Movement through cell membranes
Objectives
• You should be able to:
• Define the term "diffusion” and give
examples.
Diffusion
• Diffusion is the spreading out of particles from a region of
high concentration to a region of low concentration.
• Diffusion moves down a concentration gradient.
• It is a passive process – no energy input is required!
Diffusion animation 1
Diffusion animation 2
Diffusion in everyday life
• Smell of perfume
• Stink bomb
• Sugar in tea
Example of diffusion in living things
• In the alveoli oxygen diffuses from the
alveoli to the blood capillaries and carbon
dioxide diffuses out
Animation
Check your learning..
• Define diffusion?
• Why is diffusion called a passive process?
• Give some examples of diffusion in
everyday life
• Give an example of diffusion in a living
thing/
Objectives
• You should be able to:
• Define the term “osmosis” and give
examples.
• Investigate osmosis
Permeability
Membranes can be
Permeable – let everything in and out
Semi Permeable - let only some substances in and
out
Impermeable – let nothing in and out
Selective permeability of membranes
Cell membranes and organelle membranes
are selectively permeable
Diffusion animation 3
Osmosis is a type of diffusion, so what do you think
happens to water molecules during this process?
osmosis
water
molecule
semipermeable membrane
Osmosis is the movement of water molecules from a region
of high water concentration to a region of low water
concentration across a semipermeable membrane.
Osmosis animation 1
Osmosis animation 2
What is happening here?
To investigate osmosis
To be a fair test:
Use the same size beakers with equal amounts of water.
Equal lengths of visking tubing.
Volume of liquid in visking tubing equal.
Find the mass of each tube before you start and note it.
Leave them for the same amount of time.
To investigate osmosis
Results:
Remove the bags, dry them and record the mass of
each bag.
Also note any changes in fullness of each bag.
Can you..
• Explain what a semi permeable membrane
is?
• Tell what osmosis is?
• Explain why osmosis is a special case of
diffusion?
A bag made from a semipermeable membrane is tied
to a glass tube and filled with a strong sugar solution.
This bag is placed in a weak sugar solution.
strong sugar
solution
weak sugar
solution
What happens to the water molecules in the weak solution?
What happens to the liquid inside the semipermeable bag?
Today’s objectives
• By the end of today’s class you should be
able to:
• Describe what happens to plant cells in
different types of solutions
• Know what a cell with turgor is
If a plant cell is surrounded by a solution which has
a higher water concentration:
Water from outside will flow into the cell by osmosis,
•
the vacuole becomes bigger and the cytoplasm swells
•
The cell membrane is pushed out towards the cell wall
– the cell is called “Turgid”.
Turgor
• Def Turgor Pressure is the pressure of the cytoplasm
and the vacuole pushing against the cell wall of the
plant. (Also called “turgor”)
Lettuce leaves are
crisp when the cells
are turgid!
If a plant cell is surrounded by solution which is
less concentrated with water
• the water inside the cell would move out by osmosis
• the cell wall stays intact but the membrane shrivels up
away from it
Wilting
See animations
Check your learning..
• Define osmosis
• What is a semi- permeable membrane?
• What happens to a plant cell when it is put
in a solution of higher water conc?
• What is a cell like this called?
• What would happen to an animal cell in a
solution of higher water conc?
• What happens to a plant cell when it is put
in a solution of lower water conc?
Learning objectives
• To learn about how osmosis is used to
prevent food spoilage and 2 examples of
this
Osmosis and Food Preservation
• Bacteria are a most common causes of
food spoilage
• Bacterial cells have a cell membrane
• If a food is placed in a sugary or salty solution
then any bacteria present will lose the water in
their cells to the solution outside
• The bacteria cells shrivel and die!
Examples
• Fish and Meat (bacon) may be stored in a
salty solution
• Jams, marmalades and tinned fruits are
stored in a sugary solution
Syllabus Can you?.....
• Define the term: Selectively permeable
• Explain the role of selectively permeable
membranes.
• Define the terms: osmosis & diffusion
• Give examples of diffusion and osmosis.
• Define the term: turgor.
• Explain turgidity in plant cells.
• Describe the application of high salt or sugar
concentration in food preservation.