OSMOSIS
Water as a Solvent
• Water is important for all living organisms as many substances are
able to dissolve in it (it is a solvent)
• This makes it incredibly useful and essential for all life on Earth
• Water is important as a solvent in the following situations within
organisms:
• Dissolved substances can be easily transported around organisms eg xylem and phloem of plants and dissolved food molecules in the
blood
• Digested food molecules are in the alimentary canal but need to
be moved to cells all over the body - without water as a solvent this
would not be able to happen
• Toxic substances such as urea and substances in excess of
requirements such as salts can dissolve in water which makes them
easy to remove from the body in urine
• Water is also an important part of the cytoplasm and plays a role in
ensuring metabolic reactions can happen as necessary in cells
• All cells are surrounded by a cell membrane which
is partially permeable
• Water can move in and out of cells by osmosis
• Osmosis is the diffusion of water molecules from
a dilute solution (high concentration of water) to
a more concentrated solution (low concentration of
water) across a partially permeable membrane
• In doing this, water is moving down its concentration
gradient
• The cell membrane is partially permeable which
means it allows small molecules (like water) through
but not larger molecules (like solute molecules)
• Osmosis is the net movement of water molecules
from a region of higher water potential (dilute
solution) to a region of lower water
potential (concentrated solution), through
a partially permeable membrane
• It can get a little confusing to talk about the
'concentration of water' when we also talk about
solutions being ‘concentrated’ (having a lot of
solute in them), so instead we can say that
a dilute solution has a high water potential (the
right-hand side of the diagram below) and
a concentrated solution has a low water
potential (the left-hand side of the diagram below)
• Osmosis is a special form of diffusion and always
involves the movement of H2O across a membrane.
Osmosis is:
 the movement of H2O
 across a selectively permeable membrane
 down a water potential gradient.
• In the picture below
• - The concentration of sugar molecules is higher on
• the concentrated solution (L) and lower on the diluted
one (R).
• - The concentration of water molecules is higher on the
(R) and lower on
• the (L) (a lot of place is taken up by sugar molecules).
• we can say:
• that a diluted solution (R) has a high
water potential and a concentrated
• solution (L) has a low water potential.
• There is a water potential gradient
between the 2 sides. The water
• molecules diffuse down this gradient,
from a high water potential (R) to a
• low water potential (L).
Experiments on osmosis
Experiments on osmosis
Osmosis in Plant Tissues
• When water moves into a plant cell, the vacuole gets
bigger, pushing the cell membrane against the cell
wall
• Water entering the cell by osmosis makes the
cell rigid and firm
• This is important for plants as the effect of all the cells
in a plant being firm is to provide support and
strength for the plant - making the plant stand
upright with its leaves held out to catch sunlight
• The pressure created by the cell wall stops too much
water entering and prevents the cell from bursting
• If plants do not receive enough water the cells cannot
remain rigid and firm (turgid) and the plant wilts
• A plant cell with the vacuole pushing out on the cell wall is
said to be turgid and the vacuole is exerting turgor pressure
on the inelastic cell wall.
• If all the cells in a leaf and stem are turgid, the stem will be
firm and upright and the leaves held out straight. If the
vacuoles lose water for any reason, the cells will lose their
turgor and become flaccid
• If a plant has flaccid cells, the leaves will be limp and the stem
will droop. A plant which loses water to this extent is said to be
‘wilting’ .
• Root hair cells are in contact with water trapped between soil
particles. When the water potential of the cell sap is lower
than that of the soil water, the water will enter the cells by
osmosis providing the plant with the water it needs
•
The importance of water potential
and osmosis in animal cells and
tissues
It is vital that the fluid which bathes cells in animals,
such as tissue fluid or blood plasma, has the same
water potential as the cell contents.
• This prevents any net flow of water into or out of the
cells.
• If the bathing fluid has a higher water potential (a
weaker concentration) than the cells, water will move
into the cells by osmosis causing them to swell up.
• As animal cells have no cell wall and the membrane
has little strength, water would continue to enter and
the cells will eventually burst (a process called
haemolysis in red blood cells).
Plasmolysis
• Interpretation