Problem 166 - Objective Biology for NEET

Question

Assertion: To make one molecule of glucose 6 turns of the Calvin cycle required Reason: One turn of Calvin cycle fix one molecule of \(\mathrm{CO}_{2}\)

Step-by-Step Solution

Answer

The assertion is correct. To make one molecule of glucose, which has six carbon atoms (C6), six turns of the Calvin cycle are required since each turn of the cycle fixes one molecule of CO2.

Step 1: Understanding the assertion and the reason

The assertion is about how many turns of the Calvin cycle are required to produce a glucose molecule. It is stated as six. The reason provided is that each turn of the Calvin cycle fixes one molecule of CO2.

Step 2: Validate the assertion based on the reason

The Calvin cycle indeed uses one molecule of carbon dioxide in each turn. As per this information, since a glucose molecule consists of six carbons, using one carbon atom from each turn (each CO2 molecule), it would require six turns of the Calvin cycle to produce one glucose molecule.

Step 3: Conclusion

Since each turn of the Calvin cycle incorporates one molecule of carbon dioxide (which has one carbon atom), and since glucose requires six carbon atoms, it is accurate to say that six turns of the Calvin cycle are required to produce one molecule of glucose from CO2.

Key Concepts

Photosynthesis

Photosynthesis is the fundamental process by which plants, algae, and certain bacteria convert light energy into chemical energy. It is essentially solar power conversion taking place in the chloroplasts of plant cells, where sunlight is harnessed to transform carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂).

The process encompasses two main stages: the light-dependent reactions and the Calvin cycle, also known as light-independent reactions. The light-dependent reactions generate ATP and NADPH, which are then utilized in the Calvin cycle for the synthesis of glucose. Understanding this process is crucial for comprehending how plants grow and how they play a vital role in maintaining the balance of oxygen and carbon dioxide in Earth's atmosphere.

Carbon Fixation

Carbon fixation is a process central to the Calvin cycle, taking place within the stroma of the chloroplasts. During carbon fixation, CO₂ from the atmosphere is attached to a five-carbon sugar called ribulose bisphosphate (RuBP). This reaction is catalyzed by an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO.

The resulting six-carbon compound is unstable and quickly breaks down into two three-carbon molecules of 3-phosphoglycerate (3-PGA). Carbon fixation is the first step in the transformation of inorganic carbon into organic forms, which is essential for life on Earth. The Calvin cycle repeats this process multiple times to accumulate enough fixed carbon to move on to the next stage of glucose synthesis.

Glucose Synthesis

Glucose synthesis in the Calvin cycle is a complex process involving the transformation of 3-PGA through a series of reactions using ATP and NADPH from the light-dependent reactions. The end goal is to produce the six-carbon sugar, glucose.

As previously stated, each turn of the Calvin cycle fixes one molecule of CO₂. Since a glucose molecule contains six carbon atoms, this step has to be repeated six times to produce one molecule of glucose. This is a simplification, as the actual process involves not just the carbon-fixation step, but also reduction and regeneration of the CO₂ acceptor, RuBP. Through these stages, the fixed carbon is gradually built up into a glucose molecule, which the plant can then use for energy or convert into other carbohydrates for storage.