An overview of Membrane transport
Authors: Nimmy.M.S, Era Vaidya Malhotra, Lal bahadur singh and Vinod Kumar.
Every living cell must acquire from its surroundings the raw materials for biosynthesis and energy production, and must release to its environment the by-products of metabolism. A cell’s physical and chemical boundary defined by plasma membrane which is a bilayer of phospholipids. Pure phospholipid bilayer (without proteins) are excellent chemical barriers. If we consider the relative permeability of a pure phospholipid bilayer to various molecules and ions, a few biologically important gases (molecular oxygen, nitrogen and methane, all of which are relatively nonpolar) can cross membranes by simple diffusion. Also, it is slightly permeable to water and totally impermeable to ions and to large polar molecules. Hence the combination of lipids and proteins gives permeable nature to plasma membrane.
There are three main categories of membrane transport proteins namely channels, carriers and pumps. All transport proteins are transmembrane proteins containing multiple membrane-spanning segments that generally are α helices. Transport proteins exhibit specificity for the solutes they transport and hence there is great diversity of transport proteins in cells. In case of the model dicot Arabidopsis thaliana , out of a predicted 25,500 proteins, as many as 1800 (7%) may execute transport functions. Channels are transmembrane proteins that function as selective pores (membrane pores) through which molecules or ions can diffuse across the membrane. Transport through channels is always passive (by simple diffusion or facilitated diffusion) down the solute’s gradient of electrochemical potential. Specificity of transport by channel depends on pore size &electric charge (biophysical properties of the channel). Channel transport is limited mainly to ions or water. Channels form a hydrophilic “tube” or passageway across the membrane through which multiple water molecules or ions move simultaneously at a very rapid rate. Some channels are open much of the time (Nongated). Most ion channels open only in response to specific signals (Gated). Carriers bind and transport specific substances. Unlike channels, carrier proteins do not have pores that extend completely across the membrane. They have specific structures. The substance being transported is initially bound to a specific site on the carrier protein. Binding causes a conformational change in the protein, which exposes the substance to the solution on the other side of the membrane. Hence carriers are highly selective for a particular substrate to be transported. The protein can transport the solute in either direction with the net movement being down the concentration gradient of the solute. Because a conformational change in the protein is required to transport an ion, the rate of transport by a carrier is many orders of magnitude slower than that through a channel.
Typically, carriers may transport 100 to 1000 ions or molecules per second, while millions of ions can pass through an open ion channel in the same amount of time. Carrier- mediated transport (unlike transport through channels) can be either passive transport or secondary active transport. Passive transport via a carrier is sometimes called facilitated diffusion. Primary active transport is carried out by pumps. If we consider water transport, integral membrane proteins such as aquaporins facilitate movement of water across cell membranes. Aquaporins are a family of membrane proteins that allow water to cross biomembranes. Aquaporins are also called as water channels. They form hydrophilic transmembrane channels for the passage of water. Aquaporins are common in plant & animal membranes, and the Arabidopsis genome is predicted to encode 35 aquaporins (Maurel et al.,2008).
1. Maurel C, Verdoucq L, Luu DT, Santoni V. (2008). Plant aquaporins: membrane channels with multiple integrated functions. Annual Review of Plant Biology 59: 595-624.
2. Molecular Cell Biology (2007) Bruce Alberts
About Author / Additional Info:
Scientist at NRCPB, NewDelhi.