Transcript Key Area 2 Transport across membranes

Starter Activity:
Answer the following question in your classwork
jotter
1.
What type of fibres are the cell walls of plants
made of?
2. Name the small rings of DNA found in bacterial
cells.
3. Name the process by which yeast cells
reproduce.
4. What is the function of
a) ribosomes?
b) mitochondria?
Answers
1.
2.
3.
4.
Cellulose
Plasmids
Budding
a) protein synthesis
b) respiration
Key Area 2
Transport across cell
membranes
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the movement of
molecules across it
Success Criteria:
•
Be able to describe the structure of the cell
membrane
•
Understand the terms passive and active transport
•
Understand the term diffusion and give examples of
diffusion in cells
•
Understand the term osmosis and its effect on cells
•
Understand the idea of concentration gradient
The Function of the Cell Membrane
Twig Video Clip: The Cell Membrane
Whilst you are watching the video clip write down key words
on a show me board.
Structure of the cell membrane
• The cell membrane
is very thin.
• It is able to control
the movement of
materials into and
out of cells.
• When seen under
an electron
microscope, it can
be seen to consist
of a double layer.
Cytoplasm of cell
Cell membrane
Structure of the cell membrane
• It has a double layer
made of molecules
called phospholipids.
• Phospholipid
molecules have a
head and a tail end.
• All the tails point
inward and the
heads point
outwards.
Head
end
Tails
Head
end
Structure of the cell membrane
• There are also proteins embedded in the
membrane.
• These proteins have various functions.
• Some are involved in the transport of
materials across the membrane.
Phospholipid
double layer
proteins
Fluid mosaic model
• The membrane is a very flexible structure which
can move and change shape easily.
• It is often described as a fluid mosaic model.
• Fluid refers to the moving phospholipids and
mosaic refers to the patchy arrangement of the
proteins.
Homework task - Make a 3D cell
membrane model
•
•
•
•
•
•
Your task is to make a 3D model of the
cell membrane.
You can use any materials you can find
at home, especially recycling materials.
Your model should show the proteins
and the phospholipids with their head
and tail ends.
The component parts should be labelled
or there should be a key to identify
them.
Examples of materials you could use
include paper, cardboard, plastic bottle
lids, plasticene, playdoh, straws, cotton
buds, cocktail sticks, buttons, ribbons,
fabric, lolly sticks, dry pasta, lego,
stickle bricks, k’nex.
THE POSSIBILITIES ARE ENDLESS
SO HAVE FUN!
Selectively permeable membrane
• Permeability means the
ability of a surface to
allow molecules to pass
through it.
• The cell membrane is
usually described as
selectively permeable.
• This means that it allows
some molecules to move
through it easily, but
others move more slowly,
and some cannot move
across it at all.
Visking tubing
• Visking tubing is a selectively permeable
material that acts like a cell membrane.
• We can use it to demonstrate which
molecules are able to pass through the
membrane, and which are not.
• Your teacher will show you how to use
the visking tubing in an experiment.
Visking tubing experiment
Water
Visking tubing bag
Starch and glucose
solution
Test water for starch and glucose after 20 minutes
Visking tubing experiment
1.
2.
3.
4.
5.
6.
7.
8.
Soften a length of visking tubing under running
water until you can open it up.
Tie a firm knot in one end.
Add equal volumes of starch and glucose solutions
using droppers.
Tie a firm knot in the other end. Cut off any excess
tubing to leave 1cm tails.
Wash the bag thoroughly under running water.
Place bag in a test tube, cover with water and leave
for 20 minutes. Remove the bag.
Transfer a drop of the water to a spotting tile and
test for starch using iodine.
Test the remaining water for glucose by adding
Benedicts solution and heating to 80°C for 5
minutes.
Visking tubing experiment
Results:
Colour change
observed
Result of test –
positive or
negative
Starch test
(iodine)
Glucose test
(Benedicts)
Conclusion : The ______was able to pass through the
visking tubing but the _______was not. This is because
starch has ________ molecules but glucose has
________molecules.
Selectively Permeable Membranes
• Cell membranes allow small molecules
like glucose, oxygen and water to pass
through them freely.
• This is because the membrane has tiny
holes in it called pores that make it
permeable. Large molecules like starch
are unable to pass through.
Selectively Permeable Membranes
• Pores in the membrane are
small, so only small molecules
such as glucose, water,
oxygen and carbon dioxide can
get through.
cytoplasm
nucleus
• Large molecules such as starch
cannot pass through.
• Selectively permeable
membranes allow certain
molecules to pass through but
not others.
selectively
permeable
membrane
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the movement of
molecules across it
Success Criteria:
•
Be able to describe the structure of the cell
membrane
•
Understand the terms passive and active transport
•
Understand the term diffusion and give examples of
diffusion in cells
•
Understand the term osmosis and its effect on cells
•
Understand the idea of concentration gradient
Starter
Starter
C
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the terms passive and active
transport
• Understand the term diffusion and give
examples of diffusion in cells
Passive transport
•
passive transport is the movement of
a substance across a cell membrane
down a concentration gradient
• It does not require energy.
