Transcript Diffusion_and_osmosisIGCSEppt

Diffusion
IGCSE Science - Biology
I need some deodorant!
 Close your eyes and as soon as you
notice a change in our environment
stand up.
 I’m going to release something in
the room.
 Now you have 2 min as a group to
explain what just happened!
Diffusion
Diffusion is the movement of molecules down a
concentration gradient from an area of high
concentration to an area of low concentration,
until evenly spread out.
Why is diffusion
important?
All living cells rely on diffusion to live.
They use it for:
 getting raw materials for respiration (dissolved
substances and gases)
 removing waste products (eg. from respiration)
 photosynthesis in plants (raw materials in, waste products out)
Examples…
Respiration- gas exchange
In breathing you exchange carbon dioxide (CO2) and oxygen (O2) between
alveoli in the lungs and the blood.
This is an example of diffusion.
SEM photos of lung alveoli
Respiration- in cells
food + oxygen  carbon dioxide + water + energy
Respiration & diffusion
Photosynthesis
& Diffusion
carbon dioxide + water  oxygen + glucose
 Carbon dioxide
diffuses in
through the
stomata

Oxygen and
water diffuse out
of the stomata
Osmosis
Osmosis is a special case of diffusion.
It is the nett movement of water across a
selectively or partially permeable membrane
from a high concentration to a low
concentration.
Selectively permeable membrane ???
The cell membrane has very small holes in
it. Small molecules can pass through, but
larger ones can’t.
We say it is selectively permeable or semipermeable or partially permeable.
Water concentration gradient
In osmosis water moves both ways to balance up the
concentrations. The overall (nett) movement is to the area of low
water concentration.
Key:
Water molecule
High Water Concentration,
few molecules of solute.
Dilute solution
Low Water Concentration, many
molecules of solute.
Concentrated solution
Solute molecule
Water movement
Osmosis in action
Have a look at:
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__
how_osmosis_works.html
Why is this not funny if you
were a slug??
Osmosis in Cells
Some real time osmosis
videos to set the mood!
Group Starter Activity
Mr and Mrs Kim went on holiday to Jeju for a few days.
When they came back their favourite plant looked like this!
Explain the changes in terms of water movement
5 days ago
Today
Why is osmosis important?
Plants rely on the movement of water through them:
 Water always moves to areas of lower water concentration.
 Root hairs take in water from the soil by osmosis.
Plants- Water moving in
Water moves into a plant cell.
The cell wall stops the cell from bursting.
We say that the cell is turgid.
Turgid cell.
Vacuole is filling all the space and
pushing against cell wall
This is useful as it gives plant stems
support
Cell turgor pressure
Plants- Water moving out
When water moves out of the plant cell vacuole, by osmosis, it
becomes flaccid
Flaccid cell.
Vacuole is much smaller and
taking up less space
If a lot of water leaves the cell, the cytoplasm starts to peel away from the
cell wall and we say the cell has undergone plasmolysis.
Plasmolysis
Osmosis in Plant Cells
Low water
concentration outside
the cell
(ie concentrated
solution outside)
Equal water
concentrations inside
& out
(ie equal solute
concentrations
inside & out)
High water
concentration outside
the cell
(ie pure water or
dilute solution
outside)
Animals- Water moving in
Animal cells have no cell wall to stop the swelling, just a flexible
cell membrane.
If an animal cells are put in pure water (HWC) they take in water by
osmosis until they burst.
haemolysed
normal
turgid then burst
Animals- Water moving out
If an animal cells are put in a concentrated solution (LWC) they lose water
by osmosis until they become shrunken or haemolysed.
haemolysed
normal
http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/Osmosis_Anima
tion.htm
Quick Quiz- Osmosis
1.
What is a partially permeable membrane?
A membrane that only lets small molecules through it
2.
Which substance moves by osmosis?
Water, H2O
3.
What happens to plant cells that take up water by osmosis, and what do you call the
cell?
The cells become ‘full’ of water and are called turgid
4.
What happens to plant cells that lose water by osmosis, and what do you call the cell?
The cytoplasm comes away from the cell wall and the cells are
called plasmolysed.
5.
What does ‘haemolysed’ mean? How does it happen?
Animals cells which have lost water become shrunken and are
called haemolysed.
Osmosis
Two sugar solutions are separated
by a semi-permeable membrane
like Visking or dialysis tubing.
The larger sugar molecules are
too big to pass through the
membrane but the water
particles can pass.
On both sides of the
membrane particles move
randomly hitting the membrane
and each other.
Dilute sugar solution
Concentrated sugar solution
Osmosis
Osmosis is like diffusion except
there is a membrane in the way.
If a water molecule on either
side hits the membrane it will
bounce off - if it hits a hole it
will go through.
The sugar molecules can’t
pass and just bounce off.
Water molecules from both
sides pass through the
membrane but there is a nett
movement from left to right.
Nett movement is down the
water gradient.
Dilute sugar solution
Concentrated sugar solution
Osmosis
Osmosis is the diffusion of
molecules from an area of high
concentration to an area of lower
concentration through a partially
permeable membrane.
In terms of water potential we
can say that osmosis allows
the movement of water
particles from areas of high
water concentration or
“potential” to areas of low
water concentration or
“potential”, across a semipermeable membrane.
Nett movement is down the
water gradient.
Dilute sugar solution
Concentrated sugar solution
Now try your osmosis
practicals