Download SUB-UNIT 1.2: TRANSPORT ACROSS MEMBRANES

SUB-UNIT 1.2: TRANSPORT ACROSS
MEMBRANES
Learning Outcomes
• By the end of the topic, you should be able to • define diffusion
• describe examples of substances which enter
and leave the cell by diffusion,
• discuss the importance of diffusion to cells.
• state that osmosis is a ‘special case’ of diffusion
of water
• explain osmotic effects in plant and animal cells
• discuss the effects of placing plant and animal
cells in hypertonic, hypotonic and isotonic
solutions.
• explain the meaning of the terms plasmolysed,
turgid and flaccid.
Cells and Diffusion
As the ammonium hydroxide (an alkali) moves through it
turns the pH paper blue. This demonstrates that the gas is
spreading out from a region of high to low concentration.
Importance of diffusion
dissolved
food
oxygen
nucleus
cell membrane
cytoplasm
carbon dioxide
Human cheek cell
Importance of diffusion
The cell membrane freely allows oxygen
to diffuse into the cell.
high
high
high
low
low
high
high
low
low
high
high
Oxygen moves
from a high
concentration
to a low
concentration.
Importance of diffusion
The cell membrane freely allows carbon
dioxide to diffuse out of the cell.
low
low
low
high
high
low
low
high
high
low
low
Carbon dioxide
moves from a
high
concentration to
a low
concentration.
Diffusion in Paramecium
one cell.
• A unicellular animal has only _____
The animal is constantly respiring. This
oxygen and
means that it is using up ________
carbon
dioxide
producing _________
__________.
• The concentration of oxygen inside the
less
unicellular animal is _______
than the
concentration in the water outside.
into the cell
Oxygen therefore diffuses _____
to allow continued respiration.
Diffusion in Paramecium
Higher concentration of
oxygen outside cell
oxygen
oxygen
CO2
CO
oxygen
2
Lower
concentration
of oxygen
oxygen
CO2
CO2
Higher
concentration
of CO2
CO2
oxygen
oxygen
Lower concentration of
CO2 outside cell
CO2
oxygen
oxygen
CO2
Diffusion in Paramecium
• At the same time the cell is making CO2.
The CO2 concentration becomes greater
inside the cell than in the water
_______
_________.
As a result CO2 diffuses
outside
_____
out of the cell.
• If it wasn’t for diffusion unicellular
organisms wouldn’t get any oxygen or be
able to get rid of waste CO2.
Diffusion in Amoeba
Food vacuole
containing soluble
food
diffusion
of food
Diffusion in Amoeba
• Amoeba get their food by phagocytosis The
food becomes enclosed in a food vacuole and
digestive enzymes break it down into small
soluble products.
• The dissolved food then moves by diffusion from
a _____
high concentration (inside the food vacuole)
to a _____
low concentration (in the cytoplasm)
______
down a concentration gradient.
• Therefore diffusion allows all parts of the cell to
get food.
Importance in Multicellular Organisms
oxygen
carbon
dioxide
Diffusion in multicellular animals
• Blood returning to the lungs from cells has a
higher CO2 concentration and lower oxygen
concentration than the air in the lungs.
lungs
• CO2 diffuses from the _____
blood into the ________
lungs
and oxygen diffuses from the ________
into the
________.
blood
• Diffusion is also essential for dissolved food (e.g.
glucose
___________
and __________
_______ )
amino acids
moving from the blood to respiring cell.
• Urea also moves in the opposite direction by
diffusion.
lung
High
O2
conc
O2
Low
CO2
conc
O2
CO2
Low
O2
conc
High
CO2
conc
CO2
cells
Diffusion in Green Plants
• Green plants need diffusion to get the raw
materials needed for photosynthesis
carbon
dioxide
( ___________
________
and
_________)
water
• CO2 diffuses from the air, through the
stomata and into the air spaces in the leaf.
• Water moves from the soil into the roots,
then enters the xylem and then enters the
leaf cells.
Importance of diffusion
Water molecules are also free to move
in and out of the cell.
However a cell needs to have some
control which substances can enter and
leave the cell.
If not, then it could gain substances it
doesn’t need or even lose substances it
does need due to diffusion.
Cell Membrane
Cell membrane
with pores =
SELECTIVELY
PERMEABLE
water molecules
small area of
cell enlarged
Oxygen molecules
Carbon dioxide molecules
Glucose molecules
Starch/salt molecules
Model Cell experiment
You will need to set up:
Method
a length of visking tubing
1. Fill tubing with
starch and glucose
solution
a boiling tube
a test tube rack
a filter funnel
a paper towel
a plastic syringe, a dropper
a dimple tray
stopclock
starch and glucose solution
Iodine solution
Benedict’s solution
water bath
2. Wash and dry bag
3. Place in boiling tube
4. Immediately test
sample of water for
glucose and starch
5. Test every 4 minutes
Importance of diffusion
Large molecules like
starch cannot pass
through the cell
membrane.
Molecules of soluble
food e.g. glucose
can pass through.
The membrane is
selectively
permeable.
