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Vacuole contracting

Vacuole contracting

Cell walls

(a) HYPOTONIC figure 5-3

(b) HYPERTONIC

(a) HYPOTONIC figure 5-3

These two photographs show cells in the skin of a red onion. (a) In a hypotonic environment, the cells are pressed against the cell walls. (b) In a hypertonic environment, the cells contract and pull away from the cell walls.

figure 5-4

(a) In an environment that is isotonic to the cytosol, a human red blood cell keeps its normal shape—round and dimpled.

(b) In a hypotonic environment, the cell gains water and swells. (c) In a hypertonic environment, the cell loses water and becomes shriveled.

Cell walls

(b) HYPERTONIC

Other cells, including many of those in multicellular organisms, respond to hypotonic environments by pumping solutes out of the cytosol. This lowers the solute concentration in the cytosol, bringing it closer to the solute concentration in the environment. As a result, water molecules are less likely to diffuse into the cell.

Most plant cells, like animal cells, live in a hypotonic environment. In fact, the cells that make up plant roots may be surrounded by water. This water moves into plant cells by osmosis. These cells swell as they fill with water until the cell membrane is pressed against the inside of the cell wall, as Figure 5-3a shows. The cell wall is strong enough to resist the pressure exerted by the water inside the expanding cell. The pressure that water molecules exert against the cell wall is called turgor pressure (TER-gor PRESH-er).

In a hypertonic environment, water leaves the cells through osmosis. As shown in Figure 5-3b, the cells shrink away from the cell walls, and turgor pressure is lost. This condition is called plasmolysis (plaz-MAHL-uh-sis), and is the reason that plants wilt if they don't receive enough water.

Some cells cannot compensate for changes in the solute concentration of their environment. Red blood cells in humans, for instance, lack contractile vacuoles, solute pumps, and cell walls. As shown in Figure 5-4, these cells lose their normal shape when they are exposed to an environment that is not isotonic to their cytosol. In a hypertonic environment, water leaves the cells, making them shrink and shrivel. In a hypotonic environment, water diffuses into the cells, causing them to swell and eventually burst. The bursting of cells is called cytolysis (sie-TAHL-uh-sis).

(a) ISOTONIC

(b) HYPOTONIC

(c) HYPERTONIC

(a) ISOTONIC

(b) HYPOTONIC

(c) HYPERTONIC

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