AP Bio Chap 7Osmosis and Diffusion

So, how does a membrane regulates what goes in and out?

Depends on:1) Lipid solubility - Hydrophobic molecules, such as hydrocarbons, O2, CO2

pass freely - Ions, polar molecules need transport molecules (proteins)

with hydrophilic channels or actually bind to the carrier protein to pass through

- Aquaporins facilitate water passage2) Size of the molecule3) Concentration of the molecule

Cell membranes are semipermeable.

Types of transport: PASSIVE TRANSPORT – no energy required

1) Diffusion - movement of a substance from greater to lesser concentration (down its concentration gradient)

• will continue until dynamic equilibrium; no more NET movement

• most common method of movement for nonpolar molecules across the membrane

• most efficient when large surface area, well-defined concentration gradient, short distance.

Diffusion

Molecules of dye

Fig. 7-11a

Membrane (cross section)

WATER

Net diffusion Net diffusion

(a) Diffusion of one solute

Equilibrium

(b) Diffusion of two solutes

Fig. 7-11b

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

The diffusion of one solute is unaffected by the diffusion of another solute.

2) 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

• Direction of water flow is determined by the number (not kinds) of solute particles (molecules and ions)

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Lowerconcentrationof solute (sugar)

Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Fig. 7-UN3

Environment:0.01 M sucrose

0.01 M glucose

0.01 M fructose

“Cell”

0.03 M sucrose

0.02 M glucose

What will happen here if the sucrose cannot diffuse? Glucose, fructose, and water can.

Fig. 7-UN4

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

REMEMBER!

WATER ALWAYS FLOWS INTO A HYPERTONIC SITUATION!

WATER ALWAYS FLOWS INTO A HYPERTONIC SITUATION!

What happens to animal cells in the following situations?

• Hypertonic environment – lose water, shrivel• Hypotonic environment – water moves in, cell

swell and possibly bust• Adaptations to contend with this: - contractile vacuoles in protists - membranes less permeable to water - isotonic internal conditions to their

environment

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

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 fluid from the cell.

Water Balance of Cells with Walls

• Cell walls help maintain water balance• A plant cell in a hypotonic solution swells until the wall

opposes uptake; the cell is now turgid (firm)• If a plant cell and its surroundings are isotonic, there is no

net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

• In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

???• The ideal environment for animal cells is

________________________.• The ideal environment for plant cells is

________________________.

• Hypertonic or hypotonic environments create osmotic problems for organisms

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

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