How do ions pass through the cell membrane?
The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot. Integral membrane proteins enable ions and large polar molecules to pass through the membrane by passive or active transport.
The channels can be opened (or closed) according to the needs of the cell. Transmembrane proteins, called transporters, use the energy of ATP to force ions or small molecules through the membrane against their concentration gradient.
- Sodium ions pass through specific channels in the hydrophobic barrier formed by membrane proteins. This means of crossing the membrane is called facilitated diffusion, because the diffusion across the membrane is facilitated by the channel. In this case, sodium must move, or be pumped, against a concentration gradient.
- The water wants to flow from the higher concentration, which is outside of the cell, to the lower concentration, which is inside of the cell. The cell membrane helps to regulate and slow down the flow of water into the cell. This is yet another way that the cell membrane helps maintain homeostasis.
- Small, nonpolar molecules are hydrophobic, so they can easily cross the phospholipid bilayer of the plasma membrane. Polar molecules and ions are hydrophilic, so they cannot very easily cross the hydrophobic portion of the plasma membrane (formed by the phospholipid tails).
The Sodium-Potassium Pump. The process of moving sodium and potassium ions across the cell membrance is an active transport process involving the hydrolysis of ATP to provide the necessary energy. It involves an enzyme referred to as Na+/K+-ATPase.
- The Na+/K+ pump is found in the membranes of many types of cells. In particular, it plays a very important role in nerve cell membranes. In order to move the ions (Na+ and K+) againts their gradients, energy is required. This energy is supplied by ATP (adenosine triphosphate).
- If this pump stops working (as occurs under anoxic conditions when ATP is lost), or if the activity of the pump is inhibited (as occurs with cardiac glycosides such as digitalis), Na+ accumulates within the cell and intracellular K+ falls. This causes depolarization of the resting membrane potential.
- The inside of the cell has a low concentration of sodium ions, and the outside of the cell has a higher concentration of sodium ions. There are extra positive charges on the inside of the cell in the form of Na+ ions, and these Na+ ions line up along the membrane.
Updated: 2nd October 2019