Biology Now, 2e

Engineering Life ■ 65

At first, the

molecules

of food

coloring are

concentrated

in one region.

Net movement of

the food coloring

is from regions of

high concentration

to regions of low

concentration.

Diffusion ceases

when food-coloring

molecules are

evenly distributed.

At equilibrium, just

as many molecules

move into any given

region as leave that

region, so there is

no net change in

concentration.

A primary type of passive transport is

diffusion, the movement of a substance from

a region of higher concentration to a region

of lower concentration (Figure 4.5). Water,

oxygen, and carbon dioxide usually enter and

leave cells by simple diffusion: These small,

uncharged molecules slip between the large

molecules in the phospholipid bilayer without

much hindrance.

Water moves in and out of cells (and compart-

ments inside cells) by osmosis. Osmosis is a form

of simple diffusion because the water molecules

are moving from areas of higher concentration

to areas of lower concentration (Figure 4.6).

Osmosis is critical for cellular processes because

most cells are at least 70 percent water, and

nearly all cellular processes take place in a

water-rich environment.

Cells must maintain a stable internal water

concentration to function properly, but the

concentration of water in most cells changes

from moment to moment. For example, when

salt molecules move into a cell, the concentration

of water in the cell decreases because additional

molecules have been added. In response, water

Figure 4.5

Food coloring in water illustrates

diffusion

Q1: Is the dye at equilibrium in any of these

glasses? Describe how the first glass will look

when the dye is at equilibrium with the water.

Q2: Will diffusion mix the molecules of dye

evenly through the water, or is it necessary to

shake the container to get a uniform mixture?

Q3: Will diffusion mix the dye faster in hot

water than in cold water? Why or why not?

(Hint: Review the discussion of the behavior

of water molecules at different temperatures

in Chapter 3.)

Figure 4.6

In osmosis, water diffuses across a semipermeable

membrane

Osmotic movement of water between a cell and the external environment is

critical for maintaining a constant water concentration in the cell, which it

needs to function properly.

Q1: What would the second diagram look like if the pores in the

semipermeable membrane were too small to allow water molecules to

pass through?

Q2: What would the second diagram look like if the pores were large

enough to let both water molecules and sugar molecules through?

Q3: The fluid in an IV bag is isotonic to blood. What change would you

see in the red blood cells of a patient if a bag of a hypertonic solution

was used in error?

Water

molecule

Water

molecule

Sugar

molecule

Semipermeable

membrane

Semipermeable

membrane

After a period of time, water molecules have

moved by osmosis from the right side of the

membrane (which had a higher concentra-

tion of water) to the left side of the

membrane. The concentration of water on

the two sides of the membrane is now the

same, and the movement of water molecules

is now the same in both directions.

Sugar has just been added to a beaker of water. This beaker

is divided by a semipermeable membrane—that is, a

membrane with pores large enough to allow water molecules

to pass through, but too small for sugar molecules to pass.