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Chapter 3
Cell Passive Transport
(Osmosis, Cell Tonicity)
Slide 132-147
Water Balance of Cells Without Walls

Tonicity is the ability of a surrounding solution
to cause a cell to gain or lose water
 Isotonic solution: Solute concentration is the
same as that inside the cell; no net water
movement across the plasma membrane
 Hypertonic solution: Solute concentration is
greater than that inside the cell; cell loses
water
 Hypotonic solution: Solute concentration is
less than that inside the cell; cell gains water
2
Hypertonic solutions are those in which more solute (and hence
lower water potential) is present.
Hypotonic solutions are those with less solute (again read as higher
water potential). Isotonic solutions have equal (iso-) concentrations
of substances. Water potentials are thus equal, although there will
still be equal amounts of water movement in and out of the cell, the
net flow is zero.
Water relations and cell shape in blood cells.
Water relations in a plant cell.
3
Osmosis and tonicity
https://www.youtube.com/watch?v=niuZJ2f2Cks
4
Keeping water balance

Cell survival depends on balancing
water uptake & water loss
freshwater
balanced
saltwater
5
1Keeping

right amount of water in cell
Freshwater


KABOOM!
a cell in fresh water
high concentration of water
around cell
cell gains water
 example: Paramecium


problem: cells gain water,
swell & can burst


No problem,
here
freshwater
water continually enters
Paramecium cell
solution: contractile vacuole

pumps water out of cell
6
2Keeping
right amount of water in cell
saltwater

Saltwater
I’m shrinking,
I’m shrinking!
a cell in salt water
 low concentration of water
around cell


cell loses water
example: shellfish
 problem: cell loses water

I will
survive!
in plants: plasmolysis
 in animals: shrinking cell


solution: take up water
7
Keeping right amount of water in
3
cell
balanced

Balanced conditions

no difference in concentration
That’s
of water between cell &
better!
environment
 cell in equilibrium
 example: blood
 problem: none


water flows across
membrane equally,
in both directions
volume of cell doesn’t
change
I could
be better…
8
Solute Concentration
(b) Isotonic
surroundingsH2O
(a) Hypertonic
surroundings
(c) Hypotonic
surroundings
Animal
cell:
plasma
membrane
H2O
H2O
Plant
cell:
H2O
plasma
membrane
cell wall
wilted
Net movement of
water out of cell
H2O
H2O
Balanced water
movement
turgid
Net movement of
water into cell
9
Osmosis and Tonicity
https://www.youtube.com/watch?v=Y_w07A7chnk
10
Hypertonic-Isotonic-Hypotonic
11
12
13
Isotonic, Hypotonic, Hypertonic IV Solutions
Made Easy | Fluid Electrolytes Nursing Students
https://www.youtube.com/watch?v=51FkahHUBwc
14
Diffusion vs Osmosis
•
Passive diffusion is the net movement of
gases or small uncharged polar molecules
across a phospholipid bilayer membrane
from an area of higher concentration to
an area of lower concentration .
Examples of gases that cross membranes
by passive diffusion include N2, O2, and
CO2; examples of small polar molecules
include ethanol, H2O, and urea.
•
•
Osmosis is the diffusion of water across a
membrane from an area of higher water
concentration (lower solute concentration)
to lower water concentration (higher
solute concentration). Osmosis is powered
by the potential energy of a
concentration gradient and does not
require the expenditure of metabolic
energy.
In an isotonic environment, both the water
and solute concentration are the same
inside and outside the cell and water goes
into and out of the cell at an equal rate.
15
Hypertonic-Hypotonic
•
If the environment is hypertonic, the
water concentration is greater inside the
cell while the solute concentration is
higher outside (the interior of the cell is
hypotonic to the surrounding hypertonic
environment). Water goes out of the cell.
•
In an environment that is hypotonic,
the water concentration is greater
outside the cell and the solute
concentration is higher inside (the
interior of the cell is hypertonic to the
hypotonic surroundings). Water goes
into the cell.
16
Hypertonic Solutions: contain a high concentration of solute
relative to another solution (e.g. the cell's cytoplasm). When
a cell is placed in a hypertonic solution, the water diffuses
out of the cell, causing the cell to shrivel.
Hypotonic Solutions: contain a low concentration of solute
relative to another solution (e.g. the cell's cytoplasm). When
a cell is placed in a hypotonic solution, the water diffuses
into the cell, causing the cell to swell and possibly explode.
Isotonic Solutions: contain the same concentration of solute
as another solution (e.g. the cell's cytoplasm). When a cell is
placed in an isotonic solution, the water diffuses into and
out of the cell at the same rate. The fluid that surrounds the
body cells is isotonic.