Membrane Structure and Function

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5

Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another

No work must be done to move substances down the concentration gradient

The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen

Slide 37

(b) Diffusion of two solutes

Fig. 7-11b

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

Slide 38

Effects of Osmosis on Water Balance

Osmosis is the diffusion of water across a selectively permeable membrane

Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

Slide 39

Lower

concentration

of solute (sugar)

Fig. 7-12

H2O

Higher concentration

of sugar

Selectively

permeable

membrane

Same concentration

of sugar

Osmosis

Slide 40

Water Balance of Cells Without Walls

Tonicity is the ability of a solution to cause a cell to gain or lose water

Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water

Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

Slide 41

Fig. 7-13

Hypotonic solution

(a) Animal

cell

(b) Plant

cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

Slide 42

Hypertonic or hypotonic environments create osmotic problems for organisms

Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments

The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Video: Chlamydomonas

Video: Paramecium Vacuole

Slide 43

Fig. 7-14

Filling vacuole

50 µm

(a) A contractile vacuole fills with fluid that enters from

a system of canals radiating throughout the cytoplasm.

Contracting vacuole

(b) When full, the vacuole and canals contract, expelling