pathophysiology

The plasma membrane is a phospholipid bilayer with attached or embedded proteins. A phospholipid molecule has a polar head and nonpolar tails. When phospholipids are placed in water, they naturally form a spherical bilayer. The polar heads, being charged, are hydrophilic (attracted to water). They position themselves to face toward the watery environment outside and inside the cell. The nonpolar tails are hydrophobic (not attracted to water). They turn inward toward one another, where there is no water.

The plasma membrane keeps a cell intact. It allows only certain molecules and ions to enter and exit the cytoplasm freely. Therefore, the plasma membrane is said to be selectively permeable. Small, lipid-soluble molecules, such as oxygen and carbon dioxide, can pass through the membrane easily. The small size of water molecules allows them to freely cross the membrane by using protein channels called aquaporins. Ions and large molecules cannot cross the membrane without more direct assistance.

Sodium is the most common extracellular cation (meaning it has a positive charge) in physiological systems. The reason why is relatively simple, it is not permeable (actually, it is not very permeable). Therefore, in order for sodium to move in-and-out, it needs a pump or a transport protein.