1

Osmosis:Passive Transport

Question:Are the cells in an

elephant bigger, smaller, or about the same size as those in a mouse?

2

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

3

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