C3 and C4 Pathways

Plants use photosynthesis to convert light energy into chemical energy. In this process, plants produce glucose by using atmospheric carbon dioxide and water in the presence of sunlight. As a result, oxygen is released in the atmosphere as a by-product.
Furthermore, the whole photosynthesis process is divided into two broad phases, photochemical and biosynthetic. C3 and C4 pathway, the topic of this article are two ways of biosynthetic process. In a biosynthetic process, plants use water and carbon dioxide to produce carbohydrate.

  • C3 Pathway

Also known as the Calvin cycle, Melvin Kelvin first discovered this process. Majority of the plants on the earth uses this process. Furthermore, the first stable product produced by this process is three compound carbon. Hence, the name C3 pathway.
Moreover, this cycle occurs in three steps, these are –

  • Carboxylation – This step fixates CO2 into a steady organic intermediate. Here CO2 is used to carboxylate RuBP. RuBisCO enzyme catalyses this process.
  • Reduction – This procedure leads to the creation of glucose. It involves two molecules of NADPH for reduction and two molecules of ATP for phosphorylation.
  • Regeneration – Regeneration of CO2 acceptor molecule is necessary for a C3 pathway to continue its operations.
  • C4 Pathway

C4 pathway of photosynthesis occurs before the C3 pathway. Even though the C3 cycle is found in every plant, the C4 pathway is primarily noticeable in plants of the tropical region.
Furthermore, this process is also known as the Hatch and Slack pathway. The first stable product of this process is a four-carbon compound, hence the name.
Moreover, there are two C4 cycle steps which every plant follow; these are –

  • At first, when CO2 enters a mesophyll cell, phosphoenolpyruvate (PEP) carboxylase enzymes adds it to the three-carbon PEP. This produces a four-carbon compound called oxaloacetate. This substance is then turned into an organic acid called malate.
  • Subsequently, malate is conveyed to the bundle sheath cells deep into the leaf where the oxygen concentration is low. Then malate is disintegrated, which releases a molecule of CO2. This CO2 enters the Calvin cycle or C3 pathway afterwards where, the rubisco enzyme transforms it into sugar.

Furthermore, the C4 cycle in plants is useful in hot and arid conditions as plants close their stomata to save water.
Before moving forward, here are some C4 cycle notes for a quick recap –

  1. C4 cycle is not seen in every plant.
  2. C3 and C4 pathways are not same. They have different purpose and characteristics.
  3. C4 cycle is primarily seen in plants in the tropical region.
  4. Moreover, this process helps plants to conserve water.
  5. Examples of C4 pathway plants are crabgrass, corn, sugarcane, etc.

Difference Between C3 and C4 Pathway
Some of the key differences between these two pathways are –

  • First stable compound

The first compound produced in a C3 cycle is a 3-carbon substance named 3-phosphoglyceric acid. On the other hand, in C4 pathways, the primary stable compound is a 4-carbon compound known as oxaloacetate acid.

  • Presence in plants

C3 cycle is found in every plant. But C4 pathway diagram is found primarily in plants of the tropical region.

  • Fundamental carbon dioxide acceptor

In the case of C3, it is Ribulose Bi Phosphate (RUBP). For C4, it is phosphoenolpyruvate (PEP).

  • Carboxylase enzyme

In C4 plants it is PEP carboxylase and rubisco. However, in the case of C3, it is only rubisco.

  • Carbon fixation

Only one carbon fixation occurs in a C3 cycle. In the C4 cycle, double carbon fixation occurs.

FAQ (Frequently Asked Questions)

  1. What are the C3 and C4 cycle?

C3 and C4 cycle are two parts of the overall photosynthesis process. More preciously, these are two sections of the biosynthesis process. In this process, plants use water and carbon dioxide to make carbohydrate.

  1. What is C4 pathway cycle?

C4 pathway cycle is a part of the biosynthesis process, and it occurs before the C3 period. This process is also known as the Hatch and Slack pathway. The first stable product of this process is a four-carbon compound (oxaloacetate acid), hence the name.

  1. Which plants use the C4 cycle?

Notably, plants from the tropical region use this process. Owing to lack of water in such harsh conditions, these plants use this process to conserve water.

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