Carrier vs Channel Proteins
It is necessary to transport substances across the cell membrane, in order to keep the cells active and alive. These substances are basically transported by membrane transport proteins in the plasma membrane of cells. There are two types of membrane transport proteins; carrier proteins and channel proteins, which are implicated in the transport of water soluble and insoluble substances across the cell membrane. These proteins basically allow passing polar molecules like ions, sugars, amino acids, nucleotides, and metabolites across the plasma membrane.
What are Carrier Proteins?
Carrier proteins are the integral proteins which extend into the lipid bilayer of cell membrane, and serve as channels for water soluble substances such as glucose and electrolytes. When transporting the solutes, carrier proteins bind solute on one side of a membrane, undergo conformational changes, and release them on the other side of the membrane. These proteins can mediate both active and passive transport. During the passive transport, molecules diffuse along the concentration gradient without consuming energy. Active transport is the movement of solute particles against the concentration gradient, and it needs energy. Carrier proteins act like enzymes. They bind only specific molecules, and the mode of attachment is similar to that between the active site of an enzyme and its substrate. Examples for some carrier proteins include; Glucose Transporter 4 (GLUT-4), Na+-K+ ATPase, Ca2+ ATPase etc.
What are Channel Proteins?
Channel proteins are ion selective, and contain a pore in which solute pass at high flux rates when the channel is open. The main characteristics of channel protiens include solute selectivity, a rapid rate of solute permeation, and gating mechanisms that regulate solute permeation. Some important channel proteins include; dihydropyridine receptor, Ca2+ channel protein, slow Na+ channel protein, fast Na+ channel proteins, Nicotinic Acetylcholine (nACh) receptor, N-methyl-D-asparate etc.
What is the difference between Carrier and Channel Proteins?
• Solutes diffuse through the pore of channel proteins, whereas career proteins bind solutes on one side of membrane and release it on the other side.
• Compared with channel proteins, carrier proteins have very slow transport rates (on the order of 1000 solute molecules per second).
• Unlike carrier proteins, channel proteins contain a pore, which facilitates the solute transportation.
• Unlike channel proteins, carrier proteins have alternate solute-bound conformations.
• Channel proteins are lipoproteins, while carrier proteins are glycoproteins.
• Carrier proteins can mediate both active and passive transport, while channel proteins can mediate only passive transport.
• Channel proteins are synthesized on ribosomes bound to endoplasmic reticulum, while carrier proteins are synthesized on free ribosomes in the cytoplasm.
• Carrier proteins can transport molecules or ions against the concentration gradient, while channel protein cannot.
• Carrier proteins move across the membrane, whereas channel proteins do not move while transporting molecules or ions.
• Channel proteins only pass water soluble molecules, while carrier proteins transport both water soluble and insoluble substances.