OSMOSIS troubles a lot of people, so, for those who are still confused ...
back to Help_centre main.
This page maintained by Dr KNI Bell (questions&suggestions
welcome)
No, OSMOSIS was not an Egyptian pharoah.
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STUDY ADVICE AGAIN: [1] start with definitions (yucky-coloured glossary
pages in your text, other glossaries, the dictionary), THEN [2] use the
INDEX and go to detailed explanations. [3] It's often useful to see the
same thing in two different texts. [4] Find joy in whatever you do.
To get you started, here are some of our yucky-coloured definitions
(mostly from Dr. Whittick's Glossary). |
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diffusion:
the movement of a substance from an area of high concentration to an area
of lower concentration until an equilibrium is reached.
OSMOLARITY: an absolute measure of the concentration of solute
in a solution. In contrast, the "-tonic" terms are relative,
always in the context of comparison of 2 (not 1, not 3) solutions; if
considering two solutions, A of low and B of high osmolarity, A will be
hypotonic to B. (The term "osmolarity" is usually deferred until
1002, but in a way it's a much simpler concept and therefore some students
will find TONICITY easier to deal with if they learn osmolarity first.)
hypertonic:
concerning osmosis, a term used to compare one solution with
another, a hypertonic solution has a higher concentration of solutes,
when separated by a membrane that is permeable to the solvent and impermeable
to the solute. In this instance the solvent will flow from the region
of lower solute concentration (hypotonic) to the region of higher concentration
(hypertonic) by the process of osmosis. e.g. most fresh water protists
are hypertonic when compared with their environment.
hypotonic:
(see hypertonic for a detailed description) a solution that has a lower
concentration of solutes than a hypertonic one.
isotonic:
(see hypertonic or hypotonic) when two solutions separated by a membrane
permeable to the solvent but not the solutes are of equal concentration
this results in no net movement of solvent across the membrane.
osmosis:
the phenomenon whereby water flows across a selectively permeable membrane
from a hypotonic environment to a hypertonic environment.
selectively
permeable: applies to membranes which allow one
substance to pass, but impede another. (You need to establish what
the membrane
is and isn't permeable to, recognising that you are interested in the
difference between "pass through easily" and "hardly
ever get past".) Usually, biological membranes (and things
that mimic their performance) are permeable to water but not permeable
to (or much less permeable than) solutes (e.g. sugars, ions).
osmoregulation:
the ability of a cell or an organism to regulate its internal concentration
of water or salts in order to maintain it self in a an specific
environment e.g. the contractile vacuole of Euglena actively expels
water from the cell which enters as a result of the cell being hypertonic
in comparison with the fresh water environment.
...some terms relating to plant cells
plasmolysed:
when a living cell with a cell wall is placed in a hypertonic solution
it loses water by osmosis, if sufficient water is lost the cell membrane
is pulled away from the cell wall and the cell is said to be plasmolysed.
plasmolysis:
the act of becoming plasmolysed, see above.
turgid: means
firm, the opposite of flaccid, cells with wall become firm as a result
of the uptake of water by osmosis,
turgor: (pressure)
the force exerted on a cell wall by a cell membrane as a result of osmotic
uptake of water.
... some terms relating to transport of other substances
active
transport: the movement
of a substance across a cell membrane against its concentration
gradient, which requires energy from the cell
passive transport:
a form of transport which requires no energy from the organism, i.e. diffusion
of a substance from an area of high concentration to one of a lower one.
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OSMOSIS IS IMPORTANT BECAUSE IT DETERMINES WHERE WATER GOES, HOW IT CAN BE
CONTROLLED
WATER is an essential substance in life and it must be managed. This management
has to rely on the laws of physics.
A key observation that is often overlooked is: water is NEVER actively
transported. There are no enzymes to actively transport water, only
enzymes to actively transport solutes that affect osmosis. Organisms manage
water by managing OSMOSIS.
(Downhome example no. 1) Moving water? It's kind of
like moving a pig. Ever try to move a pig? Pigs are not only stubborn and suspicious,
they are large and very strong. I had a friend Jerry who was farming out in
St. George's Bay. Lovely area.
One day, Jerry had finished building a beautiful new pigpen -- elevated
floor, good roof, shade area for those hot days, etc., everything a pig could
want, even an ocean view -- for his pig, and cheerfully walked over to the pig
with a piece of rope to lead the pig happily to its beautiful new home. Dogs
know about leashes, but pigs -- as I said, large, strong, stubborn.