• The two types of passive transport
are:
1. Diffusion
2. Osmosis
Diffusion
• Diffusion is the movement of molecules
in a liquid or gas from high to low
concentration until they are evenly spread
out.
• Diffusion moves down a concentration
gradient.
Diffusion clip
Diffusion in liquids
• Diffusion in liquids can be seen by
adding dye to a beaker of water.
Add dye
to one
side
only
Water
Do not shake or stir. Leave for 20
minutes.
Diffusion in liquids
Red
dye in
water
Red dye molecules and water molecules have
moved until they are evenly spread – this is
DIFFUSION.
Direction of diffusion
Diffusion always occurs from high concentration
of a molecule to low concentration of that
molecule.
Concentration Gradient
The difference in concentration of two
solutions is called a concentration gradient.
In diffusion, molecules will always move down
the concentration gradient from high
concentration to low concentration.
Concentration Gradient
•Like a ball on a slope, molecules diffuse down a
concentration gradient from high to low.
high
ground
ball
ball rolls down gradient
ball stops
gradient
(slope)
low
ground
Concentration Gradient
• A concentration gradient exists when there is
a difference in concentration from one area
to another.
• Molecules move down a concentration
gradient from high to lower concentration.
The molecules will stop moving when the two
concentrations are equal.
Cells and diffusion
• Many substances can enter or leave
cells by diffusion.
• This happens across the cell membrane.
• Animal cells take in glucose, oxygen and
amino acids by diffusion.
• Carbon dioxide and waste materials
leave animal cells by diffusion.
Diffusion in an animal cell
Glucose
Oxygen
Amino acids
Carbon dioxide
Waste
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the terms passive and active
transport
• Understand the term diffusion and give
examples of diffusion in cells
Starter
Starter
Diffusion
Waste/carbon
dioxide
Cell
membrane
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the term osmosis and its
effect on cells
• Understand the idea of concentration
gradient
• Water can also Osmosis
diffuse into and out
of cells across the
cell membrane.
• Osmosis is the
Low water
special diffusion of concentration
water from an area
of high
High water
concentration to an
concentration
area of low
concentration
through a selectively
permeable
Twig Video Clip - Osmosis
membrane.
Visking tubing
• Visking tubing is a selectively permeable
material that can be used to show the effect
of osmosis on cells.
• The visking tubing behaves like a cell
membrane, so we can use it to make model
cells.
TEACHER DEMONSTRATION
Osmosis experiment
A
B
Visking tubing bag
10% sugar solution
water
Boiling tube
1. Wash and dry bags when filled.
2. Weigh both bags.
3. Place in test tubes for 20 minutes then dry and reweigh.
Results
A
Mass of bag
and contents
at start (g)
Mass of bag
and contents
after 20
minutes (g)
Difference in
mass (g)
B
Conclusion
• Bag A increased/decreased in mass.
This was because water moved in/out
by osmosis.
• Bag B increased/decreased in mass.
This was because water moved in/out
by osmosis.
• Water always moves from ________
water concentration to ______ water
concentration.
Answer the following questions in
sentences:
1.
Why was the visking tubing bag dried in a
paper towel before being weighed?
2. Why was visking tubing used in this
experiment? What property does it have
that makes it a good model cell?
3. What would happen to an onion cell placed in
pure water?
4. What would happen to a cheek cell placed in
10% sucrose solution?
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the term osmosis and its
effect on cells
• Understand the idea of concentration
gradient
Starter
Starter
C
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the term osmosis
Understand the effect of osmosis on
plant cells
Osmotic Effect On Cells
Water concentrations
• If we think about solutions in terms of
their water concentrations, it is easier to
recognise which direction water
molecules will flow in.
• A dilute sugar solution will have a high
concentration of water, whereas a
concentrated sugar solution will have a
lower water concentration.
Concentrated
Sugar Solution
Dilute Sugar
Solution
Low water concentration
High water concentration
High sugar concentration
Low sugar concentration
Water Concentrations
Cell in pure water
Direction of water
movement
H2O
concentration >
cell
The surrounding solution has a higher water
concentration than the water concentration
within the cell, so water enters by osmosis.
Water Concentrations
Cell in concentrated
salt solution
H2O
concentration <
cell
The surrounding solution has a lower water
concentration than the water concentration within
the cell, so water leaves the cell by osmosis.
Water Concentrations
Cell in solution with
equal concentration
to cell contents.
H2O concentration =
cell
The surrounding solution has a water
concentration that is equal to the water
concentration within the cell, so there is no
gain or loss of water by osmosis.
Investigating osmosis in potato tissue
1. Copy the title and the aim from the
instruction card.
