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Osmosis:Passive Transport
Question:Are the cells in an
elephant bigger, smaller, or about the same size as those in a mouse?
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Cell Size
Surface area (plasma membrane surface) is determined by
A = L x W
Volume of a cell is determined by V = L x W x H
Therefore, volume increases FASTER than the surface area
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Factors Affecting Cell Size
When the surface area is no longer great enough to get rid of all the wastes and take in enough nutrients, food and water…
then the cell MUST divide
Therefore, the cells of an organism are close in size
4
Cell Size
Question:Are the cells in an elephant bigger, smaller, or about the same size as those in a mouse?
5
Cell Size
About the same size, but …
The elephant has MANY MORE cells than a mouse!
•Osmosis is a special case of diffusion
•Osmosis involves the diffusion of water through a membrane
•The membrane may be: Artificial (non-living) - Cellophane Natural - Cell Membrane
What is Osmosis?
•The membrane must allow water molecules to diffuse through.
The cell membrane is permeable to H2O.
• If a dilute solution is separated from a concentrated solution...
water will pass from the dilute to the concentrated solution.
Permeability
Osmosisdilute
solutionconcentrated
solution
membrane
More water passes fromdilute to concentrated ...
...until concentrationsbecome equal
level rises
level falls
Common misconception:
Water does happen to pass in BOTH directions...it just passes faster from the dilute to the concentrated solution.
Remember...Water molecules undergo diffusion.
How does the H2O pass through?
H2O
membranewater sugar solution
There are as many water molecules on the right as there are on the left. Many
of them on the right are attached to sugar molecules and are not free to
move.
Because there are more freely moving water molecules on the left,
more H2O diffuses through the pores of the membrane from left to right.
Next slide
Cytoplasm functions as afairly concentrated solution.
Osmosis in Animal Cells
There is a greater concentration of free
water molecules outside the cell than inside…
...so water diffuses into the cell by osmosis
and the cell swells up
If osmosis continued, the animal cell would burst!
This would be bad news for animals
Consequently, there are processes in the animal’s body which control osmosis...
This is done by keeping the concentration of body fluids inside the cell
the same as it is outside
In fresh water fish, blood is more concentrated than the surrounding water
As a result the fish takes in water by osmosis
Not so much through the skin - (too) thick – but through the gills, which have a very thin membrane
Trout
Osmosis Adaptation: Fresh water fish - Gills
Perch
The fish gets rid of the excess water by means of its kidneys
gills absorb water by osmosis
blood carries waterto kidneys
kidneys expel excess water ...
...in the form of dilute urine
heart
Osmosis Adaptation: Contractile Vacuole
Found in unicellular protists like paramecia
Regulate water intake by pumping out excess H2O(homeostasis)
Keeps the cell from lysing (bursting)
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Contractile vacuole animation
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A solution can be…
1. Hypotonic2. Isotonic3. Hypertonic
Tonicity
Hypo = less, (think hypothermia) Tonic = dissolved particles
Hypotonic Solution = Solute concentration lower outside than inside the cell
The outside of the cell has LESS solute, MORE free H2O
What happens to a cell in a Hypotonic Solution?
http://www.chem.ufl.edu/~itl/4411/colligative/FG13_022.GIF
Water moves into cell from solution.
Cell expands (and may burst)
What happens to a cell in a Hypertonic Solution?
The concentration of solutes is the same on either side of the membrane.
Both sides have the same amount of solutes and free H2O!
No NET water movement.
This is the ideal situation!
What happens to a cell in a Isotonic Solution?
Hyper = more (think hyperactive) Tonic = dissolved particles
Hypertonic Solution: Solute concentration higher outside the cell than inside the cell.
The outside of the cell has MORE solute,
LESS free H2O
What happens to a cell in a Hypertonic Solution?
http://cdn.innovateus.net/preset_4/hypertonicbloodcell.png
Water moves out of cell into solution.
Cell shrinks and becomes shriveled
What happens to a cell in a Hypertonic Solution?
Comprehension Question:
What would happen if a nurse gave a patient an IV with a
pure water solution?Draw it, explain it!
Comprehension Question:
Survival Tip: If stranded at sea, people are cautioned not to drink the ocean water, no matter how thirsty they may
become.
Draw it, explain it!
Osmosis and Plant Cells
Cytoplasm functions as afairly concentrated solution.
Plant cells are surrounded by a strong, rigid cell wall which
prevents the cell from taking on too much water and exploding.
• Cell membrane acts as a selectively permeable membrane
• The vacuole contains a solution of salts and sugars
• If there is water outside the cell, it will diffuse by osmosis through the plasma membrane into the vacuole
• Cell wall is freely permeable to water
The vacuole will expand, pushing the cytoplasm outwards against the cell wall
• The cellulose cell wall cannot stretch, so the vacuole cannot continue to expand
• But the pressure of the vacuole against the cell wall makes the cell very firm
A plant cell in this condition is said to be turgid
If all the cells in the plant are turgid, the plant willbe firm and upright and the leaves would be
expanded
cell wall cytoplasm and cell
membrane
vacuole
The cell absorbs waterby osmosis .... ....but the cell wall stops
the cell from expanding any more
Plant cells
These cells are short of water; the tissue is limp and the plant is
wilting
The cells have taken upwater by osmosis; the
cells are turgid and thetissue is firm
Limp (Plasmolyzed) VS. Turgid Tissue
(1)Limp/ Plasmolyzed
(2) turgid
The color of cell sap in these two cells differ. What does this say about the concentrations of the
cell sap?What will happen?
Osmosis between Plant Cells
If the concentration of the cell sap is greater in one cell than in its neighbor, water will pass by
osmosis from the less concentrated to the more concentrated.
cell sap more concentrated
cell sap less concentrated
Plasmolysis in Red Onion Cell
http://www.youtube.com/watch?v=Sw87Q53NZLI
http://www.youtube.com/watch?v=gYbt7hhIxPo
http://www.youtube.com/watch?v=x4il3cHhqhY
Web Videos
Exposure to extremely hypertonic environments can
kill a plant.
EX: • Your dog creates spots of dead lawn where he pees.
• The urine is hypertonic to the interior of the cells that make up the blade of grass
• Large amounts of water are drawn out of the grass, killing it!
Vocabulary
1. Osmosis2. Contractile Vacuole3. Hypotonic Solution4. Hypertonic Solution5. Isotonic Solution6. Plasmolyzed7. Turgid