So Jerry pulled and pulled on this rope, pulled as hard as he could,
and the pig pulled the other way, or maybe it just stood still pondering the
contribution of cometary debris to the chemical composition of the atmosphere
of Io, it's hard to tell with pigs. So, after Jerry skidding his feet on the
mud for a while, all that happened was he got tired and upset. Offended perhaps.
The pig wasn't upset, it simply wasn't interested in going anywhere it hadn't
seen yet, certainly not at the end of a rope. No sale, you might say. Jerry
gave up and went to get the tractor, by golly he was going to just haul that
pig over there and a diesel International tractor with 60-inch wheels and a
differential lock was just the item to show that pig who was boss. Off he went.
Jerry was more partial to tractors anyhow -- easier to understand.
But Jerry's daughter Elizabeth liked animals, and took a jug of
pig food, and held it out for the pig and spread a little on the ground. The
pig followed the trail all the way to the new pigpen. Only took a minute or
two.
Food speaks the pig's language; rope doesn't. Jerry came roaring back on the
tractor, and the move had been made.
-- So, just as the pig couldn't be pulled by a rope, water can't be actively
transported. Just as the pig would move spontaneously to food, water can be
induced to flow in the direction of solute gradients. And many solutes can be
actively transported, and cells do this all the time.
(My colleagues may want to shoot me for that, but just as
a building requires scaffolding during its construction, conceptualisation can
benefit from analogies. If the analogy doesn't work for you, don't worry, just
move on.)
We leave the barnyard and move on to the technical terms.
| Coffeecup analogies (remember, coffee is mostly
water):
OSMOLARITY is a number referring
to the number of moles of solute in a solution (we usually get to this
in Biology 1002, but in fact it is a simple concept and for many people
it's a better starting point).
----For a cup of coffee with no solutes,
osmolarity = 0. Add solutes, osmolarity goes up. If you
take one spoon of sugar and your friend takes two spoons, the osmolarity
of your friend's coffee is higher. And therefore, your coffee would be
hypotonic to your friend's, or your friend's would be
hypertonic to yours. (That's not so hard.)
----Though your coffee would be hypotonic
to your friend's, your coffee would be hypertonic to the no-sugar cup.
That's not so hard either: the "-tonic" terms are always relative
(describing one thing relative to one other), not absolute. So if someone
asks you "is this hypotonic" you can only ask "compared
to what?" Only osmolarity is absolute: comparing any solution A to
any solution B with a greater osmolarity, A will be hypotonic to B and
B hypertonic to A, and comparing A to any solution with a lesser osmolarity
A will be hypertonic.
-------
----Finally, what will water do ... i.e.
how can you use these concepts to understand osmosis?
----Every molecule
in a liquid or in solution is mobile, and it can move in any direction.
However, the rate of movement is affected by the presence of other molecules.
DIFFUSION is the net movement, the change in distribution over time. E.g.,
in the case of sugar put into water, supposing molecular motion of water
results in, say, 335 movements toward the sugar (where the solute concentration
is higher) and, say, 319 movements away, then the net movement is 16 toward
and that has the effect of reducing the solute concentration there; the
sugar does the same; eventually this process would eliminate all gradients
until the sugar molecules and water molecules were uniformly distributed.
The bias in movement of molecules is caused by the bias in solute concentration
(no bias would mean no net movement).
----Next,
suppose you put two different solutions in a coffee mug
(as we just did) but with a divider that was permeable to
water but not to sugar (a selectively
permeable membrane), and you put
sweet coffee on the right and much sweeter coffee on the left. (Is the
sweet coffee hyp[O or ER]tonic to the much sweeter coffee?
hypOtonic<--drag mouse to see answer)
---- Now
you've sorted out which is hypotonic to which, can you identify the direction
of OSMOSIS, the direction of net movement of water across the selectively
permeable membrane? (Osmosis will be in the direction
of the sweeter coffee, i.e. net water movement will be from the solution
with less sugar (hypotonic, more water) to the solution with more sugar (hypertonic, less water).<--drag
mouse to see answer) Osmosis tends to equalise
solute concentrations, because net movement of water is from lower osmolarity
to higher osmolarity. |
| If you'd like another
example: think about adding sugar to a glass of water:
2. throw in the sugar cube and don't stir. Imagine what
happens: does the sugar remain in a cube? No, you can't lift the cube
out again.
3. throw in the sugar, stir well, and wait 2 days. Does
the sugar come out of solution and form grains again? (No, it stays dissolved).
What does this tell us?
- Unstirred, the water and the sugar molecules spontaneously diffused
to form a syrup. The water concentration was initially zero (or very
low) in the sugar grains, and the sugar concentration was initially
zero (low) in the coffee. Water moved down its concentration gradient,
and sugar moved down its concentration gradient.
