One molecule of glucose requires 2 ATP to get phosphorylated to form Fructose 1--6 Biphosphate in glycolysis. How many ATP are used in the same process if the substrate is fructose ?

Options

A. 1
B. 2
C. 0
D. 4

Explanation

To solve the problem of how many ATP molecules are used when fructose is the substrate in the process of phosphorylation to form fructose 1,6-bisphosphate, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Process**:
- In glycolysis, glucose is phosphorylated to form fructose 1,6-bisphosphate. This process involves two main phosphorylation steps, each requiring ATP.

2. **Phosphorylation of Glucose**:
- When glucose is used as the substrate, the first phosphorylation step converts glucose to glucose 6-phosphate using 1 ATP. The enzyme involved is hexokinase.
- The second phosphorylation step converts glucose 6-phosphate to fructose 1,6-bisphosphate, which uses another ATP. The enzyme involved is phosphofructokinase.

3. **Using Fructose as the Substrate**:
- If fructose is used as the substrate instead of glucose, the first step of isomerization (which is necessary when starting from glucose) is skipped because fructose is already in the correct form.
- The first phosphorylation step will convert fructose to fructose 6-phosphate, which uses 1 ATP (again, catalyzed by hexokinase).
- The second phosphorylation step will convert fructose 6-phosphate to fructose 1,6-bisphosphate, which again uses 1 ATP (catalyzed by phosphofructokinase).

4. **Total ATP Usage**:
- Therefore, whether starting from glucose or fructose, the total ATP used in both cases is 2 ATP molecules.

### Final Answer:
**2 ATP molecules are used when fructose is the substrate to form fructose 1,6-bisphosphate.**

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