Osmosis

Osmosis (oz-mocsis) is the diffusion of water molecules from a region of higher water concentration to a region of lower water concentration across a selectively permeable membrane, such as a cell membrane. In the following example, assume that the selectively permeable membrane is permeable to water molecules (the solvent) but impermeable to sucrose molecules (the solute).

In solutions, a higher concentration of solute (sucrose in this case) means a lower concentration of water; a lower concentration of solute means a higher concentration of water. This is because the solute molecules take up space that water molecules would otherwise occupy.

Just like molecules of other substances, molecules of water will diffuse from areas of higher concentration to areas of lower concentration. In figure 3.24, the presence of sucrose in compartment A means that the water concentration there is less than the concentration of pure water in compartment B. Therefore, water diffuses from compartment B across the selectively permeable membrane and into compartment A. In other words, water moves from compartment B into compartment A by osmosis. Sucrose, on the other hand, cannot diffuse out of compartment A because the selectively permeable membrane is impermeable to it.

Selectively permeable membrane

® Sugar molecule <* Water molecule

Selectively permeable membrane

® Sugar molecule <* Water molecule

Figure 3.24

Figure 3.24

Osmosis. (1) A selectively permeable membrane separates the container into two compartments. At first, compartment A contains water and sugar molecules, whereas compartment B contains only water. As a result of molecular motions, water diffuses by osmosis from compartment B into compartment A. Sugar molecules remain in compartment A because they are too large to pass through the pores of the membrane. (2) Also, because more water is entering compartment A than is leaving it, water accumulates in this compartment. The level of liquid rises on this side.

Note in figure 3.24 that as osmosis occurs, the level of water on side A rises. This ability of osmosis to generate enough pressure to lift a volume of water is called osmotic pressure.

The greater the concentration of nonpermeable solute particles (sucrose in this case) in a solution, the lower the water concentration of that solution and the greater the osmotic pressure. Water always tends to diffuse toward solutions of greater osmotic pressure.

Since cell membranes are generally permeable to water, water equilibrates by osmosis throughout the body, and the concentration of water and solutes everywhere in the intracellular and extracellular fluids is essentially the same. Therefore, the osmotic pressure of the intracellular and extracellular fluids is the same. Any solution that has the same osmotic pressure as body fluids is called isotonic.

Solutions that have a higher osmotic pressure than body fluids are called hypertonic. If cells are put into a hypertonic solution, there will be a net movement of water by osmosis out of the cells into the surrounding solution, and the cells shrink. Conversely, cells put into a hypotonic solution, which has a lower osmotic pressure than body fluids, tend to gain water by osmosis and swell. Although cell membranes are somewhat elastic, the cells may swell so much that they burst. Figure 3.25 illustrates the effects of the three types of solutions on red blood cells.

It is important to control the concentration of solute in solutions that are infused into body tissues or blood. Otherwise, osmosis may cause cells to swell or shrink, impairing their function. For instance, if red blood cells are placed in distilled water (which is hypotonic to them), water will diffuse into the cells, and they will burst (hemolyze). On the other hand, if red blood cells are exposed to 0.9% NaCl solution (normal saline), the cells will remain unchanged because this solution is isotonic to human cells. Similarly, a 5% solution of glucose is isotonic to human cells. (The lower percentage is needed with NaCl to produce an isotonic solution, in part because NaCl ionizes in solution more completely and produces more solute particles than does glucose.)

Filtration

Molecules move through membranes by diffusion or osmosis because of their random movements. In other instances, molecules are forced through membranes by the process of filtration (fil-tracshun).

Filtration is commonly used to separate solids from water. One method is to pour a mixture of solids and water onto filter paper in a funnel (fig. 3.26). The paper serves as a porous membrane through which the small water molecules can pass, leaving the larger solid particles behind. Hydrostatic pressure, which is created by the weight of water due to gravity, forces the water molecules through to the other side. An example of this is making coffee by the drip method.

In the body, tissue fluid forms when water and dissolved substances are forced out through the thin, porous walls of blood capillaries, but larger particles such as blood protein molecules are left inside (fig. 3.27). The force for this movement comes from blood pressure, generated largely by heart action, which is greater within the vessel than outside it. (Although heart action is an active body process, filtration is still considered passive because it can occur due to the pressure caused by gravity alone.) Filtration is discussed further in chapters 15 (p. 606) and 20 (p. 831).

H What kinds of substances most readily diffuse through a cell membrane?

Explain the differences among diffusion, facilitated diffusion, and osmosis.

Distinguish among hypertonic, hypotonic, and isotonic solutions.

Explain how filtration occurs in the body.

Shier-Butler-Lewis: Human Anatomy and Physiology, Ninth Edition

I. Levels of Organization

3. Cells

© The McGraw-Hill Companies, 2001

Figure 3.25

Red Blood Cells Hypotonic Solution

(c) Cell in isotonic solution

(b) Cell in hypotonic solution

(c) Cell in isotonic solution

(a) Cell in hypertonic solution

Figure 3.25

(a) If red blood cells are placed in a hypertonic solution, more water leaves than enters, and the cells shrink (8,200x). (b) In a hypotonic solution, more water enters than leaves, and the cells swell, become spherical, and may burst (8,200x). (c) In an isotonic solution, equal volumes of water enter and leave the cells, and their sizes and shapes remain unchanged (8,200x).

Water and solids

Figure 3.26

Smaller molecules

Water and solids

Figure 3.26

In this example of filtration, gravity provides the force that pulls water through filter paper, while tiny openings in the paper retain the solids. This process is similar to the drip method of preparing coffee.

Larger molecules

Smaller molecules

Larger molecules

Capillary wall ' . Tissue fluid

Figure 3.27

Capillary wall ' . Tissue fluid

Figure 3.27

In this example of filtration, blood pressure forces smaller molecules through tiny openings in the capillary wall. The larger molecules remain inside.

Active Transport

When molecules or ions pass through cell membranes by diffusion, facilitated diffusion, or osmosis, their net movement is from regions of higher concentration to regions of lower concentration. Sometimes, however, the net movement of particles passing through membranes is in the opposite direction, from a region of lower concentration to one of higher concentration.

Sodium ions, for example, can diffuse slowly through cell membranes. Yet, the concentration of these ions typically remains many times greater outside cells

Binding site

Binding site

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