- Note that this movement of molecules would continue until there was
no gradient, i.e. until the coffee is uniformly sweet and there are
no undissolved (grains of) sugar left.
- In this case diffusion of BOTH the water and the solute (sugar in
this case) contributed to elimination of all concentration gradients.
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- any area that has a higher concentration of solute (sugar) can be
called hypertonic compared to any area that has a lower
concentration of solute (hypotonic), and ...
- ... if these areas are separated by a selectively permeable membrane
that prevents the movement of solutes, the water alone will cross the
membrane and the volumes on each side of the membrane will change.
- Pressure can also influence the balance of water movement across
SPM: If the volumes are constrained, as in a plant cell that has
a cell wall, the pressure inside the cell (if that is hypertonic compared
to its environment) will rise, and as it rises the rate of diffusion
out of the cell will rise until it matches the diffusion into the cell.
At that point, the pressure in the cell equals (balances) the osmotic
pressure generated by the solute differences. If the cell wall is not
strong enough, the cell ruptures before this point is reached. That
is why protozoan ectoparasites of fish can be fought by rapid changes
from seawater to fresh and vice versa.
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Downhome example no.2: When you salt fish you
use salt or a brine to 'draw' water out of the fish; the brine is hypERtonic
to the fish, and the fish is hypOtonic to the brine, so the water in
the fish diffuses toward the brine faster than the water in the brine
diffuses toward the fish -- net movement is out of the fish.
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Cell survival depends on balancing water uptake and loss . Cells
can burst if they take on too much water, or collapse if they lose too
much. Downhome example again: you salt fish
to preserve it, by reducing the water content to a point where the bacteria
that would spoil it cannot live.
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Now think about animal cells and plant cells in a hypotonic environment (take
an extreme hypotonic environment, like distilled water). Both will take on water
because osmosis will be in the direction of higher osmolarity. Generally, the
plant cell will take on water until the pressure inside the cell causes a balancing
of the flux of water into and out of the cell. The animal cell will take on
water until it ruptures.
And finally, it's good to know what you don't know, so here's something for
you to find out. If the selective permeability of a membrane is based
mostly on size, and solutes like Na+ and CL- are smaller than H2O, how can
you have a membrane that allows water to pass but not solutes?
From KNIB&others' html lecture material, to put Osmosis into the general
context of transport
HOW MOLECULES GET IN AND OUT OF CELLS:
(obviously, if molecules can't get in and out of cells,
then cells can't obtain nutrients or release wastes, and so they die. From
the cell's point of view this is a serious matter. You are made of cells,
so this matters to you too.)
| Important words: DIFFUSION,
ACTIVE TRANSPORT, PASSIVE TRANSPORT, OSMOLARITY & OSMOSIS (always relates
to water movement & solute, solvent), CELL MEMBRANE, PERMEABILITY, ...
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A CELL MEMBRANE IS SELECTIVELY PERMEABLE
(some molecules cross more easily than others). The selective permeability of
a membrane depends on:
- Permeability of the phospholipid bilayer itself
(p 136 b)
- Transport proteins that bridge the phospholipid bilayer
(p 136 b)
- properties of molecules: size, polarity, charge
SOME MOLECULES CROSS EASILY AND SPONTANEOUSLY. These
tend to diffuse until concentrations are equal:
- Nonpolar (hydrophobic) molecules dissolve in the membrane and cross it
with ease (e.g., hydrocarbons, 02, C02).
- Small, polar uncharged molecules (e.g., H20, ethanol)
rapidly pass through the plasma membrane.
SOME MOLECULES DO NOT CROSS EASILY, HAVE
TO BE HELPED ACROSS:
- Larger, polar uncharged molecules (e.g., glucose) will not easily pass through
the phospholipid bilayer
- All ions, even small ones (e.g., Na+, H+)
are hydrophilic and therefore have difficulty
penetrating the hydrophobic layer
Transport proteins: TO HELP SOME MOLECULES
ACROSS. Transport proteins allow a way
for hydrophilic substances to avoid (bypass) the hydrophobic core of the bilayer.
Transport proteins thus make biological membranes permeable to specific ions
and certain polar molecules of moderate size
What are transport proteins? Integral membrane proteins that transport
specific molecules or ions across biological membranes (see Campbell, Figure
8.8a). Often anchored to cytoskeleton. Often controlled.
How do transport proteins work?
- Some provide a hydrophilic tunnel through the membrane (passive transport,
no energy cost).
