Introduction
Reminder 1:
Osmosis as the net movement of water molecules from a region
of higher water potential (from a more dilute solution) to a region of
lower water potential (a more concentrated solution), through a
partially permeable membrane (concentrated refers to dissolved molecules
or ions).
Reminder 2:
Active transport is the movement of particles (molecules or
ions) through a cell membrane from a region of lower concentration to a
region of higher concentration using energy from respiration.
Within cell membranes there are carrier proteins
use energy from respiration to transport molecules or ions across
the membrane, against the concentration gradient, therefore cells
that use active transport usually have more mitochondria for
respiration compared to other cells.
Know and understand that water often moves across boundaries by
osmosis - a special case of particle (water) diffusion down a
concentration gradient.
Know that osmosis is the diffusion
or net bulk movement of water from a dilute to a
more concentrated solution through a partially
permeable membrane (also described as a partly/semi-permeable membrane) that allows the passage of
very small molecules like water (diagram on right).
(Caution: Don't assume other
small molecules cannot pass through a membrane, the term osmosis refers to
the passage of water,
but things are more complicated when dealing with other small molecules
and ions passing through membranes).
You can also express osmosis as the
movement of water from a higher concentration to a lower concentration -
this is effectively from a lower solute concentration to a higher solute
concentration.
A partially permeable membrane has
extremely small pores or holes that only allow the tiniest of molecules like
water through e.g. even
relatively small molecules like sucrose will not pass through a partially permeable
membrane (see experiment in section
2b. below).
Particle theory: All the
particles in a solution are moving at random because of their kinetic
energy store, so if a water molecule (blue
circle) hits the right spot on the
membrane, it can diffuse through. However, a larger molecule (purple
circles) like a
sugar molecule, will just bounce back off the surface of the membrane.
The purple particles also represent a higher
concentration of small molecules or ions on the left, so the higher
concentration of water on the right will ensure there is a net diffusion
of water from right to left through the semi-permeable membrane.
In osmosis only
water gets through a partially permeable membrane and depending on the relative concentration of dissolved
substances on either side of the membrane, osmosis can happen in either direction
- meaning water can diffuse through the membrane in either direction.
The greater the concentration of water, the more dilute
the solution, the greater the water potential.
The lower the concentration of water, the more
concentrated the solution, the lesser the water potential.
Therefore in osmosis through a semi-permeable
membrane, water moves from a region of higher water potential, to a
region of lower water potential.
Although the water molecules
(and any other particles) are moving around at random, there will be a net
transfer of water in one direction at a time through a partially permeable
membrane.
The net direction of diffusion of
water is from a
less concentrated solute solution (more water molecules) to a more concentrated
solute solution (less water molecules) i.e. from the
higher concentration of water molecules to a lower concentration of water
molecules across the membrane.
You can also express this as water
moving from a higher concentration to a lower concentration,
OR, from water moving from a
higher water potential to lower water potential.
Therefore a more concentrated
solution becomes more dilute in the process.
This osmosis diffusion can occur in either direction depending on the
relative concentration of the solutes in the cell fluids or tissue fluids
and concentrated solutions e.g. of sugars on either side of a partially
permeable membrane.
But, whatever, the more
concentrated solution will tend to become diluted by water
passing through the partially permeable membrane.
The movement of water in and out of cells
The soft cell wall, or outer
membrane of an animal
cell, acts as a partially permeable membrane.
The water surrounding cells, the
tissue fluid, contains the dissolved molecules the cell needs to survive e.g.
sugars, amino acids, oxygen, as well as waste carbon dioxide etc.
(a) If the cells are short of water
('partially dehydrated'), the concentration of dissolved substances
increases, so water diffuses through the cell membrane into the cells to dilute the cell
fluids until equilibrium is established.
The term water potential
is used to describe these situations.
You can talk about the water
potential gradient across a partially permeable membrane.
Water will diffuse from a high
water potential to a low water potential.
In situation (a), the fluid
outside the cells has a high water potential than the solute
solution in the cells, so water moves into the cells.
(b) Conversely, if the cell solution
is too dilute, then water will diffuse out by osmotic action across the
semi-permeable membrane of the cell wall.
In situation (b), the solution
inside the cells has a high water potential than the fluid outside
the cells, so water moves out of the cells.
Therefore the diagram on the right
could represent the passage of water (blue circles) in or out of a cell,
depending on the relative concentrations of water on either side of the
membrane.
Osmosis - plant cells and water potential
(i) When you water a plant it
increases the water potential of the soil around it.
Therefore the plant cells will draw
water in by osmosis until they become turgid - fatter and
swollen.
The cell fluids (contents of the
cell) will push against the cell wall, known as turgor pressure,
and this helps support the plant tissues (therefore the plant as
a whole).
(ii) If the soil is very dry, lacking
in water, the plant starts to wilt and the water potential of the plant
is greater than the surrounding soil.
The result is the plant cells become
flaccid and begin to lose water.
The plant doesn't droop (flop)
completely and retains much of its shape because the strong cellulose
cell wall is relatively inelastic and helps the plant retain its shape.
Osmosis - animal cells and water
potential
In the case of animal cells, they do
not have strong walls and can respond adversely to change in the ambient
water pressure.
If animal cells are surrounded by
a solution of greater water potential (less concentrated in
solutes), they can absorb so much water by osmosis that they burst -
which kills the cells - see red blood cell example below.
In extreme cases you can die of
over-hydration, but its a complicated effect that reduces the level
of salt (= sodium ions) in the blood to dangerous levels.
If cells are surrounded by a too
concentrated salt solution, they lose so much water they can shrink
and shrivel up and the dehydrated kills the cells - see red blood
cell example below.
Osmosis is important for the function
of many animal organs
It isn't just all about individual
cells. e.g. water is absorbed into the
bloodstream from the large intestine to form faeces in the appropriate
physical state!
The function of the kidney and the formation of urine,
all involve osmotic transportation of water.
See also
Homeostasis - osmoregulation - ADH - water control
gcse biology revision notes
and
Transport and gas exchange in plants,
transpiration, absorption of nutrients, leaf and root structure
