Calcium is pumped back into sarcoplasmic reticulum by
**Core Concept**
The sarcoplasmic reticulum (SR) is a specialized form of smooth endoplasmic reticulum found in muscle cells, playing a crucial role in regulating muscle contraction and relaxation. Calcium ions (Ca2+) are pumped back into the SR through a specific pump, enabling the muscle to relax.
**Why the Correct Answer is Right**
The correct pump responsible for pumping Ca2+ back into the SR is the **Sarcoplasmic Reticulum Calcium-ATPase (SERCA)**. This enzyme utilizes the energy from ATP hydrolysis to transport Ca2+ ions back into the SR, reducing the cytosolic concentration of Ca2+ and facilitating muscle relaxation. This process is essential for the proper functioning of skeletal and cardiac muscle cells.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the pumping of Ca2+ back into the SR. Other pumps, such as the sodium-calcium exchanger (NCX), can also influence cytosolic Ca2+ levels but are not responsible for pumping Ca2+ back into the SR.
**Option B:** This option is incorrect as it refers to a different cellular process. The sodium-potassium pump (Na+/K+-ATPase) is essential for maintaining the resting membrane potential and regulating the concentration of sodium and potassium ions in the cell, but it is not involved in Ca2+ pumping.
**Option C:** This option is incorrect as it is not directly involved in the pumping of Ca2+ back into the SR. The ryanodine receptor (RyR) is a channel that releases Ca2+ from the SR, but it is not responsible for pumping Ca2+ back into the SR.
**Option D:** This option is incorrect as it refers to a different cellular process. The calcium channel (L-type calcium channel) is involved in the influx of Ca2+ into the cell, but it is not responsible for pumping Ca2+ back into the SR.
**Clinical Pearl / High-Yield Fact**
SERCA is an essential enzyme for maintaining proper muscle function, and its dysfunction can lead to muscle diseases such as muscular dystrophy. Understanding the role of SERCA in regulating Ca2+ levels is crucial for diagnosing and treating muscle disorders.
**Correct Answer: C. Sarcoplasmic Reticulum Calcium-ATPase (SERCA)**