![]() In contrast, channel proteins (see the next section) form open pores through the membrane, allowing the free diffusion of any molecule of the appropriate size and charge.Ĭarrier proteins are responsible for the facilitated diffusion of sugars, amino acids, and nucleosides across the plasma membranes of most cells. They then undergo conformational changes that allow the molecule to pass through the membrane and be released on the other side. Carrier proteins bind specific molecules to be transported on one side of the membrane. Two classes of proteins that mediate facilitated diffusion are generally distinguished: carrier proteins and channel proteins. Facilitated diffusion therefore allows polar and charged molecules, such as carbohydrates, amino acids, nucleosides, and ions, to cross the plasma membrane. Instead, their passage is mediated by proteins that enable the transported molecules to cross the membrane without directly interacting with its hydrophobic interior. However, facilitated diffusion differs from passive diffusion in that the transported molecules do not dissolve in the phospholipid bilayer. No external source of energy is provided, so molecules travel across the membrane in the direction determined by their concentration gradients and, in the case of charged molecules, by the electric potential across the membrane. The passage of these molecules across the membrane instead requires the activity of specific transport and channel proteins, which therefore control the traffic of most biological molecules into and out of the cell.įacilitated Diffusion and Carrier Proteinsįacilitated diffusion, like passive diffusion, involves the movement of molecules in the direction determined by their relative concentrations inside and outside of the cell. Consequently, larger uncharged polar molecules such as glucose are unable to cross the plasma membrane by passive diffusion, as are charged molecules of any size (including small ions such as H +, Na +, K +, and Cl -). Other biological molecules, however, are unable to dissolve in the hydrophobic interior of the phospholipid bilayer. Thus, gases (such as O 2 and CO 2), hydrophobic molecules (such as benzene), and small polar but uncharged molecules (such as H 2O and ethanol) are able to diffuse across the plasma membrane. Importantly, only small, relatively hydrophobic molecules are able to diffuse across a phospholipid bilayer at significant rates ( Figure 12.15). ![]() ![]() Passive diffusion is thus a nonselective process by which any molecule able to dissolve in the phospholipid bilayer is able to cross the plasma membrane and equilibrate between the inside and outside of the cell. The net flow of molecules is always down their concentration gradient-from a compartment with a high concentration to one with a lower concentration of the molecule. No membrane proteins are involved and the direction of transport is determined simply by the relative concentrations of the molecule inside and outside of the cell. During passive diffusion, a molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane. The simplest mechanism by which molecules can cross the plasma membrane is passive diffusion.
0 Comments
Leave a Reply. |