BIPIN.S

NATURAL SCIENCE

REG No.13971012

Difference in concentration between solutions on

either side of semi permeable membrane called

Osmotic gradient

Osmosis is the spontaneous net movement

of solvent molecules through a partially

permeable membrane into a region of higher solute

concentration, in the direction that tends to equalize

the solute concentrations on the two sides.

Osmosis provides the primary means by

which water is transported into and out of cells

Concentration gradient

Concentration Gradient - change in the concentration of a substance from one area to another.

OsmosisOsmosis is the movement of WATER across a semi-permeable membraneAt first the concentration of solute is very high on the left.But over time, the water moves across the semi-permeable membrane and dilutes the particles.

Osmosis – A Special kind of Diffusion

Diffusion of water across a selectively permeable membrane (a barrier

that allows some substances to pass but not others). The cell

membrane is such a barrier.

Small molecules pass through – ex: water

Large molecules can’t pass through – ex: proteins and complex

carbohydrates

Hypotonic – The solution on one side of a membrane where the solute

concentration is less than on the other side. Hypotonic Solutions contain a

low concentration of solute relative to another solution.

Over time molecules will move across the membrane until the

concentration of solutes is equal on both sides. This type of solution

is called ISOTONIC.

• Cytoplasm is a solution of water and solids

(solutes dissolved in the water).

• Water moves into and out of cells because of the different

concentrations of the solutes.

• Different kinds of cells react differently depending on the solution they

are in.

• Below are examples of red blood cells in different types of solutions and

shows what happened to the red blood cells.

There is a greater concentration of free water molecules outside the cell than inside

so water diffuses into the cellby osmosis

and the cell swells up

11

cell wall

cytoplasm and cell membrane

vacuole

The cell absorbs waterby osmosis .... ....but the cell wall stops the

cell expanding any more

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For osmosis we talk about the potential

water molecules have to move – the

OSMOTIC POTENTIAL. Distilled water has the

highest potential (zero).

When water has another substance dissolved in

it, the water molecules have less potential to

move. The osmotic potential is NEGATIVE.

The osmotic potential of a cell is

known as its WATER POTENTIAL. For

animal cells, the water potential is

the osmotic potential of the

cytoplasm.

An animal cell with water

potential –50 is

placed in a solution…

If the osmotic potential

of the solution is less negative than the water

potential of the

cytoplasm(the solution is

hypotonic), net endosmosiswill occur, i.e. water will

move into the cell from

the solution. The result

will be haemolysis (the cell

will burst)Water potential of cytoplasm = -50

Osmotic potential of solution= -20

If the osmotic potential of

the solution is more negativethan the water potential of

the cytoplasm (the solution

is hypertonic), net exosmosis will

occur. The result will be

crenation (the cell will shrivel

up)

Water potential of cytoplasm= -50

Osmotic potential of solution = -80

If the osmotic

potential of the

solution is the same as the

water potential

of the

cytoplasm (the

solution is

isotonic), there

will be no net osmosis.

Water potential of cytoplasm= -50

Osmotic potential of solution= -50

In animal cells, the water potential is equal

to the osmotic potential of the cytoplasm,

but this is different in plant cells…

Plant cells have a cell wall, which exerts an

inward pressure when the cell is turgid. This

is known as the pressure potential.

The water potential of an animal cell is

equal to the osmotic potential of the

cytoplasm plus the cell wall pressure:

W.P.= O.P. + P.P.

A plant cell with water

potential –50 is placed in a

solution…

If the solution is

hypotonic, net

endosmosis occurs

and the cell

becomes fully

turgid.

Water potential of cytoplasm = -50

Osmotic potential of solution = -20

If the solution is

hypertonic, net

exosmosis occurs and

causes plasmolysis (the

cell membrane pulls

away from the cell

wall. The cell wall

stays intact).Water potential of cytoplasm = -50

Osmotic potential of solution = -80

If the solution is

isotonic, no net osmosisoccurs. The cell is

not plasmolysed,

but it is not fully

turgid either.

Water potential of cytoplasm = -50

Osmotic potential of solution = -50

Conclusion:

Osmosis is a vital process in biological systems,

as biological membranes are semi permeable.

Osmosis is responsible for the ability of plant roots to draw water from the soil.

Osmosis is a vital process in both plants and animals

to maintain their own life process.

Reference:

Borg, Frank (2003). "What is osmosis? Explanation and

understanding of a physical phenomenon“ Wiley international

publications.

Kramer, Eric; David Myers. "Osmosis is not driven by water

dilution". Trends in Plant Science 18 (4): 195–197.

Kosinski, R. J. "Challenging misconceptions about

osmosis.". Association for Biology Laboratory Education 30: 63–

87.