1. Diffusion, Osmosis and the SF Bay

2. Solutions Define solution
A homogeneous mixture in which one substance is dissolved in another.
Ex. Kool-aid, saltwater
Define solute
The substance in a solution that is dissolved.
Ex. Sugar, salt
Define solvent
The substance in a solution that does the dissolving.
Ex. water

3. Selectively Permeable Membrane
Diffusion
Define diffusion
The process by which molecules of a substance move across a concentration gradient, from areas of higher concentration of that substance to areas of lower concentration
Define concentration gradient
The distinguishing line between higher and lower concentrations
Define selectively permeable membrane
A biological membrane that allows some substances to pass through, but not others
Selectively Permeable Membrane

4. Diffusion
Factors that determine whether diffusion occurs across a membrane include:
Unequal concentration of substances on either side of a membrane
Membrane permeability
If molecules can penetrate a membrane, diffusion will continue until equilibrium is reached!

5. Osmosis Define osmosis
The diffusion of water through a selectively permeable membrane from an area of greater water concentration to an area of lesser water concentration.

6. Osmosis
Problems
Cell cytoplasm is filled with solute particles (sugars, salts, proteins etc.)
Water concentration is lower inside the cell than in fresh water
Cell membrane is permeable to water, but not to solutes
If water moves in, the cell will expand and burst!
Solutions
Some cells do not come into contact with fresh water
Plants cells and bacteria have cell walls to prevent cells from expanding
Some unicellular organisms pump excess water out of cell

7. Passive Transport Define passive transport
The transport of molecules, down a concentration gradient, from a region of higher concentration to a region of lower concentration
Energy is NOT required for this process to occur
Ex. diffusion, osmosis and facilitated diffusion

8. Facilitated Diffusion
Define facilitated diffusion
The transport of molecules across a membrane in the direction of lowest concentration by a carrier protein
Explanation:
Some molecules are too big or have to wrong charge to be permitted to freely pass through a cell membrane
The molecules move down a concentration gradient, so energy is NOT required!

9. Active Transport Define active transport
The transport of molecules against a concentration gradient, from a region of low concentration to a region of high concentration
Examples:
Molecules are carried through membrane-associated pumps
Endocytosis
The molecules move against a concentration gradient, so energy IS required!

10. Applying the Concepts-Hypotonic Solution
1. Hypotonic solution-a solution with a lesser concentration of solute compared to the concentration of solute inside the cell
Example: a single celled salt water organism is placed in brackish water
Direction of water flow?
Direction of solute flow?
What will happen to the cell?
Water flows into the cell.
The solute does not move because of the selectively permeable membrane.
The cell will expand and could burst!
brackish water

11. Applying the Concepts-Hypertonic Solution
2. Hypertonic solution-a solution with a greater concentration of solute compared to the concentration of solute inside the cell
Example: a single celled fresh water organism is placed in salt water
Direction of water flow?
Direction of solute flow?
What will happen to the cell?
Water flows out of the cell.
The solute does not move because of the selectively permeable membrane.
The cell will shrink!
salt water

12. Applying the Concepts-Isotonic Solution
3. Isotonic solution-a solution with an equal concentration of solute compared to the concentration of solute inside the cell
Example: a single human red blood cell is placed in a 0.9% saline (salt) solution
Direction of water flow?
Direction of solute flow?
What will happen to the cell?
There is no net movement of water in a particular direction.
The solute does not move because of the selectively permeable membrane.
The cell size stays the same!
0.9% saline
Osmosis Animation

13. Diffusion Demonstration
Set-up #1
The cornstarch represents the living cell. The iodine solution represents the environment.
Example: a single celled salt water organism placed in brackish water.
There is a lower concentration of iodine inside the cell, so the iodine moves in.
There is no net movement of cornstarch because of the selectively permeable membrane.
This causes the liquid inside the plastic bag to change color and the plastic bag to expand.
This is an example of a_____________ environment.
Iodine solution
Cornstarch
solution
hypotonic

14. Diffusion Demonstration
Set-up #2
The cornstarch represents the environment. Iodine represents the living cell.
Example: a single celled fresh water organism placed in salt water.
There is greater concentration of iodine inside the cell, so iodine moves out.
There is no net movement of cornstarch because of the selectively permeable membrane.
This causes the liquid outside the plastic bag to change color and the plastic bag to shrink.
This is an example of a _______________ environment.
Cornstarch solution
Iodine
solution
hypertonic