The Cell Membrane
Structure of cell membrane
Structure of cell membrane
Fluid Mosaic model – Composed of protein and lipid molecules
Surface proteins – give support
- carry molecules across membrane
- enzymes to speed up reactions
Channel proteins – allow
small molecules to pass
through membrane
Phospoholipid bilayer
– molecules can
move and swap
places
Relative water concentrations
Water concentration - compares concentration of solutions
Lower solute
concentration
Higher water
concentration
Lower water
concentration
Higher solute
concentration
hypotonic
Solution A is _________________
to solution B
hypertonic
Solution B is _________________
to solution A
More concentrations
More concentrations
Lower solute
concentration
Higher water
concentration
Lower water
concentration
Higher solute
concentration
hypotonic
Solution X is _________________
to solution Y
hypertonic
Solution Y is _________________
to solution X
Lower solute concentration
Higher water concentration
Higher solute concentration
Lower water concentration
HYPOTONIC
hypotonic = more water
hypertonic = less water
HYPERTONIC
Osmosis is a special kind of
diffusion, where:
Water molecules move from a
region of high water
concentration to a region of low
water concentration across a
selectively permeable membrane.
Let’s take a look
Selectively permeable membrane
Sucrose
molecules
Low water
concentration
High water
concentration
Water
molecules
Inside the cell
Outside the cell
Selectively permeable membrane
Sucrose
molecules
Water molecules are small enough to
pass across the selectively permeable
membrane
Water
molecules
Inside the cell
Outside the cell
Selectively permeable membrane
Water molecules
passing into the
cell
Inside the cell
Outside the cell
Selectively permeable membrane
Sucrose
molecules
Osmosis continues until the water
molecules are evenly spread inside and
outside of the cell
Water
molecules
Inside the cell
Outside the cell
SELECTIVELY PERMEABLE MEMBRANE
Solute molecules
are too large to
pass through the
pores of the
membrane
Net movement
of small water
molecules
PURE
WATER
SOLUTION
(SOLVENT IS WATER)
Osmosis
• Diffusion of water through cell membranes
• Diffuses from an area of high water
concentration [HWC] to an area of low water
concentration [LWC]
• The difference in concentration between the
areas is called the concentration gradient
Osmosis
• Set up test tubes shown
Salt solution
water
Cylinder of potato
Weigh dried cylinders
Leave for 30 minutes
Dry then re-weigh
Molecular Model of Osmosis
Model cell
A
0.1 M sucrose
solution
0.5M sucrose
solution
Molecular Model of Osmosis
Model cell B
0.5M sucrose solution
0.5M sucrose solution
Molecular Model of Osmosis
Model Cell C
1.0M sucrose solution
0.5M sucrose solution
Direction of Osmosis?
1
High water concentration
Low water concentration
(100% water)
2 10% sucrose solution (90% water)
3
(90% water)
10% sucrose solution
(100% water)
4
distilled water
(90% water)
5
10% starch solution
distilled water
(80% water)
20% sucrose solution
(99.5% water)
0.5% sucrose solution
(99% water)
1% starch solution
(100% water)
6
animal cell
distilled water
Animal cells in water
Plant cells in water
turgid
plasmolysed
In distilled water, an animal cell will gain
water & burst
Animal Cells
In strong salt/sugar solution, an
animal cell will lose water & shrink
In distilled water, plant cells gain water
& become swollen (turgid)
Plant Cells
In strong salt/sugar solution, plant cells lose
water, the cytoplasm & vacuole shrink in from the
cell wall – cell becomes plasmolysed
The plant cell has a rigid cell
wall that helps the plant cell
keep its shape.
The animal cell does not.
Unicellular animals
Contractile vacuole with
canals to collect water
When water passes in, vacuole
expands then bursts when full
When one vacuole is emptying,
the other is filling
This is osmoregulation
Osmosis and Energy Production
The world's first test plant to
harness osmotic power, a new
emission-free source of energy,
opened recently in Norway.
Near Oslo, the facility will
exploit the energy produced
when fresh water meets
seawater.
Fresh water and salt water is guided into
two separate chambers, divided by an
selectively permeable membrane.
The fresh water moves by osmosis
towards the seawater. The flow puts
pressure on the seawater side, and that
pressure can be used to drive a turbine,
producing electricity.
Problems……..
• The plant at Oslo produces 2 kilowatts
of electricity – enough to run a coffee
machine!!
• Very Expensive to build
But
Huge potential with further research.
Active Transport
ACTIVE TRANSPORT
low
concentration
high
concentration
PLASMA MEMBRANE
Carrier protein takes
up particles on one
side of the plasma
membrane and ……….
energy
from respiration
…..releases them
on the other side
low
concentration
gradient
high
It takes energy to move molecules against their
concentration gradient.
Active Transport is…..
The movement of molecules against a
concentration gradient, this means from a low
concentration to a high concentration.
Energy is needed for active transport.
Carrier proteins pump molecules e.g. into the cell
in order to maintain a high concentration inside
the cell or out of the cell in order to maintain a
low concentration in the cell.
Low conc. of
A outside cell
Plasma membrane
protein
Active transport in
High conc. of
A inside cell
• Animation: How the Sodium Potassium
Pump Works
• Nerve Cells: Sodium ions are actively
transported out of the cell and
potassium ions transported into the cell.
• Cells in our thyroid gland actively uptake
iodine. They use it to make hormones.
More oxygen
Less oxygen
More respiration
Less respiration
More energy produced
Less energy produced
More active transport
of molecules
Less active transport
of molecules
Hyperlink: Summary of Diffusion, Osmosis, Active
Transport
• Membrane Transport