Diffusion and Osmosis

Net diffusion of a molecule or ion through a cell membrane always occurs in the direction of its lower concentration. Nonpolar molecules can penetrate the phospholipid barrier, and small inorganic ions can pass through channels in the membrane. The net diffusion of water through a membrane is known as osmosis.

A solution consists of the solvent, water, and solute molecules that are dissolved in the water. The molecules of a solution (solvent and solute) are in a constant state of random motion as a result of their thermal (heat) energy. If there is a concentration difference, or concentration gradient, between two regions of a solution, this random motion tends to eliminate the concentration difference as the molecules become more diffusely spread out (fig. 6.2). Hence, this random molecular motion is known as diffusion. In terms of the second law of thermodynamics, the concentration difference represents an unstable state of high organization (low entropy) that changes to produce a uniformly distributed solution with maximum disorganization (high entropy).

As a result of random molecular motion, molecules in the part of the solution with a higher concentration will enter the area of lower concentration. Molecules will also move in the opposite direction, but not as frequently. As a result, there will be a net movement from the region of higher to the region of lower concentration until the concentration difference no longer exists. This net movement is called net diffusion. Net diffusion is a physical process that occurs whenever there is a concentration difference across a membrane and the membrane is permeable to the diffusing substance.

■ ft In the kidneys, blood is filtered through pores in capillary walls to produce a filtrate that will become urine.

^ Wastes and other dissolved molecules can pass through the pores, but blood cells and proteins are held back. Then, the molecules needed by the body are reabsorbed from the filtrate back into the blood by transport processes. Wastes generally remain in the filtrate and are thus excreted in the urine. When the kidneys fail to perform this function, the wastes must be removed from the blood artificially by means of dialysis. In this process, waste molecules are removed from the blood by having them diffuse through an artificial porous membrane. The wastes pass into a solution (called a dialysate) surrounding the dialysis membrane. Molecules needed by the body, however, are kept in the blood by including them in the dialysate. This prevents their net diffusion by abolishing their concentration gradients.

Chapter Six

Higher j Lower concentration | concentration

Higher j Lower concentration | concentration

Net diffusion

Equal concentrations

No net diffusion

■ Figure 6.2 Diffusion of a solute. (a) Net diffusion occurs when there is a concentration difference (or concentration gradient) between two regions of a solution, provided that the membrane separating these regions is permeable to the diffusing substance. (b) Diffusion tends to equalize the concentrations of these regions, and thus to eliminate the concentration differences.

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