Many cells maintain size and composition using active transport.
Many cells use active transport to maintain their size. Consider human red blood cells placed in a variety of different solutions (Fig. 5.14). If a red blood cell is placed in a hypertonic solution (one with a higher solute concentration than that inside the cell), water leaves the cell by osmosis and the cell shrinks. By contrast, if a red blood cell is placed in a hypotonic solution (one with a lower solute concentration than that inside the cell), water moves into the cell by osmosis and the cell lyses, or bursts. Animal cells solve the problem of water movement in part by keeping the intracellular fluid isotonic (that is, at the same solute concentration) as the extracellular fluid. Cells use the active transport of ions to maintain equal concentrations inside and out, and the sodium-potassium pump plays an important role in keeping the inside of the cell isotonic with the extracellular fluid.
FIG. 5.14 Changes in red blood cell shape due to osmosis. Red blood cells shrink, swell, or burst because of net water movement driven by differences in solute concentration between the inside and the outside of the cell.
Quick Check 3 In the absence of the sodium-potassium pump, the extracellular solution becomes hypotonic relative to the inside of the cell. Poisons such as the snake venom ouabain can interfere with the action of the sodium-potassium pump. What are the consequences for the cell?
Quick Check 3 Answer
If the sodium-potassium pump is made inactive by poison, the cell will swell and even burst, as the intracellular fluid becomes hypertonic relative to the outside of the cell and water moves into the cell by osmosis.
Human red blood cells avoid shrinking or bursting by maintaining an intracellular environment isotonic with the extracellular environment, the blood. But what about a single-celled organism, like Paramecium, swimming in a freshwater lake? In this case, the extracellular environment is hypotonic compared with the concentration in the cell’s interior. As a result, Paracemium faces the risk of bursting from water moving in by osmosis. Paramecium and some other single-celled organisms contain contractile vacuoles that solve this problem. Contractile vacuoles are compartments that take up excess water from inside the cell and then, by contraction, expel it into the external environment. The mechanism by which water moves into the contractile vacuoles differs depending on the organism. The contractile vacuoles of some organisms take in water through aquaporins, while the contractile vacuoles of other organisms first take in protons through proton pumps, with water following by osmosis.