Download The Membrane: Achieving Balance

Osmosis:
Striking a Balance
Maintaining A Balance
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Cells are surrounded by watery solutions
and are filled by watery solutions.
A solution is a mixture in which one or
more substances (Solutes) are dissolved
in another substance (Solvent)
The concentration of a solute is important
to organisms.
Organisms cannot live unless the
concentration of dissolved substances
stays within a narrow range.
Balance and Homeostasis
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This balance of conditions is called
Homeostasis
The movement of water across the
membrane helps to balance the
concentrations of dissolved substances in
a cell
The diffusion of water across a selectively
permeable membrane is called osmosis
Osmosis
Like other molecules the movement of
water is controlled by differences in
concentration (a concentration gradient)
 Water will move in the direction where
there is a high concentration of solute
(hence a lower concentration of water)
 A simple rule to remember is SALT
SUCKS
 Salt is a solute, when it is concentrated
inside or outside a cell, it will draw water
in its direction
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Tonicity
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The measurement of solute concentration
on either side of a membrane is known as
Tonicity
The tonicity of a cell’s environment affects
its rate of osmosis.
Isotonic
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In an isotonic solution, the concentration of
solute is the same on both sides of the
membrane (inside the cell and outside).
A cell placed in an isotonic solution
neither gains or loses water.
Most cells in the body are in an isotonic
solution.
Why?
Red Blood Cells
Plant Cells
Hypertonicity
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A hypertonic solution
is one that has a high
solute (Less water)
concentration.
Cells in a hypertonic
solution will lose
water.
Hypertonicity
As a result of a difference in
structure, plant and animal cells
respond differently
Animal Cells shrink crenate
Plant cells, maintain shape
because of their cell wall, the cell
membrane breaks away from the
cell wall resulting in Plasmolyis
Red Blood Cells
Plant Cells
Hypotonicty
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A hypotonic solution is
one that has less
solute (more water).
When placed in a
hypotonic solution,
cells tend to gain
water and swell
Hypotonicty
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If an animal cell swells too much it will
burst. This is called Lysis.
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Plant cells rarely burst because of their
cell wall. As water fills the cell the
membrane is pushed up against the wall
(turgor pressure) making the plant cell
turgid.
Red Blood Cells
Plant Cells
Hypotonicity
Many single cell organisms live in
hypotonic solution so they may be in
danger of constantly bursting.
 To help these organisms they have a
contractile vacuole to pump water out
of the cell.
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Questions
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How can we determine which solution is
isotonic to a given set of cells?
Is the concentration the same for all cells?