Diffusion and
Osmosis
The Cell Membrane
� The cell membrane is:
�Selectively permeable
� Permeable = Pass through (Latin)
� Cell membrane = Gate-Keeper that determines what can and can’t enter the cell.
The Gate Keeper: The Cell
Membrane
� Who comes in for the
party?
� Small molecules like
water, carbon dioxide,
or oxygen can easily
pass through the
pores of the
membrane.
� Who gets bounced?
� Large molecules like
glucose
� Charged molecules
like salts.
Molecule Movement
� Recall cells are small because they need to
obtain sufficient nutrients and dispose of their
wastes.
� This is done by the cell membrane by three
means:
1. Diffusion
2. Osmosis
3. Active Transport (Senior Science)
Definitions.
� Solution = A homogenous, liquid mixture of two or more substances.
� Solvent = The dissolving agent of a solution. Water is the most versatile solvent known.
� Solute = Substance that is dissolved in a solution.
Diffusion
� The process by which molecules tend to move
from an area of higher concentration to an area
of lower concentration.
� Concentration= amount of substance in a given
volume.
� Think of the perfume example (high� low)
� Think of the food-dye example (high� low)
Why Diffusion?
� Kinetic molecular theory.
� Molecules are always moving
� Collisions occur between molecules, which causes the molecules to move away from one another.
� If there is space these molecules want to spread out evenly.
What direction will the molecules move?
Diffusion and the Cell
� Movement of molecules by diffusion is how the cell fulfills its needs.
� Nutrients: Low concentration in the cells, high concentration outside of the cells.
� What direction will the nutrients diffuse?
� From outside the cell to inside the cell.
� High to Low Concentration!!
Diffusion and the Cell
� Wastes: High concentration in the cell, low outside of the cell.
� What direction will the wastes move?
� From inside the cell to outside the cell.
� High to Low concentration
A special case of diffusion:
Osmosis
� Water moves through the pores of the cell
membrane like other molecules.
� The movement of water from a high
concentration to a low concentration (thru a
semi-permeable membrane) is called: Osmosis
� Osmosis = movement of water (the solvent)
� Diffusion = movement of dissolved materials
(the solutes)
Osmosis is the movement of water (red dots) through a semipermeable membrane to a higher concentration of
solutes (blue dots).
Activity — Predicting
Movement of Water� This “U” tube has a
selectively permeable membrane separating side A and B.
� Side A has a 40% solution of sugar and side B has a 60% solution of sugar.
� Which side has more water?
� Which way will the water move?
Selectively Permeable Membrane
Osmosis – a cure for the flu?
� Who here has taken penicillin to combat sickness?
� Penicillin weakens bacterial cell walls
� Water is at higher conc. outside the cell…which way will water move?
� With a weak cell wall, the bacterial cell swells and…POP!
**Summary**
� Both osmosis and diffusion involve the movement of molecules from an area of high concentrations to a area of low concentrations.
� Osmosis = movement of water (the solvent)
� Diffusion = movement of dissolved materials (the solutes)
Water Balance
� The cell membrane doesn’t actually control the movement of water.
� The environment around the cell determines the direction of water movement.
� For example, a cell is placed in a salt water tide pool. The sun evaporates some water.
� The concentration of water inside the cell is higher than outside.
� What happens?
� Water leaves the cell. � Possible shrinking of the cell.
Water Balance
� On the contrary imagine it rains.
� A higher concentration of water outside the cell is the result.
� What happens?
� Water moves into the cell
� Possible swelling of the cell.
Water Regulation
� Without other mechanisms the cell would burst or shrivel.
� Plant vacuoles store water even when the environment is dry.
� The cell wall is rigid to prevent bursting or shriveling.
� Animals can increase or decrease their amount of water intake and outtake (Urine concentration/amount of water that is drank)
Hypertonic Solution – More water inside cell than outside
Plasmolysis
Plasmolysis
Cells shrink
and die
Iso-osmotic Solution – same concentration of water inside and outside cell
Animal cell
Plant cell
No net gain or
loss of water
Hypotonic Solution – more water
outside cell than inside
Cytolysis –
cell swells and
bursts
Builds up turgor
pressure – cell
becomes stiff,
keeping plant
upright
� What would happen to a cell placed in pure water?
90% water
10% solute100% water
Cell’s are about 90% water.
Human Red Blood Cell Before: After
� What would happen to a cell
placed in a 20% salt solution?
90% water
10% solute
Cell’s are about 90% water.
80% water
Human Red Blood Cell Before: After
Potatoes and Corn Starch…..
� Finish your Experiments…
� Remember Unit Test on Friday
� Microscopes & Cells
� Structures/Functions
� Cellular Respiration/Photosynthesis
� Cell Size
� Osmosis/Diffusion
� Single Celled Organisms.
� Similar to Quiz Yesterday