Quiz on functions of
components of cell membrane
Crossing membranes
(passive processes)
Starting activity:
What do you remember
about diffusion?
•Define it...
•What affects its rate?
•What state can it occur
in?
pages 22-27
Diffusion
The net movement of molecules or ions from a
region of their high concentration to a region of
their lower concentration, down a
concentration gradient.
What substances have to
diffuse into and out of living
cells?
Metabolism
Cross membranes
The rate of diffusion
•
•
•
•
•
•
Temperature
Concentration gradient
Stirring/moving
Surface area
Distance/thickness
Size of molecule
Diffusion
Lipid soluble, small, uncharged molecules can diffuse across
the phospholipid bilayer down a concentration gradient –
random process relying on kinetic energy of particles
Substances e.g. CO2, O2, H2O, steroid hormones (lipid soluble)
Larger or charged molecules
Facilitated diffusion
Facilitated diffusion
Large or polar molecules can’t
pass easily through the bilayer
These molecules move through
gaps in channel proteins
Transport depends on a
concentration gradient
(facilitated = made easy)
Substances e.g. glucose, sodium
ions, amino acids
Summary table
Substance moved by
Simple diffusion
Examples
Facilitated diffusion using
channel proteins
Facilitated diffusion using
carrier proteins
Compare and contrast facilitated diffusion and
simple diffusion
Osmosis – a special case
The net movement of water molecules from a
region of higher water potential to a region of
lower water potential (down a water potential
gradient) across a partially permeable membrane.
Water potential = a measure of
the concentration of water
molecules which are able to
diffuse.
Key definitions
• Solvent – a liquid that dissolves solids
• Solute – a solid that dissolves in a liquid
• Solution – a liquid containing dissolved solids
Water Potential
• Water potential (Ψ = greek letter psi) of a solution is the
tendency of water to diffuse out of it (unit is one of
pressure) i.e. how frequently water molecules are hitting
the membrane – kPa, kilopascals
• Pure water has a water potential of 0
• Adding a solute to pure water decreases the
concentration of water molecules in a given space - the
chance of water molecules leaving the solution is less
likely as the water molecules are attracted to the solute
molecules and move less freely
• Solutions have a negative water potential
– 17g sucrose in 1dm3 water Ψ = -130kPa
Osmosis – a special case of diffusion
• Although water molecules are polar they are small enough
to pass through the membrane
• Osmosis is…..the diffusion of water from a high water
potential to a low water potential (down its water potential
gradient) across a partially permeable membrane
Understanding water potential and
osmosis
•
•
•
•
•
Units = Greek letter psi ψ
Measured in kPa.
Pressure created by water molecules
Pure water has a ψ 0 kPa.
As the water potential decreases value for ψ
becomes more negative.
• The more solute dissolved, the lower the
water potential of the solution.
Diagram 4 from books page 27
Osmosis in animal and plant cells
crenated
haemolysed
Incipient plasmolysis
plasmolysed
Potato Cylinders Investigation
Results
Examine the potato cylinders and measure their mass (g)
Copy and complete this table.
How could we display this on a graph?
Sucrose Concentration (M)
0.0
Initial mass (g)
Final mass (g)
Mass change
(+/- g)
Percentage
mass change
0.1
0.2
0.4
0.6
0.8
1.0
Discussion
1.
2.
Why do you calculate the percentage change in mass rather than the
actual change in mass
Estimate the concentration of the potato from the graph.
3. From your knowledge of osmosis in plant cells, what do you suppose has
happened to the cells of the potato tissue (a) in 1M sucrose, (b) in water?
4. How would these changes in the cells account for any changes in size of
the potato cylinder?
5.
What do you think limits the change in mass of the potato cylinder which
was immersed in water?
6.
How could the results of this experiment be used to explain the
extension growth of plant shoots and roots ?
7. How could the experiment be modified to find out the osmotic
concentration of cell sap ?
Red onion practical
Crossing membranes
(active processes)
Diffusion alone will not meet the needs of a cell.
• Speed
• Concentration gradient (glucose, magnesium ions)
• Ensuring a one way flow
• Moving large amounts
Active transport
The movement of molecules or ions across membranes,
which uses ATP to drive protein ‘pumps’ within the
membrane. Eg Na⁺ in nerve axons
• Some carrier proteins act as pumps
• Complementary shape to molecule they carry
The molecule can fit on
just one side ensuring
unidirectionality of movement
Active transport requires energy:
• It acquires energy from respiration so cells
that carry out a lot of A.T. will need lots of
mitochondria.
– What materials are required for aerobic
respiration to occur?
• Root cells actively transport ions e.g. NO₃⁻, K⁺
into roots.
– This process stops when soil is waterlogged such
as in peat bogs. Can you suggest why?
Moving large amounts – bulk transport
Pinocytosis (small molecules / liquids)
Phagocytosis (large solids)
Animation
Movement across membranes
Summary of movement across membranes:
Passive processes
Diffusion
Facilitated diffusion
Osmosis
Active processes
Active transport
Endocytosis and exocytosis
Tasks to try:
1. Explain why active transport allows
substances to be accumulated in an area,
whereas facilitated diffusion doesn’t.
2. Answer Q’s 10, 11 &12
Homework: Due next lesson
Finish Q’s above and Q9 from booklet