Osmosis

A solution is composed of a solute dissolved in a solvent. In the sugar water described in Figure 5-1, the solute was sugar and the solvent was water, and the solute molecules diffused through the solvent. It is also possible for solvent molecules to diffuse. In the case of cells, the solutes are organic and inorganic compounds, and the solvent is water. The process by which water molecules diffuse across a cell membrane from an area of higher concentration to an area of lower concentration is called osmosis (ahs-MOH-sis). Because water is moving from a higher to lower concentration, osmosis does not require cells to expend energy. Therefore, osmosis is the passive transport of water.

Direction of Osmosis

The net direction of osmosis depends on the relative concentration of solutes on the two sides of the membrane. Examine Table 5-1. When the concentration of solute molecules outside the cell is lower than the concentration in the cytosol, the solution outside is hypotonic to the cytosol. In this situation, water diffuses into the cell until equilibrium is established. When the concentration of solute molecules outside the cell is higher than the concentration in the cytosol, the solution outside is hypertonic to the cytosol. In this situation, water diffuses out of the cell until equilibrium is established.

Observing Diffusion

Materials disposable gloves, lab apron, safety goggles, 600 mL beaker, 25 cm dialysis tubing, funnel, 15 mL starch solution (10 percent), 20 drops Lugol's solution, 300 mL water, 100 mL graduated cylinder, 20 cm piece of string (2)

Procedure

1. Put on your disposable gloves, lab apron, and safety goggles.

2. Pour 300 mL of water in the 600 mL beaker.

3. Add 20 drops of Lugol's solution to the water.

4. Open the dialysis tubing, and tie one end tightly with a piece of string.

5. Using the funnel, pour 15 mL of 10 percent starch solution into the dialysis tubing.

6. Tie the other end of the dialysis tubing tightly with the second piece of string, forming a sealed bag around the starch solution.

7. Place the bag into the solution in the beaker, and observe the setup for a color change.

Analysis What happened to the color in the bag? What happened to the color of the water around the bag? Explain your observations.

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TABLE 5-1 Direction of Osmosis

Condition Net movement of water

TABLE 5-1 Direction of Osmosis

Condition Net movement of water

External solution into the is hypotonic to cell cytosol

h2o » i h20

External solution out of is hypertonic to the cell cytosol

h2o 4 » h2o

External solution none is isotonic to cytosol

H2O ^ ^ H2O

When the concentrations of solutes outside and inside the cell are equal, the outside solution is said to be isotonic to the cytosol. Under these conditions, water diffuses into and out of the cell at equal rates, so there is no net movement of water.

Notice that the prefixes hypo-, hyper-, and iso- refer to the relative solute concentrations of two solutions. Thus, if the solution outside the cell is hypotonic to the cytosol, then the cytosol must be hypertonic to that solution. Conversely, if the solution outside is hypertonic to the cytosol, then the cytosol must be hypotonic to the solution. Water tends to diffuse from hypotonic solutions to hypertonic solutions.

How Cells Deal with Osmosis

Cells that are exposed to an isotonic external environment usually have no difficulty keeping the movement of water across the cell membrane in balance. This is the case with the cells of vertebrate animals on land and of most other organisms living in the sea. In contrast, many cells function in a hypotonic environment. Such is the case for unicellular freshwater organisms. Water constantly diffuses into these organisms. Because they require a relatively lower concentration of water in the cytosol to function normally, unicellular organisms must rid themselves of the excess water that enters by osmosis.

Some of them, such as the paramecia shown in Figure 5-2, do this with contractile vacuoles (kon-TRAK-til VAK-yœ-OL), which are organelles that remove water. Contractile vacuoles collect the excess water and then contract, pumping the water out of the cell. Unlike diffusion and osmosis, this pumping action is not a form of passive transport because it requires the cell to expend energy.

figure 5-2

The paramecia shown below live in fresh water, which is hypotonic to their cytosol. (a) Contractile vacuoles collect excess water that moves by osmosis into the cytosol. (b) The vacuoles then contract, returning the water to the outside of the cell. (LM 315x)

Vacuole filling with water

Vacuole filling with water

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