From where is RBC membrane cholesterol replenished?

The source of RBC membrane cholesterol replenishment is the plasma. Red blood cells (RBCs) are in constant exchange with the plasma, which is rich in lipoproteins that transport cholesterol. As the membrane cholesterol depletes due to various processes, it is replenished primarily through the uptake of cholesterol from the plasma. This uptake occurs via specific receptors that facilitate the movement of cholesterol from apolipoproteins and lipoproteins circulating in the blood onto the RBC membrane.

Other sources such as mitochondria or the cytoplasm do not play a significant role in sourcing membrane cholesterol for red blood cells. The mitochondria are involved in energy production and lipid synthesis but are not directly responsible for cholesterol replenishment in the RBC membrane. Similarly, while the cytoplasm contains various lipids, it does not provide a direct pathway for replenishing membrane cholesterol in the same manner that plasma does. The EMB pathway, which involves energy metabolism, is also not related to the replenishment of membrane cholesterol.