- Some bind to a substance and physically move it across the membrane (active
transport, costs ATP)
- Generally specific for the substance they translocate.
Passive transport is diffusion across a membrane
(Campbell
Fig.8.9)
DIFFUSION = The spreading out
of a substance as a result of the movements of individual molecules -- this
implies a net movement of a substance down its concentration gradient (Campbell),
until there is no gradient left. Or (Whittick) the net
movement of a substance from an area of high concentration to an area of lower
concentration until an equilibrium is attained. If you like, you can think of
it as the destruction of a gradient by equalisation of the values all along
it. This occurs molecule by molecule.
- Results from random molecular motion, even though the net movement
may be biased down the gradient
- Diffusion continues until a dynamic equilibrium is reached. At equilibrium
the molecules continue to move, but there is no net directional movement.
- Diffusion does not entail a cost to the cell, except where it has to be
countered
- ANALOGY for 'gradient': pour honey onto a plate. As you add it, there
is more honey where you pour it; the difference between honey levels at any
two places is the gradient; if two places are the same there is no gradient;
over time the honey flows down whatever gradients they are, until all gradients
have become zero. Then take OUT a spoonful of honey: this establishes a new
gradient, etc.
Concentration gradient = increase or
decrease (in concentration) over distance
Net directional movement = Overall movement away
from the center of concentration, which results from random molecular movement
in all directions. A molecule has a higher probability of moving from
a high concentration to a low concentration than vice versa.
In the absence of other forces or constraints (e.g., pressure) a substance
will diffuse from where it is more concentrated to where it is less concentrated.
- A substance diffuses down its own concentration gradient.
- It is not affected by the gradients of other substances in the same
place.
Much of the traffic across cell membranes occurs by diffusion and is thus
a form of passive transport.
"PASSIVE TRANSPORT is Diffusion of a substance across a [biological] membrane"
(Campbell p 137a)
- Spontaneous process which is a function of a concentration gradient when
a substance is more concentrated on one side of the membrane.
- does not require cell to expend energy.
- Rate of diffusion is regulated by the permeability of the membrane, so
some molecules diffuse more freely than others.
- Water diffuses freely across most cell membranes.
"Osmosis
is the passive transport of water" (Campbell p 137d). Important words hypertonic,
hypotonic, isotonic:
- HypERtonic solution = solution with a greater
solute concentration than that inside a cell. It will 'draw' water out of
a cell.
- HypOtonic solution = solution with a lower
solute concentration than that inside a cell. Cell will tend to 'draw' water
from the solution.
- ISOtonic solution = A solution with an equal
solute concentration compared to that inside a cell.
- -->!! these words always refer to a comparison
!!<--
i.e. if you have two solutions that are different, the one with the lower solute
concentration is hypOtonic to the other, and the one with the higher solute
concentration is hypERtonic to the other. (Greek hypo- = under, hyper-=over.
Hence hypodermic needle ... a hypERdermic needle would be one that never gets
under your skin. Gk. 'iso' = 'same'.)
Osmosis = (see Campbell,
Fig.8.10) Diffusion of water across a selectively permeable membrane. Water
diffuses down its concentration gradient, i.e. from the hypOtonic zone to the
hypERtonic zone.
Downhome example: When you salt fish you use salt or
a brine to 'draw' water out of the fish; the brine is hypERtonic to the fish,
and the fish is hypOtonic to the brine, so the water in the fish diffuses
toward the brine faster than the water in the brine diffuses toward the fish
-- net movement is out of the fish.
Example: If two solutions of different concentrations are separated by a
selectively permeable membrane that is permeable to water but not to solute,
water will diffuse from the hypotonic solution to the hypertonic solution.
The direction of osmosis is determined by the difference in total solute concentration.
If two isotonic solutions are separated by a selectively permeable membrane,
water molecules diffuse across the membrane in both directions at an equal
rate. There is no net movement of water.
Osmotic pressure = Measure of the tendency for a solution to take up water
when separated from pure water by a selectively permeable membrane. Osmotic
pressure can be measured using an osmometer.
In one type of osmometer, pure water is separated from a solution by a selectively
permeable membrane that is permeable to water but not solute. The tendency
for water to move into the solution by osmosis is counteracted by applying
enough pressure with a piston so the solutions volume will stay the same.
The amount of pressure required to prevent net movement of water into the
solution is the osmotic pressure.
Cell survival depends on balancing water uptake and loss . Cells can
burst if they take on too much water, or collapse if they lose too much. Downhome
example again: you salt fish to preserve it, by reducing the water content
to a point where the bacteria that would spoil it cannot live.
