Movement across Cell
– be able to describe how substances move across cell
membranes and identify the 5 ways in which substances do
– To be able to explain
• diffusion as the passive movement of substances in the
direction of a concentration gradient
• the role of carrier and channel proteins in facilitated
• osmosis as a special case of diffusion across a partially
• Active transport as the movement of molecules or ions
through a membrane by carrier proteins against a
concentration gradient,
• Cell membranes are a barrier to most
• This property allows materials to be
concentrated inside cells, excluded
from cells, or simply separated from
the outside environment.
• This is compartmentalisation and is
essential for life, as it enables reactions
to take place that would otherwise be
• Eukaryotic cells can also
compartmentalise materials inside
• Materials need to be able to enter and
leave cells
• There are five main methods by which
substances can move across a cell
1. Lipid Diffusion
2. Osmosis
3. Passive Transport
4. Active Transport
5. Vesicles
Lipid Diffusion
(or Simple Diffusion)
m olec ule
m em brane
• A few substances can diffuse directly through the lipid
bilayer part of the membrane.
• The only substances that can do this are lipid-soluble
molecules such as steroids, or very small molecules,
such as H2O, O2 and CO2.
• Lipid diffusion is a passive diffusion process, no energy
is involved and substances can only move down their
concentration gradient.
• Lipid diffusion cannot be controlled by the cell, in the
sense of being switched on or off.
• Osmosis is the diffusion of water across a
• It is in fact just normal lipid diffusion, but since water
is so important and so abundant in cells the
diffusion of water has its own name - osmosis.
• The contents of cells are essentially solutions of
numerous different solutes, and the more
concentrated the solution, the more solute
molecules there are in a given volume, so the fewer
water molecules there are.
• Water molecules can diffuse freely across a
membrane, but always down their concentration
gradient, so water therefore diffuses from a dilute to
a concentrated solution.
m em brane
w ater
s olute
m olec ules
m olec ules
net m ovem ent of w ater
dilute s olution
c onc entrated s olution
low c onc entration of s olute
high c onc entration of s olute
high c onc entration of w ater
low c onc entration of w ater
high w ater potential ( )
low os m otic pres s ure (O P )
low w ater potential ( )
high os m otic pres s ure (O P )
• Dissolved substances attract a ‘cloud’ of
polar water molecules around them. (Usually
held by weak hydrogen bonds)
• The water molecules can no longer move
• The more concentrated the solution the more
water molecules are ‘tied up’
• When a solution is separated from pure
water or a more dilute solution by a
membrane permeable to water, the free water
molecules are still able to diffuse, moving
across the membrane.
• The lipid bilayer of the plasma
membrane is theoretically impermeable
to water because they are polar
• Due to the fluid properties of the
membrane, and the fact that water is a
very small molecule, as well as there
being protein lined pores, there is
unrestricted water movement across
the membrane
Water Potential
• The water potential of a solution is the
name given to the tendency of water
molecules to enter or leave a solution
by osmosis
• ‘Water potential’ is really a measure of
the free kinetic energy of the water
• The Greek letter psi (symbol ψ) is used
to represent water potential
Solute potential
• This is the effect of the amount of
dissolved solute present.
• It is shown by the symbol ψs
• This used to be referred to as the
Osmotic pressure
• A solution with a high solute potential
would have a high inflow of water
molecules to dilute it.
Pressure potential
• Pressure potential (or turgor pressure)
is the mechanical pressure acting on a
• If a pressure greater than atmospheric
pressure is applied to a solution then a
pressure potential is created in the
• Pressure potential is represented by
the symbol ψp
• Visking tubing containing sucrose solution is
lowered into pure water
• Uptake of water over time due to high solute
potential. The water molecules in the pure
water have a high water potential.
• Visking tubing becomes stretched by high
internal pressure
• Net water uptake ceases due to high
hydrostatic pressure
• The pressure potential (ψp) offsets the solute
potential (ψs)
Water potential = solute potential + pressure potential
ψs + ψp
• 100% pure water has ψ = 0, which is the
highest possible water potential, so all
solutions have ψ < 0, and you cannot
get ψ > 0.
• Once a solute is dissolved in water, the
water molecules become less mobile
and are less likely to diffuse.
