When 54 molecules of `CO_2` fixed by `RuBisCO` in a `C_3` plant , number of `G_3 -P` participate in regeneration phase respectively

Options

A. 90,18
B. 54,54
C. 60 , 48
D. 18, 90

Answer

To solve the question regarding the number of G3P molecules that participate in the regeneration phase when 54 molecules of CO2 are fixed by RuBisCO in a C3 plant, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Role of RuBisCO**:
RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme that catalyzes the first step of carbon fixation in the Calvin cycle of photosynthesis. It helps in fixing CO2 into an organic molecule.

2. **Calvin Cycle Overview**:
In the Calvin cycle, for every 6 molecules of CO2 fixed, 12 molecules of G3P (glyceraldehyde-3-phosphate) are produced. Out of these, 2 G3P molecules are used to form glucose, while the remaining G3P molecules are used in the regeneration phase.

3. **Calculate G3P Production**:
If 54 molecules of CO2 are fixed, we can calculate the number of G3P produced:
\[ \text{G3P produced} = \left(\frac{12 \text{ G3P}}{6 \text{ CO2}}\right) \times 54 \text{ CO2} = 108 \text{ G3P} \]

4. **Determine G3P Used in Regeneration**:
Out of the 108 G3P produced, 2 G3P are used to form glucose. Therefore, the number of G3P that participate in the regeneration phase is:
\[ \text{G3P for regeneration} = 108 \text{ G3P} - 2 \text{ G3P} = 106 \text{ G3P} \]

5. **Final Calculation**:
The regeneration phase of the Calvin cycle requires additional G3P to regenerate RuBP (ribulose bisphosphate). For every 6 G3P molecules, 1 RuBP is regenerated. Therefore, the number of G3P molecules that participate in the regeneration phase can be calculated as:
\[ \text{G3P for regeneration} = \frac{108 \text{ G3P}}{6} = 18 \text{ G3P} \]
The remaining G3P molecules (90) will be used in the regeneration phase.

6. **Conclusion**:
Therefore, the number of G3P molecules that participate in the regeneration phase is 18, and the total number of G3P produced is 90.

### Final Answer:
The correct answer is **Option D: 18, 90**.