An acetylcholine receptor (green) forms a gated ion channel in the plasma membrane. This receptor is a membrane protein with an aqueous pore, meaning it allows soluble materials to travel across the plasma membrane when open. When no external signal is present, the pore is closed (center).

Ion channels may be classified by gating, i.e. what opens and closes the channels. For example, voltage-gated ion channels open or close depending on the voltage gradient across the plasma membrane, while ligand-gated ion channels open or close depending on binding of ligands to the channel.

When ion channels are in their open state, they conduct electrical current by allowing specific types of ions to pass through them, and thus, across the plasma membrane of the cell. Gating is the process by which an ion channel transitions between its open and closed states.

The appropriate stimulus opens chemical, mechanical, or light gated ion channels. 2. If the stimulus opens Na+ channels, Na+ moving into the cell causes the membrane potential to become more positive, resulting in depolarization; the inside of the membrane becomes more positive and the outside becomes more negative.

This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium.

There are three main types of gated channels: chemically-gated or ligand-gated channels, voltage-gated channels, and mechanically-gated channels. Ligand-gated ion channels are channels whose permeability is greatly increased when some type of chemical ligand binds to the protein structure.

The Na+ channel’s selection of Na+ over K+ depends on ionic radius; the diameter of the pore is sufficiently restricted that small ions such as Na+ and Li+ can pass through the channel, but larger ions such as K+ are significantly hindered (Figure 13.27).

Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells.

It is important to remember that aquaporins do not actively transport water across the cell membrane; instead they facilitate the diffusion of water across the cell membrane.

There are 12 known aquaporins, of which four aquaporins are located in the kidney: AQP1 is located in the kidney capillary endothelia; AQP2 is located in the intracellular and apical kidney CD cells; AQP3 is located in the kidney; and AQP11 is located in the proximal tubule.

Aquaporins (AQPs) are integral membrane proteins and found in all living organisms from bacteria to human. AQPs mainly involved in the transmembrane diffusion of water as well as various small solutes in a bidirectional manner are widely distributed in various human tissues.

Although many aquaporins function as always-open channels, a subgroup of aquaporins, particularly in plants have evolved a sophisticated molecular mechanism through which the channel can be closed in response to harsh conditions of the environment, under which exchange of water can be harmful for the organism.

Cell-membrane water permeability varies considerably from cell to cell; high permeability denotes a fluid lipid bilayer and expression of AQPs. Low water permeability occurs when there is no aquaporin expression and membrane is rich in cholesterol.

Water moves in and out of cells by osmosis through the cell membrane. For example, aquaporins are needed when water must be retrieved when urine is concentrated in the kidney. Aquaporins selectively conduct water molecules in and out of cells, while preventing the passage of ions and other solutes.

The presence of aquaporins (proteins that form water channels in the membrane) should speed up the process of osmosis.

Water is a charged molecule, so it cannot get through the lipid part of the bilayer. In order to allow water to move in and out, cells have special proteins that act as a doorway.

The physical driving force of osmosis is the increase in entropy generated by the movement of free water molecules. There is also thought that the interaction of solute particles with membrane pores is involved in generating a negative pressure, which is the osmotic pressure driving the flow of water.

In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semipermeable membrane is present, so only the solvent molecules are free to move to equalize concentration.