What does diffusion have to do with the cell membrane? Suppose a substance is present in unequal concentrations on either side of a cell membrane, as shown in Figure 7–15. If the substance can cross the cell membrane, its particles will tend to move toward the area where it is less concentrated until it is evenly distributed. Once the concentration of the substance on both sides of the cell membrane is the same, equilibrium is reached.

Even when equilibrium is reached, particles of a solution continue to move across the membrane in both directions. However, because almost equal numbers of particles move in each direction, there is no further net change in the concentration on either side.

Diffusion depends on random particle movements. Therefore, substances diffuse across membranes without requiring the cell to use additional energy. The movement of materials across the cell membrane without using cellular energy is called passive transport.

FIGURE 7–15 Diffusion Diffusion is the process by which molecules of a substance move from an area of higher concentration to an area of lower concentration. It does not require the cell to use energy. Predict How would the movement of solute particles seen here be different if the initial area of high concentration had been on the inside of the cell instead of the outside?

d

Facilitated Diffusion Since cell membranes are built around lipid bilayers, the molecules that pass through them most easily are small and uncharged. These properties allow them to dissolve in the membrane's lipid environment. But many ions, like Cl⁻, and large molecules, like the sugar glucose, seem to pass through cell membranes much more quickly than they should. It's almost as if they have a shortcut across the membrane.

How does this happen? Proteins in the cell membrane act as carriers, or channels, making it easy for certain molecules to cross. Red blood cells, for example, have protein carriers that allow glucose to pass through them in either direction. Only glucose can pass through these protein carriers. These cell membrane channels facilitate, or help, the diffusion of glucose across the membrane. This process, in which molecules that cannot directly diffuse across the membrane pass through special protein channels, is known as facilitated diffusion. Hundreds of different proteins have been found that allow particular substances to cross cell membranes. Although facilitated diffusion is fast and specific, it is still diffusion, so it does not require any additional use of the cell's energy.

In Your Notebook Explain how you can demonstrate diffusion by spraying air freshener in a large room.

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