Functions: Transpo Systems An essential role of biomembranes is to allow movement of all compounds necessary for the normal function of a cell across the membrane barrier. These compounds include a vast array of substances like sugars, amino acids, fatty acids, steroids, cations and anions to mention a few. These compounds must enter or leave the cells in an orderly manner for normal functioning of the cell. A. 1. Ion Channels Ion channels are transmembrane channels, pore-like structures composed of proteins. Specific channels for Na+, K+, Ca++, and Cl- have been identified. Cation conductive channels are negatively charged within the channel and have an average diameter of about 5 to 8 nm. All ion channels are basically made up of transmembrane subunits that come together to form a central pore through which ions pass selectively. All channels have gates and are controlled by opening and closing. Types of Gates Two types of gated channels. They are a. Ligand-gated channels: In this, a specific molecule binds to a receptor and opens the channel. Example: Acetylcholine receptor is present in the postsynaptic membrane. It is a complex of five subunits, having a binding site for acetylcholine. Acetylcholine released from the presynaptic region binds with the binding site of the postsynaptic region, which triggers the opening of the channel and influx of Na+. b. Voltage-gated channels: These channels open or close in response to changes in membrane potential. Some propeies of ion channels * Composed of transmembrane protein subunits. * Highly selective. * Well regulated by the presence of "gates". * Two main types of gates: Ligand-gated and voltage-gated. * Activities are affected by ceain drugs. * Mutations of genes encoding transmembrane proteins can cause specific diseases. 2. Ionophores Ceain microorganisms can synthesize small organic molecules, called ionophores, which function as shuttles for the movement of ions across the membrane. Structure: These ionophores contain hydrophilic centres that bind specific ions and are surrounded by peripheral hydrophobic regions. Types: Two types: (a) Mobile ion carriers: Like valinomycin (Refer uncouplers of oxidative phosphorylation). (b) Channel formers: Like gramicidin. 3. Water Channels (Aquaporins) In ceain cells, e.g. in red blood cells, and cells of the collecting ductules of the kidney, the movement of water by simple diffusion is enhanced by movements of water through water channels, composed of tetrameric transmembrane proteins called aquaporins. About five distinct types of aquaporins have been recognised. CLINICAL ASPECT Recently mutations in the gene encoding AP-2 (Aquaporin 2) protein, have been shown to be the cause of one type of nephrogenic diabetes insipidus. 4. Gap Junction Ceain cells develop specialized regions on their membranes for intercellular communications which are in close proximity. Function: They mediate and regulate the passage of ions and small molecules upto 1000 to 2000 mol wt, through a narrow hydrophilic core connecting the cytosol of adjacent cells. Structure: They are primarily composed of protein, called connexon which contains four membrane-spanning a-helices. the mitochondrial membrane also has paicularly energy coupling mechanism.Ref: MN Chatterjea Textbook of Medical Biochemistry, 8th edition