2. Prepare a results table.
3. Set up the experiment by following
the instructions on the card.
Results
Bathing
solution
Water
Strong
sucrose
solution
Conclusion
Evaluation
Initial
mass (g)
Final
mass (g)
% Change
in mass
Plant Cells and Osmosis
Pure water: there is more water
outside of the cell than inside,
therefore water will move into
the cell by osmosis. This causes
the cell to swell and become
turgid.
Strong salt solution: there is
more water inside the cell than
outside, therefore water will
move out of the cell by osmosis.
This causes the cell to become
softer or flaccid.
Cell in pure
water
Cell in
strong salt
solution
Plant Cells and Osmosis
Plant Cells and Osmosis
Cell in pure
water - turgid
Normal cell
Cell in strong
salt solution plasmolysed
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the term osmosis
Understand the effect of osmosis on
plant cells
Starter
Starter
A
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the term osmosis
Understand the effect of osmosis on
animal cells
Viewing plasmolysed cells
• In this practical, you will expose onion cells to
a solution of salt to observe plasmolysis. You
will also prepare cells in water for comparison
(as a control experiment).
• You will view the cells under the microscope
and make drawings of what you see.
• You will need a pencil and a circular object (eg
beaker) to draw round.
• Follow the instructions on the card provided.
Plasmolysis in onion cells
• The photo shows
onion cells which
have been
bathed in strong
salt solution and
have become
plasmolysed.
• Notice how the
cytoplasm and
cell membrane
have shrunk
inwards away
from the cell
wall.
cell wall
cell membrane
nucleus
cytoplasm
Osmosis in red cabbage cells
1.
2.
3.
4.
5.
6.
Your task in this experiment is to identify
liquids A and B by looking at the results.
Label two boiling tubes A and B.
Half fill tube A with liquid A and tube B with
liquid B.
Cut 10 discs from red cabbage using a cork
borer.
Wash the leaf discs in a small beaker of
water and dry them gently.
Add 5 discs to each test tube.
Leave for 20 minutes and observe any colour
change in the liquids.
Osmosis in red cabbage cells
Results:
Test tube A
Test tube B
Appearance of
liquid
Conclusion:
Which test tube contained strong sucrose solution?
How do you know this?
Which test tube contained water?
How do you know this?
Animal Cells & Osmosis
• The effects of osmosis on animal cells
are totally different to plant cells
because animal cell structures are
different.
• Animals cells do not have:
Cell walls
Vacuoles
Animal Cells & Osmosis
• Red blood cells (RBCs) float in a liquid
called plasma which is the same water
concentration as the cells.
• RBCs in plasma do not change size because
the water has no concentration gradient to
follow.
• RBCs will change if there is a concentration
gradient for water to follow.
Animal Cells &
Osmosis
RBC in equal
concentration solution
Normal red
blood cell
Cell loses
water by
osmosis and
shrinks.
No net
osmosis,
cell stays
the same.
Cell takes
in water by
osmosis,
swells and
eventually
bursts.
Osmosis in eggs
• Your teacher will show you a demonstration
experiment involving eggs placed in different
solutions.
• Write a description of this experiment in
your notebook. Include the following:
Title
Diagram
Method
Result
Conclusion
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the term osmosis
Understand the effect of osmosis on
animal cells
Starter
Starter
Turgid
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the term active transport
and its effect on cells
• Understand the idea of concentration
gradient
Passive transport
• Diffusion and osmosis are examples of
passive transport.
• This always involves movement of
molecules down a concentration
gradient.
• Passive transport does not need any
energy input.
Active transport
• Some molecules and ions are moved across the
membrane by active transport.
• This involves movement of molecules and ions
against a concentration gradient.
• Active transport does need energy input.
• Membrane proteins are involved in this
process.
Transport across
cell membranes
(Glow)
Active transport
The sodium potassium pump
•This is an example of active transport which occurs in nerve cells.
•Sodium ions are pumped out of cells against the concentration gradient.
•Potassium ions are pumped into cells against the concentration gradient.
OUTSIDE THE CELL
Sodium
pumped
out
CELL
MEMBRANE
Membrane
proteins
Potassium
pumped in
INSIDE THE CELL
Active transport in cells
• When a cell is alive, the membrane can
use active transport to make sure that
suitable molecules are allowed to build
up in the cell, and unsuitable or harmful
molecules are kept out of the cell.
Practical application of this idea
• You will probably have had
antibiotics at some time
for an infection.
• Many antibiotics work by
destroying the membranes
of live bacteria in your
body.
• The damaged membranes
allow vital nutrients to
escape and poisonous
substances to enter and kill
the bacterial cells.
Transport across cell
membranes
Learning Intention:
Investigate the cell membrane and the
movement of molecules across it
Success Criteria:
• Understand the term active transport
and its effect on cells
• Understand the idea of concentration
gradient