• The effect of dissolving a solute in
water is to lower its water potential,
making it more negative
• When a concentrated solution of glucose is
separated from a dilute solution of glucose
by a partially permeable membrane, which
solution has:
– The higher water potential?
• Dilute glucose solution
– A higher concentration of water
• Dilute glucose solution
– Will show a net gain of water molecules?
• Concentrated glucose solution
• Water potential is measured in units of pressure (Pa,
or usually kPa)
• Because the highest water potential is pure water
when ψ = 0 all other values are negative.
• When you write about water potential, try to write in
terms of less negative and more negative, rather
than higher and lower, and then you won’t get
• If you have to use the words higher and lower, think
about temperatures. A temperature of–10 °C is
higher than one of –20 °C. In the same way, a water
potential of –10 Pa is higher than a water potential
of –20 Pa.
Cells and Osmosis
• The concentration (or solute potential) of the
solution that surrounds a cell will affect the state of
the cell, due to osmosis.
• There are three possible concentrations of solution
to consider:
– Isotonic solution  a solution of equal ψs (or
concentration) to a cell
– Hypertonic solution  a solution of higher ψs (or
concentration) than a cell
– Hypotonic solution a solution of lower ψs (or
concentration) than a cell
H y p o to n ic s o lu tio n
Is o to n ic s o lu tio n
H y p e rto n ic s o lu tio n
w a te r e nte rs, ce ll sw e lls
a nd m a y b urst (lysis).
no ne t m o ve m e nt o f
w a te r, ce ll no rm a l size
w a te r le a ve s, ce ll
shrinks a nd cre na te s
w a te r e nte rs, ce ll sw e lls
a b it a nd b e co m e s turg id .
no ne t m o ve m e nt o f
w a te r, ce ll no rm a l size
w a te r le a ve s, cyto p la sm
shrinks a nd p la sm o lyse s
An im a l
C e ll
P la n t
C e ll
• Simple animal cells (protozoans) in
fresh water habitats are surrounded by
a hypotonic solution and constantly
need to expel water using contractile
vacuoles to prevent swelling and lysis.
• Cells in marine environments are
surrounded by a hypertonic solution,
and must actively pump ions into their
cells to reduce their water potential and
so reduce water loss by osmosis.
• Young non-woody plants rely on cell turgor
for their support, and without enough water
they wilt.
• Plants take up water through their root hair
cells by osmosis, and must actively pump
ions into their cells to keep them hypertonic
compared to the soil.
• This is particularly difficult for plants rooted
in salt water.
why of a standard size were cut from a single large potato. One
a) (i)
the potato
length; were placed in
and the
–3 sucrose solution showed no further
(ii) the
cylinder inofthe
1.0 mol dm
The length of each
decrease in length after 40 minutes.
cylinder was measured every 5 minutes for the next 50 minutes. The
(b) (i) Describe the difference in the rate of decrease in length during the first 10
graph shows the changes in length at each–3sucrose concentration.
minutes between the cylinder in the 0.4 mol dm and the cylinder in the 0.8
mol dm–3 solution.
(i) (ii) Use your knowledge of water potential to explain this difference.
Explain why
a) (i) the potato cylinder in distilled water increased in length;
potato more negative water potential/hypertonic;
(accept more concentrated)
water enters by osmosis;
cells extend/are turgid; 2 max
(ii) the potato cylinder in the 1.0 mol dm–3 sucrose solution showed no further
decrease in length after 40 minutes.
little/no water remaining in potato/fully plasmolysed/all water
has moved out;
cell wall prevents further shrinkage/sucrose solution moves in;
or, water potentials are equal/equilibrium/isotonic;
no net movement of water/no further osmosis; 2 marks
(b) (i) Describe the difference in the rate of decrease in length during the first 10
minutes between the cylinder in the 0.4 mol dm–3 and the cylinder in the 0.8
mol dm–3 solution.
faster rate (of decrease) in 0.8 mol dm-3 ; 1mark
(ii) Use your knowledge of water potential to explain this difference.
bigger water potential gradient/greater difference in water
potentials (between potato and surrounding solution); 1mark
Points to note
• Try to think of osmosis in terms of water potential
rather than water or solute concentration.
• Osmosis is the movement from a high water potential
to a low water potential through a partially
permeable membrane.
• The water potential of pure water is 0. Since pure
water has the highest water potential, all other
values will be negative.
• Osmosis and water potential

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