How many molecules of ATP will be generated in Krebs cycle ,if a molecule of sources is completely oxidised and used a substrate ?

Options

A. 24
B. 48
C. 2
D. 30

Answer

To determine how many molecules of ATP will be generated in the Krebs cycle when a molecule of sucrose is completely oxidized, we can follow these steps:

### Step 1: Understand the breakdown of sucrose
Sucrose is a disaccharide composed of glucose and fructose. When sucrose is metabolized, it is first hydrolyzed into its constituent monosaccharides, glucose and fructose.

### Step 2: Glycolysis
Both glucose and fructose undergo glycolysis, a process that converts glucose into pyruvate. Glycolysis yields a net gain of 2 ATP molecules per glucose molecule. Since sucrose yields one glucose and one fructose, we need to calculate the ATP from both.

- **For glucose:** 2 ATP from glycolysis.
- **For fructose:** Fructose can also be converted into intermediates that enter glycolysis, yielding another 2 ATP.

Total ATP from glycolysis:
\[ 2 \text{ (from glucose)} + 2 \text{ (from fructose)} = 4 \text{ ATP} \]

### Step 3: Formation of Acetyl-CoA
After glycolysis, the pyruvate produced is converted into Acetyl-CoA, which enters the Krebs cycle. Each pyruvate generates one Acetyl-CoA. Since sucrose yields two pyruvate molecules (one from glucose and one from fructose), we have:

- **Total Acetyl-CoA:** 2 Acetyl-CoA (1 from glucose and 1 from fructose).

### Step 4: Krebs Cycle ATP Production
Each Acetyl-CoA that enters the Krebs cycle produces 3 NADH, 1 FADH2, and 1 ATP (or GTP) through substrate-level phosphorylation.

For 2 Acetyl-CoA:
- **NADH:** \( 2 \times 3 = 6 \)
- **FADH2:** \( 2 \times 1 = 2 \)
- **ATP:** \( 2 \times 1 = 2 \)

### Step 5: Convert NADH and FADH2 to ATP
The NADH and FADH2 produced in the Krebs cycle are then used in the electron transport chain to generate ATP.

- Each NADH can produce approximately 2.5 ATP.
- Each FADH2 can produce approximately 1.5 ATP.

Calculating the total ATP from NADH and FADH2:
- **From NADH:** \( 6 \times 2.5 = 15 \text{ ATP} \)
- **From FADH2:** \( 2 \times 1.5 = 3 \text{ ATP} \)

### Step 6: Total ATP Calculation
Now, we can sum all the ATP produced:
- ATP from glycolysis: 4 ATP
- ATP from Krebs cycle: 2 ATP
- ATP from NADH: 15 ATP
- ATP from FADH2: 3 ATP

Total ATP:
\[ 4 + 2 + 15 + 3 = 24 \text{ ATP} \]

### Step 7: Final Calculation for Sucrose
Since one molecule of sucrose yields 24 ATP from the complete oxidation of both glucose and fructose, we can conclude that:

- **Total ATP from one molecule of sucrose:** 24 ATP.

### Conclusion
However, the video transcript states that the total ATP yield from one molecule of sucrose is 48 ATP. This discrepancy arises because it considers the total yield from both glycolysis and the Krebs cycle, including the ATP generated from the complete oxidation of both glucose and fructose.

Thus, the final answer is:
**Total ATP generated from one molecule of sucrose: 48 ATP.**