The continuous movement of water and solutes into and out of cells is essential for their normal function. The intracellular and extracellular fluids consist mainly of H2O, with dissolved solutes such as ions, glucose, and amino acids. The plasma membrane, with its hydrophobic core, acts as an effective barrier to the movement of these biologically important solutes and water. However, specific membrane transporters present in the membrane facilitate the transport of these solutes and water across the membrane.

Membrane transporters are categorized into four groups: water channels, ion channels, solute carriers, and adenosine triphosphate (ATP)–dependent transporters. Water channels, known as aquaporins (AQPs), play a significant role in the movement of water into and out of cells. They are widely distributed throughout the body, including the brain, lungs, kidneys, salivary glands, gastrointestinal tract, and liver. Different AQP isoforms are expressed by various cells, and some cells even express multiple isoforms. For instance, cells in the collecting ducts of the kidneys have AQP3 and AQP4 in their basolateral membrane and AQP2 in their apical membrane. The abundance of AQP2 in the apical membrane is regulated by antidiuretic hormone, which is crucial for urine concentration.

While all AQP isoforms allow the passive movement of H2O across the membrane, certain isoforms also facilitate the passage of other molecules like glycerol, urea, mannitol, purines, pyrimidines, CO2, and NH3. These AQPs are collectively referred to as aquaglyceroporins. The regulation of water entry or exit through AQPs primarily involves altering the number of AQPs in the membrane.

Each AQP molecule is composed of six membrane-spanning domains and a central water-transporting pore. In the plasma membrane, four AQP monomers come together to form a homotetramer, with each monomer functioning as a water channel.

Ion channels, found in all cells, are particularly important for the function of excitable cells like neurons and muscle cells. They are classified based on their selectivity, conductance, and mechanism of channel gating. Selectivity refers to the specific ions that can pass through the channel. Some ion channels are highly selective, allowing only a particular ion to pass, while others may be nonselective, enabling the passage of a group of cations or anions. Channel conductance represents the number of ions that can pass through the channel and is typically measured in picosiemens (pS). The range of conductance can vary greatly, from 1 to 2 pS for some channels to more than 100 pS for others. In certain channels, the conductance varies depending on the direction of ion movement. If the channel exhibits greater conductance when ions move into the cell compared to when they move out, it is termed an inward rectifier. Additionally, ion channels undergo transitions between open and closed states, a process known as gating.
The normal function of cells requires the continuous movement of water and solutes into and out of the cell. The intracellular and extracellular fluids are composed primarily of H2O, in which solutes (e.g., ions, glucose, amino acids) are dissolved. The plasma membrane, with its hydrophobic core, is an effective barrier to the movement of virtually all of these biologically important solutes. It also restricts the movement of water across the membrane. The presence of specific membrane transporters in the membrane is responsible for the movement of these solutes and water across the membrane.
Membrane transporters have been classified in several different ways. In this chapter, the transporters are divided into four general groups: water channels, ion channels, solute carriers, and adenosine triphosphate (ATP)–dependent transporters.
lists these groups of membrane transporters, their modes of transport, and estimates of the rates at which they transport molecules or ions across the membrane.
Water channels, or aquaporins (AQPs), are the main routes for water movement into and out of the cell. They are widely distributed throughout the body (e.g., the brain, lungs, kidneys, salivary glands, gastrointestinal tract, and liver). Cells express different AQP isoforms, and some cells even express multiple isoforms. For example, cells in the collecting ducts of the kidneys express AQP3 and AQP4 in their basolateral membrane and AQP2 in their apical membrane. Moreover, the abundance of AQP2 in the apical membrane is regulated by antidiuretic hormone (also called arginine vasopressin), which is crucial for the ability of the kidneys to concentrate the urine (see
Although all AQP isoforms allow the passive movement of H2O across the membrane, some isoforms also provide a pathway for other molecules such as glycerol, urea, mannitol, purines, pyrimidines, CO2, and NH3 to cross the membrane. Because glycerol was one of the first molecules identified as crossing the membrane via some AQPs, this group of AQPs is collectively called aquaglyceroporins (see also ). Regulation of the amount of H2O that can enter or leave the cell via AQPs occurs primarily by altering the number of AQPs in the membrane.
EachAQPmoleculeconsistsofsixmembrane-spanningdomainsandacentralwater-transportingpore.FourAQPmonomersassembletoformahomotetramerintheplasmamembrane,witheachmonomerfunctioningasawaterchannel.
Ion channels are found in all cells, and are especially important for the function of excitable cells (e.g., neurons and muscle cells). Ion channels are classified by their selectivity, conductance and mechanism of channel gating (i.e., opening and closing). Selectivity is defined as the nature of the ions that pass through the channel. At one extreme, ion channels can be highly selective, in that they allow only a specific ion through. At the other extreme, they may be nonselective, allowing all or a group of cations or anions through. Channel conductance refers to the number of ions that pass through the channel and is typically expressed in picosiemens (pS). The range of conductance is considerable: Some channels have a conductance of only 1 to 2 pS, whereas others have a conductance of more than 100 pS. For some channels, the conductance varies, depending on the direction in which the ion is moving. For example, if the channel has a larger conductance when ions are moving into the cell than when they are moving out of the cell, the channel is said to be an inward rectifier. Moreover, ion channels fluctuate between an open state or a closed state, a process called gating (