Exchanging Substances
Facilitated Diffusion, Osmosis and
Active Transport
Passive Transport
Learning Objectives
• - explain what is meant by
passive transport
• - compare diffusion and
facilitated diffusion
• Identify the role of
membrane proteins in
transport
Success Criteria
• Construct definitions of
diffusion, facilitated
diffusion and osmosis
• Create a comparison table
• Draw and annotate
diagrams to show how the
processes differ
Transport
across
the
cell
membrane
All cells are surrounded by a partially-permeable membrane that controls what
substances can enter and exit the cell.
A cell needs to be able to import the
substances it needs to survive, and to export
waste materials and substances that are
needed outside the cell.
There are several methods by which substances (molecules and ions) can cross the cell
membrane:

diffusion

osmosis

active transport.
What is diffusion?
Diffusion is the net movement of particles down a concentration gradient: from a region
of high concentration to a region of low concentration.
net movement
of particles
No metabolic energy is expended during diffusion so it is an example of passive
transport.
One example of diffusion is gas exchange across respiratory surfaces, such as the lungs of
mammals and birds, and the gills of fish.
The rate of diffusion
The rate of diffusion in a given direction across an exchange surface can be summarized by
Fick’s law, which states that:
surface area × difference in conc.
rate of diffusion is
proportional to:
length of diffusion path
(membrane thickness)
Increasing the surface area across which the particles diffuse, or increasing the size of
the concentration gradient will increase the rate of diffusion.
Increasing the distance (or thickness of the membrane) over which diffusion takes place
will decrease the rate.
Plasma Membrane
Plasma membranes form boundaries between cells
and their environment.
But substances need to cross membranes because:
• Nutrients need to get in.
• Waste/hormones etc. need to get out.
OUTSIDE
INSIDE
Exchange Mechanisms
Plasma membranes are partially permeable –
they let some things through, but not others.
The substances that can’t get through, need help
crossing membranes.
The ways in which substances can cross
membranes are:
Diffusion
Facilitated Diffusion
Active Transport
Osmosis
DIFFUSION
Diffusion Recap
All particles that make up liquids and gases are in random
motion – they’re moving around very fast.
Because of this, they move from:
a region of high concentration to a region of low
concentration
High conc.
Low conc.
Diffusion Recap
The molecules will diffuse both ways, but the net (overall)
movement will be to the area of low concentration.
There is a concentration gradient
..... particles move down it
Diffusion is PASSIVE!
There is NO energy involved.
Diffusion occurs if the molecules involved can pass
freely through a membrane. So they have to be small. If
they’re not, facilitated diffusion is required!
Rates of Diffusion
The rate at which diffusion occurs is determined by
several factors:
1. The size of the concentration gradient.
The larger the difference in concentration, the faster diffusion will
occur.
2. The thickness of the exchange surface.
The thinner the exchange surface, the faster diffusion will occur.
3. The distance between the two areas.
A shorter distance = faster diffusion.
4. The size of the molecules.
Smaller molecules such as oxygen will diffuse quicker than large
molecules like proteins.
What is diffusion proportional to?
surface area x difference in concentration
length of diffusion path
FACILITATED DIFFUSION
Facilitated Diffusion
Facilitated diffusion uses the same principle as ordinary
diffusion, except that protein carriers are involved.
Small molecules like O2 and CO2 can simply diffuse across a
membrane without any help.
Larger molecules like amino acids and glucose can’t diffuse
directly through the phospholipid bilayer.
They still move down a concentration gradient, but
because they’re so big, they move through carrier
proteins or channel proteins.
Facilitated diffusion is also passive (no energy).
CARRIER or transmembrane Proteins
OUTSIDE
INSIDE
Carrier proteins move large molecules in or out of the cell
down a concentration gradient.
1. Molecules attach to the carrier protein.
2. The carrier changes shape.
3. It releases the molecule on the other side.
CHANNEL Proteins
OUTSIDE
INSIDE
Channel proteins form pores in the membrane for
CHARGED PARTICLES to move down a concentration
gradient.
Only open in response to presence of molecule
Facilitated Diffusion
• Facilitated diffusion is specific.
• i.e. A certain type of molecule will have a
corresponding carrier or channel
Glucose = glucose channel
Amino acids = amino acid channel
ACTIVE TRANSPORT
Active Transport
Active transport is different from
diffusion and facilitated diffusion
because it uses ENERGY.
Unlike diffusion and facilitated diffusion,
molecules move AGAINST a
concentration gradient.
This happens in the intestines, where the
concentration of nutrients is very high
in the cells already.
Active transport uses carrier proteins
too, but they work using ATP (energy).
Active Transport
OUTSIDE
ATP
INSIDE
1. Molecule attaches to carrier protein.
2. ATP molecule provides energy to go against gradient.
3. Carrier changes shape and molecule is released on the
other side (side with higher concentration).
Cells like epithelial cells
in the intestine have to
carry out active
transport.
They’re packed with
mitochondria to
provide the ATP
(energy) needed for
transporting nutrients
against a concentration
gradient.
Summary
• Complete a table to show similarities and differences between the
different methods of movement across a cell membrane
Simple
diffusion
Energy
required?
Substances
moving
Location of
cell
membrane
Factors
affecting
speed
Facilitated
diffusion
Active
transport
Osmosis
Starter
• Name a cellular organelle that possesses a membrane and
describe the purpose of the membrane.
