ap bio chap 7 osmosis and diffusion. so, how does a membrane regulates what goes in and out? depends...
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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)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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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