Facilitated Diffusion

Another type of passive transport is called facilitated diffusion. This process is used for molecules that cannot readily diffuse through cell membranes, even when there is a concentration gradient across the membrane. Such molecules may not be soluble in lipids, or they may be too large to pass through the pores in the membrane. In facilitated diffusion, the movement of these kinds of molecules across the cell membrane is assisted by specific proteins in the membrane. These proteins are known as carrier proteins.

The carrier proteins that serve in facilitated diffusion transport molecules from an area of higher concentration on one side of the membrane to an area of lower concentration on the other side. Because the molecules are moving from a higher to lower concentration, which does not require any additional energy, facilitated diffusion is passive transport.

Figure 5-5 shows a model of how facilitated diffusion is thought to work. According to the model, a molecule binds to a specific carrier protein that transports it. As soon as the molecule binds to the carrier protein, the carrier protein then changes shape. This altered shape may shield the molecule from the hydrophobic interior of the lipid bilayer. Once shielded, the molecule can be transported through the cell membrane. On the other side of the membrane, the molecule is released from the carrier protein, and the protein returns to its original shape. The carrier protein is now free to bind to another molecule.

A good example of facilitated diffusion is the transport of glucose. Many cells depend on glucose for much of their energy needs. But glucose molecules are too large to diffuse easily across cell membranes. When the level of glucose within a cell is lower than the level of glucose outside the cell, carrier proteins transport glucose into the cell.

figure 5-5

Facilitated diffusion occurs in four steps. O A molecule, such as glucose, binds to a carrier protein on one side of the cell membrane. © The carrier protein changes shape, shielding the molecule from the interior of the membrane. © The molecule is released on the other side of the membrane. O The carrier protein then returns to its original shape.

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