Across the cell membrane of a skeletal muscle cell in the resting state with a non electrogenic pump (1 Na+ : 1K+)
**Core Concept**
The question is testing the understanding of the sodium-potassium pump and its role in maintaining the resting membrane potential in skeletal muscle cells. The sodium-potassium pump is a non-electrogenic pump that uses ATP to transport one sodium ion out of the cell and one potassium ion into the cell against their concentration gradients.
**Why the Correct Answer is Right**
The sodium-potassium pump (Na+/K+ ATPase) is an enzyme that plays a crucial role in maintaining the resting membrane potential of skeletal muscle cells. It uses energy from ATP to transport 3 sodium ions out of the cell and 2 potassium ions into the cell, resulting in a net movement of 1 sodium ion out and 1 potassium ion in. This movement helps to maintain the resting membrane potential by reducing the concentration of positively charged sodium ions inside the cell and increasing the concentration of positively charged potassium ions outside the cell.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the sodium-potassium pump is a non-electrogenic pump, meaning it does not directly move charges across the membrane. It uses ATP to transport ions against their concentration gradients, but it does not generate a net movement of charges.
* **Option B:** This option is incorrect because the sodium-potassium pump is not an electrogenic pump. An electrogenic pump would directly move charges across the membrane, generating a net movement of positively or negatively charged ions.
* **Option C:** This option is incorrect because the sodium-potassium pump transports one sodium ion out and one potassium ion in, resulting in a net movement of 1 sodium ion out and 1 potassium ion in. This is not a 1:1 exchange.
**Clinical Pearl / High-Yield Fact**
The sodium-potassium pump is an essential component of the resting membrane potential, and its dysfunction can lead to various muscle disorders, including hypokalemia and hyperkalemia. Understanding the role of the sodium-potassium pump is crucial for diagnosing and managing these conditions.
**Correct Answer: D.**