The hydrophobic layer of the plasma membrane creates a barrier that prevents the diffusion of most substances. Exceptions are small molecules such as gases like nitric oxide (NO) and carbon dioxide (CO2), and nonpolar substances such as steroid hormones and fatty acids. Even though fatty acids can diffuse across the plasma membrane, this occurs slowly. Recent work indicates that a substantial amount of fatty acid transport is via carrier proteins.
Channels are large proteins in which multiple subunits are arranged in a cluster so as to form a pore that passes through the membrane. Each subunit consists of multiple transmembrane domains. Most of the channels that we will consider are ion channels. Another important type of channel protein is an aquaporin. Aquaporins are channels that allow water to move rapidly across cell membranes.
Movement through a channel does not involve specific binding (see facilitated diffusion below). The two factors that affect the flow of ions through an open ion channel are the membrane potential and the concentration gradient. Note that when ions move through a channel across a membrane, this changes the membrane potential (depolarization or hyperpolarization). Changes in membrane potential are used to code information, particularly in the nervous system. See the web page on Membrane Potentials.
For any ion channel, there are two important properties to consider: selectivity and gating. Selectivity refers to which ion (Na+, K+, Ca++, or Cl-) is allowed to travel through the channel. Most ion channels are specific for one particular ion. Gating refers to what opens or closes a channel. Below we classify different ion channels according to the type of gating.
A few types of ion channels are ungated, meaning they are open all the time. For instance, some K+ and some Cl- channels are ungated. By contrast, Ca++ and Na+ ion channels are NEVER ungated.
Voltage-gated ion channels open or close in response to changes in membrane potential. Voltage-gated ion channels are key in the generation of electrical signals in nerve, muscle, and cardiac cells. See the web page describing an important example, the voltage-gated Na+ channel.
Ligand-gated ion channels are opened when regulatory molecules bind to the channel protein. Many neurotransmitter receptors are ligand gated ion channels. An example is the nicotinic acetylcholine receptor. This is the receptor that is found at the neuromuscular junction on skeletal muscle cells, and also at synapses in autonomic ganglia.
Afferent neurons (sensory neurons) in the skin that respond to touch or stretch have ion channels in their sensory dendrites that open in response to pressure or other mechanical changes at the cell membrane. Mechanically-gated channels are also found in the specialized sensory cells of the auditory and vestibular system.
There are also afferent neurons that sense warm and cold and possess temperature-gated ion channels in their sensory dendrites. While these ion channel proteins are normally gated by temperature, it turns out that certain ligands also can open them. For instance, capsaicin, the molecule found in chili peppers, opens the channel that is normally opened by noxious heat, while menthol opens the channel that is normally opened by cool temperatures.
Facilitated diffusion is transport involving a carrier protein that has a specific binding site for the transported substance. An example is the movement of glucose from the extracellular fluid into cells (glucose uptake). The transport protein, known as the glucose transporter, has a specific binding site for glucose. The binding of glucose changes the conformation of the glucose transporter, which can exist in different conformations that expose the binding site to either the extracellular fluid or the cytosol.
The concentration gradient for glucose determines the rate and direction of transport. Facilitated diffusion is a passive process, meaning that it does not require ATP. With glucose uptake, glucose is transported from the extracellular fluid into the cytosol, where cells metabolize it as a source of energy. However, if the glucose concentration is higher inside the cell than outside, the direction of transport will be in the opposite direction.
Facilitated diffusion and other processes that depend on membrane transport proteins are regulated by controlling the number of transport proteins present in the membrane. For instance, glucose uptake is regulated by the hormone insulin. At low concentrations of insulin, few glucose transporters are on the plasma membrane. Insulin stimulates glucose uptake by causing vesicles containing glucose transporters to fuse with the plasma membrane, as shown in the figure.
Coupled transport is similar to facilitated diffusion in that it involves specific binding, however in this case, two substances are required to bind in order for transport to occur. As a consequence, the free energy driving the transport is the sum of the free energies for transport of both substances. If the transported substances move in the same direction across the membrane, it is called cotransport; if they move in the opposite direction, it is called countertransport.
The transport of glucose across the apical plasma membrane of epithelial cells in the small intestine is an example of cotransport. This is the first step in the absorption of glucose from the foods you eat. The transport protein is known as the sodium-glucose cotransporter. Immediately after eating a lot of carbohydrates, the concentration gradient of glucose will favor transport into cells, but as more and more glucose is absorbed, that will not be the case. However, there is always a steep concentration gradient favoring the movement of Na+ into cells, because the concentration of Na+ inside of cells is kept very low through the constant action of the sodium-potassium pump (Na+/K+-ATPase, see below). Because transport is coupled, the Na+ concentration gradient can power the movement of glucose uphill against its concentration gradient.
Because they both involve specific binding, facilitated diffusion and coupled transport show saturation. Transport depends upon a limited number of transport proteins in the membrane, each of which must bind with the transported substance for a given period of time. As the concentration of the transported substance increases, the rate of transport also increases, but then starts to level off and approach a maximum. At high concentrations, there comes a point where every transporter in the membrane is bound by the transported substance, and the transport rate cannot increase beyond this transport maximum (Vmax).
Active transport describes the process whereby the transport of specific substances is coupled to ATP hydrolysis. Because the energy for transport is derived from ATP hydrolysis, these transporters effectively move substances in one direction, regardless of the concentration gradient.
The most widespread and physiologically important active transporter in cells is the Na+/K+-ATPase, or sodium-potassium pump. This protein moves three Na+ ions out of the cell and two K+ ions into the cell with each cycle of ATP hydrolysis. The Na+/K+-ATPase is expressed in all cells, and is responsible for generating the typical Na+ and K+ gradients found across the cell membrane. These ionic gradients underlie electrical excitability in neurons and muscles. As well, the Na+ gradient is used to power coupled transport of glucose and many other substances, as discussed above. It is estimated that in a body at rest, the activity of the Na+/K+-ATPase consumes about a third of all ATP.
Other important active transporters include Ca++-ATPases and the H+/K+-ATPase. Ca++-ATPases keep the Ca++ concentration low in the cytosol. One type of Ca++-ATPase is found in the plasma membrane; another is found in the membrane of the endoplasmic reticulum and the sarcoplasmic reticulum of muscle cells. The H+/K+-ATPase or "proton pump" is responsible for acid secretion in the stomach.
ABC transporters are a family of transport proteins that depend upon ATP binding for transport. ABC stands for ATP-Binding Cassette. ABC proteins have a particular molecular structure that includes two nucleotide binding domains where ATP binds. An example is the multidrug resistance protein (MDR1). This protein uses the energy of ATP hydrolysis to pump a wide variety of nonpolar drugs and toxins out of cells. It is so named because over-expression of this protein in tumor cells confers resistance to chemotherapy drugs.
A physiologically important member of the ABC transporter family is the protein CFTR. CFTR forms a Cl- channel that is expressed on the apical plasma membrane of many epithelial cells. CFTR is the protein that is defective in the genetic disorder cystic fibrosis. Unlike most ABC transporter proteins that use the energy of ATP hydrolysis to pump substances across the membrane and out of cells, CFTR works as an ion channel that is regulated by both phosphorylation and ATP binding.
CFTR plays a key role in the secretion of ions and water across epithelia (see page on Epithelial Transport). Some bacterial toxins cause unregulated activity of CFTR, resulting in excessive secretion in the small intestine which causes diarrhea. In cystic fibrosis, CFTR channels are defective or absent, leading to decreased secretion, which causes pathology in the lungs and digestive system.