioxide combines with the RubisCO and is made into sugar through the Calvin cycle or C3. 

4)The pyruvate that is produced in the bundle sheath cells goes back into the mesophyll cells. Reacting with the ATP, it converts into PEP which is the initial starting point of the C4 cycle. 

Conclusion

Plants adapted to the C4 pathway due to the diminished supply of CO2 in hot areas. The C4 Pathway In plants is the process by which they efficiently fix carbon dioxide at low concentrations. This enables them to reduce photorespiration, and expedite the process of photosynthesis.