Golgi, mitochondria, chloroplast, nucleus, lysosomes
Compartmentalise enzyme reactions/control substances in and out
• Describe the purpose of cholesterol in the plasma membrane
Prevents phospholipid tails from packing close together and preserves fluidity of membrane
• Suggest why organisms living in polar regions have a high
proportion of cholesterol in the membrane
To keep membrane fluid and functioning correctly
• List three substances that need to be transported into animal
cells in order to survive.
Oxygen, food, minerals, water
• List two substances that need to be transported out of animal
cells in order to survive
Carbon dioxide, nitrogenous wastes
OSMOSIS – TRANSPORTING WATER
Osmosis
Learning Objectives
• - Explain what is meant by
osmosis, in terms of water
potential.
•
Success Criteria
• Identify the direction of
osmosis using water
potential values
(No calculations of water potential will be
required);
• - Recognise and explain the
effects that solutions of
different water potentials
can have upon plant and
animal cells.
• Complete a quantitative
practical task (practice)
What is osmosis?
Osmosis is the diffusion of water. It is the net movement of water
molecules from a region of high water concentration to a region
of low water concentration, through a partially-permeable
membrane.
net movement of water molecules
Osmosis is the process by which cells exchange water with their
environment, such as in the mammalian kidney.
Osmosis
Osmosis is simply the diffusion of water molecules.
The definition for osmosis is:
Water Potential
• Water is rarely pure – it is never composed of 100% H2O
molecules.
• There are usually dissolved solutes in it such as minerals and ions.
• Pure water would have a ‘water potential’ (Ψ) of zero.
• But when there are dissolved solutes in water, there’s no longer
100% water molecules – so we say the water potential is more
negative.
100% water
Ψ = zero
Ψ=
more
negative
Ψ = even
more
negative
Water Potential
Water potential is measured in kilopascals (kPa).
Water potential of pure water = 0 kPa
Water potential of water with a pinch of salt added
= -70 kPa
Just remember that the more
concentrated water is with solutes
(substances), the more negative the
water potential will be.
What is water potential?
The net movement of water by osmosis is determined by
differences in water potential between two solutions connected by
a partially-permeable membrane.
Water potential is the tendency of water molecules in a system to
move. It is denoted by the symbol Ψ and is measured in kiloPascals
(kPa).
Pure water has the highest water potential, and has a value of 0
kPa. Solutions have a lower water potential than pure water, and
have a negative water potential.
Water molecules always move from a region of high
water potential to a region of low (more negative) water
potential.
Water movement during osmosis
What is solute potential?
The water potential of a solution is affected by the amount of
solute it contains. The greater the amount of solute, the lower the
water potential.
free water
molecule
This is because water
molecules bind to the
solute molecules,
reducing the number of
water molecules that are
free to diffuse.
solute
molecule
The contribution that solutes make to the water potential of a
solution is the solute potential (ΨS), and is a negative value.
What is pressure potential?
The water potential of a solution is also affected by the pressure
applied to it. The greater the pressure, the higher the water
potential.
This is called the pressure potential (ΨP) and is always a
positive value.
In plant cells, the pressure potential is a result of the cell
wall exerting pressure on the cytoplasm.
Water potential is calculated using the following equation:
water potential
Ψ
= solute potential
+ pressure potential
=
+
ΨS
ΨP
Calculating water potential
Osmosis in plant cells
Osmosis in animal cells
Water potential of potatoes
Ψ = -40
kPa
Ψ = -30
kPa
Ψ = -36
kPa
Understanding Water Potential
• Kerboodle – 3.7 Maths Skills (Practice
Questions)
Annotate your diagram to explain
what is happening
Osmosis
Osmosis in an Animal Cell
• Copy and complete table – use a red blood cell as the example
Water potential of
external solution
compared to cell
solution
Net movement of
water
State of cell
Annotated diagram
Higher (less negative)
Equal
Lower (more negative)
Osmosis in an Animal Cell
Water potential of
external solution
compared to cell
solution
Higher (less negative)
Equal
Lower (more negative)
Net movement of water
Enters cell
Neither enters nor
leaves
Leaves cell
State of cell
Swells and bursts
No change
Shrinks
Annotated diagram
Contents of cell released
Normal RBC
Cell looks darker, as
haemoglobin more
concentrated. Cell
shrunken and shrivelled.
Ψ = -5 kPa
Ψ = -2 kPa
Ψ = -1 kPa
Ψ = -3 kPa
Ψ = -2 kPa
Ψ = -4 kPa
Draw the following diagrams, and show with
arrows, which way you think osmosis will occur.
Cells are affected by the water potential of
their surroundings
When you compare two water potentials, you can give
special names to them in terms of how different they
are:
Ψ = -2 kPa
Ψ = -2 kPa
Ψ = -10 kPa
Ψ = -10 kPa
Ψ = -2 kPa
Ψ = -2 kPa
Cell is in a
HYPOTONIC
SOLUTION
Cell is in a
ISOTONIC
SOLUTION
Cell is in a
HYPERTONIC
SOLUTION
PLENARY
Exam Q’s
1. Diffusion is a passive transport process. What does this
mean?
2. How does the thickness of an exchange surface affect
the rate of diffusion across it?
3. What happens if a cell is placed in a hypotonic
solution?
Exam Q’s
4. Pieces of potato of equal mass were put into different
concentrations of sucrose solution for three days. The
difference in mass when they were taken out is shown
in the table.
Concentration
of sucrose (%)
1
2
3
4
Mass
difference (g)
+0.4
+0.2
0
-0.2
a. Why did the pieces in 1% and 2% sucrose gain mass?
b. Why did the mass of the piece in 3% sucrose stay the
same?
c. What happens to a cell placed in a hypertonic solution?
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