Understanding the Difference between the C3 and C4 Cycles
Photosynthesis is the process by which plants convert light energy into chemical energy to produce food for themselves. The carbon fixation process within photosynthesis occurs through either the C3 or C4 cycle. The C3 cycle is present in the majority of plants, while the C4 cycle is present in some particular plant species such as maize, sugarcane, and sorghum. In this article, we’ll discuss the differences between these two cycles.
The C3 Cycle
The C3 cycle, also called the Calvin Cycle, takes place in the stroma of the chloroplast. It is named after the first molecule produced during carbon fixation, which has three carbon atoms. During this cycle, carbon dioxide enters the leaf through small pores called stomata and is stored in the spongy mesophyll cells of the leaf.
In the C3 cycle, carbon dioxide is fixed into a three-carbon compound called 3-phosphoglyceric acid. This reaction is catalyzed by an enzyme known as RuBisCO. The 3-phosphoglyceric acid then undergoes various enzymatic reactions that ultimately produce the energy-rich molecule, glucose.
Generally, the C3 cycle is slower than the C4 cycle, limiting plant productivity during arid environments where the available carbon dioxide levels decrease.
The C4 Cycle
Unlike the C3 cycle, the C4 cycle takes place in two types of cells, namely the mesophyll cells and the bundle sheath cells, both of which possess chloroplasts. In the first stage, carbon dioxide is captured by an enzyme called PEP carboxylase in the mesophyll cells to form a four-carbon compound called oxaloacetate. This process is known as “Carbon Dioxide Concentration Mechanism.”
The four-carbon compound is then converted to malate or aspartate, which is then transported to the bundle sheath cells. Here, decarboxylation occurs, releasing carbon dioxide that will be used in the C3 cycle. The C4 cycle allows plants to continue photosynthesis even at low carbon dioxide concentrations, meaning plants can thrive in arid environments.
Conclusion
While both the C3 and C4 cycles play a crucial role in the process of photosynthesis, the C4 cycle provides a distinct advantage for plants present in arid regions. Plants possessing the C4 cycle have high productivity, better water-use efficiency, and can tolerate high temperatures. Understanding these differences is crucial to comprehend the different strategies plants use in carbon fixation, ultimately helping researchers develop better crop yields and addressing food shortages in areas that do not have access to adequate water resources.
Table difference between c3 and c4 cycle
| Feature | C3 cycle | C4 cycle |
|---|---|---|
| Plants that perform cycle | Most plants, including wheat, rice, and soybeans | Certain plants, including corn, sugarcane, and sorghum |
| Location of cycle | Occurs in mesophyll cells of plant leaves | Occurs in mesophyll and bundle sheath cells of plant leaves |
| CO2 fixation | CO2 is fixed directly into a 3-carbon compound (PGA) | CO2 is fixed into a 4-carbon compound (oxaloacetic acid) before being converted to a 3-carbon compound (PGA) |
| Enzyme responsible for CO2 fixation | Rubisco | PEP carboxylase |
| Efficiency in hot and dry conditions | Less efficient due to photorespiration | More efficient due to concentrating CO2 in the bundle sheath cells and reducing photorespiration |