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Facilitated diffusion is the movement of lipid insoluble molecules across the phospholipid bilayer by the use of carrier proteins[1]. Contents
Basic FeaturesThe bilayer consists of lipids, therefore only lipid-soluble molecules can pass through it. This is mainly small polar uncharged molecules and small hydrophobic molecules. Facilitated diffusion is a passive process that requires no use of external energy[1]. The action of facilitated diffusion is spontaneous, however, the rate of the diffusion differs according to how permeable a membrane is for each substance. Most membranes are selectively permeable so different membranes have different permeability. For example, water is diffused freely across cell membranes. Other substances, however, must pass through the lipid bilayer first.[2] The molecules move across the membrane from an area of high concentration to an area of low concentration. If the solute carries a net charge, for example, molecules will move down its electrochemical gradient[3].Sugars and amino acids are examples of molecules that move across the plasma membrane using facilitated diffusion. Also, GLUT transporters are a group of carrier proteins that move glucose sugars and associated hexose sugars across the plasma membrane[1]. Facilitated is characterised by the following:
Two major types of facilitated diffusion:1. Carrier proteinsThese are proteins that span the plasma membrane (transmembrane proteins) and are also known as permeases[4]. Each protein carrier is specific to bind to a complementary molecule. On one side of the membrane (higher concentration of molecules), the molecules bind to the carrier protein. The carrier then changes conformational shape, moving the binding site from one side of the membrane to the other[5], releasing the molecules on the other side of the membrane (where there is a lower concentration of the molecules)[6]. 2. Ion channel proteinsUnlike carrier proteins, ion channel proteins have gate to control the passage of substances across the cell membrane, down their electrochemical gradient, usually by the ligand-gated ion channel, voltage-gated ion channel or intracellular messenger-gated ion channel[7]. Ion channels are integral membrane proteins (hydrophobic region) that do not require binding of solutes but only allow specific types of solutes to pass through the protein channel[8]. There are three forms of transport can with facilitated diffusion ;
Factors affecting rate of facilitated diffusion
References
What is moved across the plasma membrane by facilitated diffusion?Facilitated diffusion therefore allows polar and charged molecules, such as carbohydrates, amino acids, nucleosides, and ions, to cross the plasma membrane. Two classes of proteins that mediate facilitated diffusion are generally distinguished: carrier proteins and channel proteins.
What is moved during facilitated diffusion?Facilitated diffusion is the passive movement of molecules along the concentration gradient. It is a selective process, i.e., the membrane allows only selective molecules and ions to pass through it. It, however, prevents other molecules from passing through the membrane.
Which of the following is most likely to move through the cell membrane by facilitated diffusion?The molecule most likely to be involved in simple diffusion is water - it can easily pass through cell membranes.
Which substance uses a carrier protein to cross the plasma membrane by facilitated diffusion?Glucose uses a carrier protein to cross the plasma membrane by facilitated diffusion. common glucose transporters which help in glucose transfer are GLUTs. GLUTs are integral membrane protein.
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