The potential difference across the membrane of a relaxed
muscle fiber is called resting potential. It amounts to about

Options

A. `-70 mV`
B. `50 mV`
C. `100m V`
D. `50- 100mV`

Answer

### Step-by-Step Solution:

1. **Understanding Resting Potential**: Resting potential refers to the potential difference across the membrane of a muscle fiber when it is at rest. It is crucial for the functioning of muscle fibers and neurons.

2. **Characterization of Resting Potential**: The resting potential is characterized by a negative charge inside the cell (intracellular fluid) and a positive charge outside the cell (extracellular fluid). This difference in charge is essential for the excitability of muscle fibers.

3. **Role of Sodium-Potassium Pumps**: The resting potential is maintained by the sodium-potassium pump, which actively transports ions across the cell membrane. Specifically, it expels 3 sodium ions (Na⁺) from the intracellular fluid to the extracellular fluid and brings in 2 potassium ions (K⁺) from the extracellular fluid to the intracellular fluid.

4. **Creation of Charge Difference**: The activity of the sodium-potassium pump results in a higher concentration of sodium ions outside the cell and a higher concentration of potassium ions inside the cell. This ion distribution creates a positive environment outside the cell and a negative environment inside the cell.

5. **Immobile Negatively Charged Proteins**: Additionally, the presence of immobile negatively charged proteins inside the cell contributes to the negative charge of the intracellular fluid, further enhancing the negative resting potential.

6. **Final Value of Resting Potential**: The combined effects of the sodium-potassium pump activity and the presence of negatively charged proteins lead to a resting potential of approximately -70 millivolts (mV) inside the muscle fiber.

### Final Answer:
The potential difference across the membrane of a relaxed muscle fiber, known as resting potential, amounts to about **-70 mV**.

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