Facilitated Diffusion

The transport of glucose from the blood across plasma membranes occurs by facilitated diffusion. Facilitated diffusion, like simple diffusion, is powered by the thermal energy of the diffusing molecules and involves net transport from the side of higher to the side of lower concentration. ATP is not required for either facilitated or simple diffusion.

Unlike simple diffusion of nonpolar molecules, water, and inorganic ions through a membrane, the diffusion of glucose through the plasma membrane displays the properties of carrier-mediated transport: specificity, competition, and saturation. The diffusion of glucose through a plasma membrane must therefore be mediated by carrier proteins. In the conceptual model shown in figure 6.14, each transport carrier is composed of two protein subunits that interact with glucose in such a way as to create a channel through the membrane, thus enabling the movement of glucose down its concentration gradient.

Like the isoenzymes described in chapter 4, carrier proteins that do the same job may exist in various tissues in slightly different forms. The transport carriers for the facilitative diffusion of glucose are designated with the letters GLUT, followed by a number for the isoform. The carrier for glucose in skeletal muscles, for example, is designated GLUT4.

Molecule

Transport maximum

Saturation

--

Molecules

/

Saturation

X+ Y

/ s / ✓

//

Concentration of X

■ Figure 6.13 Characteristics of carrier-mediated transport. Carrier-mediated transport displays the characteristics of saturation (illustrated by the transport maximum) and competition. Since molecules X and Y compete for the same carrier, the rate of transport of each is lower when they are both present than when either is present alone.

- Glucose

Outside of cell

Higher concentration

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Essentials of Human Physiology

Essentials of Human Physiology

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  • fre-qalsi tesmi
    How is facilitated diffusion powered?
    8 years ago

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