9/27/2015
Diffusion
•
Movement of a substance from an area of high
concentration to an area of low concentration
(along a diffusion gradient)
•
Net movement stops when substance is evenly
distributed
Biology 102
Cells and Osmosis
Diffusion
•
Diffusion can occur across a barrier, IF the
barrier is permeable to the substance
Diffusion
•
Different substances can diffuse independently
•
May be in the same direction, may be in
different directions
Diffusion
•
This is useful for cells
•
Some substances can diffuse across the cell
membrane
•
Requires no energy
Activity
•
Predict the direction of diffusion relative to the
separation (to the left or to the right)
2M
4M
22mM
5mM
5%
1.3%
2.4M/kg
1.5M/kg
350mM
0.35M
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9/27/2015
Diffusion
•
Scenario:
Osmosis
•
Diffusion of water molecules
•
Occurs when…
•
Concentration gradient exists across a barrier
•
Barrier is not permeable to the substance
•
Concentration gradient exists
•
Lowest energy state of the system is to
equalize the gradient
•
Membrane is not permeable to substance
•
Membrane is permeable to water
•
What to do?
•
If you can’t move the substance, move the
water!
Osmosis
•
•
Osmosis
Osmosis: Movement of water from an area of low
•
solute concentration to an area of high solute
concentration
Use arrows to indicate the direction that water
will move across the membrane
•
Remember: the goal is to reach the same
concentration on both sides
Again, won’t happen if the barrier is permeable
to the dissolved substance
2M
4M
22mM
5mM
5%
1.3%
2.4M/kg
1.5M/kg
350mM
0.35M
Osmosis
Tonicity
•
This frequently happens in cells
•
•
Water diffuses in or out through a membrane
protein called aquaporin
A way to describe solutions surrounding a living
cell, and the osmotic effect on the cell
•
Three possibilities:
•
Hypertonic
•
Solution has higher osmotic concentration than cell
•
Hypotonic
•
Isotonic
•
•
Solution has lower osmotic concentration than cell
Solution has equal osmotic concentration to cell
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Tonicity
•
•
•
•
Consider a cell suspended in a solution of sucrose
Sucrose can’t get in or out through the
membrane
•
Cell: 300mM
Solution: 450mM
•
•
•
•
Hypertonic
Which way will the
water move?
•
What will happen
•
Tonicity
450mM
Cell: 300mM
Solution: 265mM
•
Hypotonic
Which way will the
water move?
•
What will happen
•
300mM
Consider a cell suspended in a solution of sucrose
Sucrose can’t get in or out through the
membrane
to cell shape?
•
Tonicity
Consider a cell suspended in a solution of sucrose
Sucrose can’t get in or out through the
membrane
Cell: 300mM
• Solution: 300mM
•
•
Consider a cell suspended in a solution of sucrose
Sucrose can’t get in or out through the
membrane
Cell: 0.85%
• Solution: 1.2%
•
•
Isotonic
Which way will the
water move?
• What will happen
to cell shape?
•
•
Tonicity?
Which way will
water move?
• What will happen
to cell shape?
•
300mM
300mM
Consider a cell suspended in a solution of sucrose
•
Sucrose can’t get in or out through the
membrane
•
•
•
1.2%
Tonicity
•
•
0.85%
•
Tonicity
•
265mM
to cell shape
Tonicity
•
300mM
•
Isotonic
•
Solution does not cause osmotic flow of water
•
No change in cell shape
Cell: 300mOs
Solution: 375mOs
Tonicity?
Which way will
water move?
What will happen
to cell shape?
300mOs
375mOs
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9/27/2015
Tonicity
•
Hypotonic
Tonicity
•
Hypertonic
•
Solution with lower osmotic concentration
than the cell
•
Solution with higher osmotic concentration
than the cell
•
Cell will swell
•
Cell will shrink
Thought Question
•
How would adding salt to the soil affect a plant?
(The Romans did this after defeating Carthage
in 149 BC)
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