Problem 165 - Objective Biology for NEET

Question

Assertion: Calvin cycle is referred as \(\mathrm{C}_{3}\) pathway Reason: In Calvin cycle the first product of \(\mathrm{CO}_{2}\) fixation is \(\mathrm{a} \mathrm{C}_{3}\) acid (PGA).

Step-by-Step Solution

Answer

The Calvin cycle is referred to as the 'C3' pathway because the first product of carbon dioxide fixation is a C3 acid, 3-phosphoglyceric acid (PGA). Hence both the assertion and reason are correct and the reason is the correct explanation for the assertion.

Step 1: Verification of the Statement

The Calvin cycle is indeed often referred to as the C3 pathway. This is because the first stable product of the Calvin cycle, after CO2 has been absorbed, is a three-carbon compound, or a 'C3' compound.

Step 2: Confirmation of the Reason

The reason given about the carbon fixation product in the Calvin cycle is correct. The first stable product of CO2 fixation is a C3 acid known as 3-phosphoglyceric acid (abbreviated as 3-PGA or PGA).

Step 3: Conclusion

Therefore, it can be concluded that because the first stable product of the Calvin cycle is a 'C3' acid (PGA), the Calvin cycle is indeed oftentimes referred to as the 'C3 pathway', confirming both the assertion and reason given in the exercise.

Key Concepts

C3 Pathway

The C3 pathway, commonly known as the Calvin cycle, plays a crucial role in the process of photosynthesis, specifically in the second stage called the light-independent reactions. These reactions occur within the stroma of chloroplasts, where atmospheric carbon dioxide (CO2) is converted into glucose, thus sustaining plant life.

In the C3 pathway, the enzyme Rubisco facilitates the incorporation of CO2 into a five-carbon sugar, ribulose bisphosphate (RuBP), and splits it into two molecules of 3-phosphoglyceric acid (3-PGA). This series of biochemical reactions is termed 'C3' because the immediate fixation product is a three-carbon compound, 3-PGA.

Each turn of the cycle incorporates one molecule of CO2 and requires the energy carriers ATP and NADPH, produced during the light-dependent reactions of photosynthesis. Multiple rounds of the cycle are needed to produce a single six-carbon glucose molecule. This pathway is fundamental to many plants, and understanding the C3 cycle can provide insights into how plants convert CO2, a simple inorganic molecule, into complex organic substances necessary for growth and survival.

CO2 Fixation in Photosynthesis

CO2 fixation is the process by which inorganic CO2 is attached to an organic molecule within a plant cell. This event marks the beginning of the process of converting CO2 into glucose, a form of stored energy that can be used by the plant and ultimately other organisms in the ecosystem.

In the context of photosynthesis, CO2 fixation takes place during the Calvin cycle, which is comprised of three main phases: carbon fixation, reduction, and regeneration of RuBP. The first phase, carbon fixation, relies heavily on the enzyme Rubisco, which connects CO2 to RuBP to form 3-PGA. The subsequent stages utilize ATP and NADPH to convert 3-PGA into glyceraldehyde 3-phosphate (G3P), from which glucose and other sugars can be synthesized.

Importance of CO2 Fixation

CO2 fixation is essential not only for the survival of plants but also for the global carbon cycle. Plants act as a significant carbon sink, removing CO2 from the atmosphere and helping to moderate global climate. Enhancing and understanding CO2 fixation therefore has implications for agricultural productivity and climate change mitigation strategies.

3-Phosphoglyceric Acid (3-PGA)

The molecule 3-phosphoglyceric acid (3-PGA) is of paramount importance in the Calvin cycle. It is the first stable product of CO2 fixation and is a three-carbon compound, from which the term 'C3 pathway' is derived.

3-PGA serves as a precursor to other organic molecules during the Calvin cycle. After its formation, 3-PGA undergoes a series of reactions using ATP and NADPH to be transformed into glyceraldehyde 3-phosphate (G3P). G3P can then be further processed to form various end products such as glucose, starch, and cellulose, which are the building blocks for plant structure and energy storage.

Role in the Calvin Cycle

The significance of 3-PGA extends beyond its role as an immediate product of CO2 fixation. It also marks the start of the energy consumption phase in the Calvin cycle, leading to the synthesis of high-energy sugars. Due to its pivotal role in the biosynthesis of carbohydrates, 3-PGA is central to the metabolism of not only plants but virtually all life on Earth that depends on photosynthesis for the creation of organic compounds.