5.4 - 5.6

Question 1

Note 1 —-»

Answer 1

One of the most important substances that cross membranes by passive transport is water.

Question 2

Osmosis

Answer 2

The diffusion of free water across a selectively permeable membrane.

Question 3

Note 2 —-»

Answer 3

A selectively permeable membrane allows some substances to cross more easily than others.

Question 4

Note 3 —-»

Answer 4

If a membrane permeable to water but not to a solute (such as glucose) separates two solutions that have different concentrations of solute. (A solute is a substance that dissolves in a liquid solvent. The resulting mixture is a solution.) The solution on the right side of the U-shaped tube initially has a higher concentration of solute than that on the left side. Water will cross the membrane until the solute concentrations are more nearly equal on both sides, as you can see in the U-tube on the right.

Question 5

Note 4 —-»

Answer 5

Polar water molecules cluster around hydrophilic (water-loving) solute molecules. The effect is that on the right side of the U-tube, there are fewer water molecules that are free to cross the membrane. The less-concentrated solution on the left side has fewer solute molecules but more free water molecules available to move. There is a net movement of water down its concentration gradient, from the solution with more free water molecules (and lower solute concentration) to that with fewer free water molecules (and higher solute concentration). The result is the difference in water levels in the U-tube.

Question 6

Predict the net water movement between two solutions—a 0.5% sucrose solution and a 2% sucrose solution—separated by a membrane not permeable to sucrose.

Answer 6

Water will move from the 0.5% sucrose solution (lower solute concentration) to the 2% sucrose solution (higher solute concentration).

Question 7

Tonicity

Answer 7

The ability of a solution surrounding a cell to cause that cell to gain or lose water.

Question 8

Note 5 —-»

Answer 8

The tonicity of a solution mainly depends on its concentration of solutes relative to the concentration of solutes inside the cell.

Question 9

Isotonic

Answer 9

Referring to a solution that when surrounding a cell, causes no movement of water into or out of the cell.

Question 10

Note 6 —-»

Answer 10

When an animal cell, such as the red blood cell shown in the top center of the figure, is immersed in a solution that is isotonic to the cell (iso, same, and tonos, tension), the cell’s volume remains constant. The solute concentration of a cell and its isotonic environment are essentially equal, and the cell gains water at the same rate that it loses it. In your body, red blood cells are transported in the isotonic plasma of the blood.

Question 11

Note 7 —-»

Answer 11

Intravenous (IV) fluids administered in hospitals must also be isotonic to blood cells. The body cells of most animals are bathed in an extracellular fluid that is isotonic to the cells. And seawater is isotonic to the cells of many marine animals, such as sea stars and crabs.

Question 12

Hypotonic

Answer 12

A solution that, when surrounding a cell, will cause the cell to take up water.

Question 13

Note 8 —-»

Answer 13

What happens when an animal cell is placed in a hypotonic solution (hypo, below), a solution with a solute concentration lower than that of the cell? As shown in the upper left of the figure, the cell gains water, swells, and may burst (lyse) like an overfilled balloon. The upper right shows the opposite case—an animal cell placed in a hypertonic solution (hyper, above), a solution with a higher solute concentration. In which direction will water move? The cell shrivels and can die from water loss.

Question 14

Hypertonic

Answer 14

A solution that, when surrounding a cell, will cause the cell to lose water.

Question 15

Note 9 —-»

Answer 15

For an animal to survive in a hypotonic or hypertonic environment, it must have a way to prevent excessive uptake or loss of water and regulate the solute concentration of its body fluids. The control of water balance is called osmoregulation. For example, in a freshwater fish, which lives in a hypotonic environment, water enters its cells by osmosis and its kidneys must work constantly to remove excess water from the body.

Question 16

Osmoregulation

Answer 16

The homeostatic maintenance of solute concentrations and water balance by a cell or an organism.

Question 17

Note 10 —-»

Answer 17

Although the plant cell swells as water enters by osmosis, the cell wall exerts a back pressure, called turgor pressure, which prevents the cell from taking in too much water and bursting. Plants that are not woody, such as most houseplants, depend on their turgid cells for mechanical support. In contrast, when a plant cell is surrounded by an isotonic solution, there is no net movement of water into the cell, and the cell is flaccid (limp). The plant itself may wilt.

Question 18

Note 11 —-»

Answer 18

In a hypertonic environment (bottom right), a plant cell is no better off than an animal cell. As a plant cell loses water, it shrivels, and its plasma membrane pulls away from the cell wall. This process, called plasmolysis, causes the plant to wilt and can be lethal to the cell and the plant. The walled cells of bacteria and fungi also plasmolyze in hypertonic environments. Thus, meats and other foods can be preserved with concentrated salt solutions because the cells of food-spoiling bacteria or fungi become plasmolyzed and eventually die.

Question 19

Explain the function of the contractile vacuoles in a freshwater Paramecium (shown in Figure 4.11A) in terms of what you have just learned about water balance in cells.

Answer 19

The pond water in which Paramecium lives is hypotonic to the cell. The contractile vacuoles expel the water that constantly enters the cell by osmosis.

Question 20

Note 12 —-»

Answer 20

Nonpolar molecules, such as O2 and CO2, can dissolve in the lipid bilayer of a membrane and diffuse through it with ease. But how do polar or charged substances make it past the hydrophobic center of a membrane? Hydrophilic molecules and ions require the help of specific transport proteins to move across a membrane. This assisted transport, called facilitated diffusion, is a type of passive transport because it does not require energy. As in all passive transport, the driving force is the concentration gradient.

Question 21

Facilitated Diffusion

Answer 21

The passage of a substance through a specific transport protein across a biological membrane down its concentration gradient.

Question 22

Note 13 —-»

Answer 22

Another type of transport protein, called a carrier protein, binds its passenger, changes shape, and releases the transported molecule on the other side. In both cases, the transport protein helps a specific substance diffuse across the membrane down its concentration gradient and, thus, requires no input of energy.

Question 23

Note 14 —-»

Answer 23

Substances that use facilitated diffusion for crossing cell membranes include a number of sugars, amino acids, ions—and even water. The water molecule is very small, but because it is polar (see Module 2.6), its diffusion through a membrane’s hydrophobic interior is relatively slow. For many cells, this slow diffusion of water is adequate. Cells such as plant cells, red blood cells, and the cells lining your kidney tubules, however, have greater water-permeability needs.

Question 24

Note 15 —-»

Answer 24

The very rapid diffusion of water into and out of such cells is made possible by a protein channel called aquaporin. In the next module, we explore the discovery of these transport proteins.

Question 25

Aquaporin

Answer 25

A transport protein in the plasma membrane of an animal, plant, or microorganism cell that facilitates the diffusion of water across the membrane.

Question 26

How do transport proteins contribute to a membrane’s selective permeability?

Answer 26

Because they are specific for the solutes they transport, the numbers and kinds of transport proteins affect a membrane’s permeability to various solutes.