Simple Diffusion

Simple diffusion is the net movement of particles from a place of high concentration to a place of lower concentration as a result of their constant, spontaneous motion. It can be observed by dropping a dye crystal in a dish of still water. As the crystal dissolves, it forms a colored zone in the water that gets larger and larger with time (fig. 3.14). The dye molecules exhibit net movement from the point of origin, where their concentration is high, toward the edges of the dish, where their concentration is low. When the concentration of a substance differs from one point to another, we say that it exhibits a concentration gradient. Particle movement from a region of high concentration toward a region of lower concentration is said to go down, or with, the gradient, and movement in the other direction is said to go up, or against, the gradient.

Diffusion occurs readily in air or water, and has no need of a membrane. However, if there is a membrane in the path of the diffusing molecules, and if it is permeable to that substance, the molecules will pass from one side of

Dye Dissolving Water

Figure 3.14 Diffusion and Concentration Gradients. Dye molecules diffusing away from a crystal dissolving in water. The direction from high concentration (near the crystal) to low concentration is described as "down the concentration gradient"; the opposite direction is described as "up the concentration gradient."

Figure 3.14 Diffusion and Concentration Gradients. Dye molecules diffusing away from a crystal dissolving in water. The direction from high concentration (near the crystal) to low concentration is described as "down the concentration gradient"; the opposite direction is described as "up the concentration gradient."

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition the membrane to the other. This is how oxygen passes from the air we inhale into the bloodstream. Dialysis treatment for kidney disease patients is based on diffusion of solutes through artificial dialysis membranes.

Diffusion rates are very important to cell survival because they determine how quickly a cell can acquire nutrients or rid itself of wastes. Some factors that affect the rate of diffusion through a membrane are as follows:

  • Temperature. Diffusion is driven by the kinetic energy of the particles, and temperature is a measure of that kinetic energy. The warmer a substance is, the more rapidly its particles diffuse. This is why sugar diffuses more quickly through hot tea than through iced tea.
  • Molecular weight. Heavy molecules such as proteins move more sluggishly and diffuse more slowly than light particles such as electrolytes and gases. Small molecules also pass through membrane pores more easily than large ones.
  • quot;Steepness" of the concentration gradient. The steepness of a gradient refers to the concentration difference between two points. Particles diffuse more rapidly if there is a greater concentration difference between two points. For example, we can increase the rate of oxygen diffusion into a patient's blood by using an oxygen mask, thus increasing the difference in oxygen concentration between the air and blood.
  • Membrane surface area. As noted earlier, the apical surface of cells specialized for absorption (for example, in the small intestine) is often extensively folded into microvilli. This makes more membrane available for particles to diffuse through.
  • Membrane permeability. Diffusion through a membrane depends on how permeable it is to the particles. For example, potassium ions diffuse more rapidly than sodium ions through a plasma membrane. Nonpolar, hydrophobic, lipid-soluble substances such as oxygen, nitric oxide, alcohol, and steroids diffuse through the phospholipid regions of a plasma membrane. Water and small charged, hydrophilic solutes such as electrolytes do not mix with lipids, but diffuse primarily through channel proteins in the membrane. Cells can adjust their permeability to such a substance by adding channel proteins to the membrane or taking them away. Kidney tubules, for example, do this as a way of controlling the amount of water eliminated from the body.

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