IB Biology
2 Cells
2.4 Cell Membranes
All syllabus statements ©IBO 2007
All images CC or public domain or link to original material.
Jason de Nys
http://www.flickr.com/photos/edsweeney/6346198056/
2.4.1 Draw and label a diagram to show the structure of membranes.
http://www.youtube.com/watch?v=w9VBHGNoFrY
2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain
the structure of cell membranes.
What happens when
you put a drop
of oil in
water?
http://www.flickr.com/photos/zorin-denu/5385963280/
The Oil droplet stays together and makes
a perfect circular shape.
The oil
molecules are
Hydrophobic
Oil Molecules are nonpolar and water
molecules are polar.
See 3.1.5
http://www.flickr.com/photos/zorin-denu/5385963280/
Phospholipid
molecules have a
polar (charged)
phosphate head and
long non-polar lipid
tails
*
*h not b!
The head is attracted
to water and the
tails are not.
http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_4.svg
When put into water, an emergent property is that
phospholipids will self-organise to keep their heads
‘wet’ and their tails ‘dry’
micelle
liposome
http://commons.wikimedia.org/wiki/File:Micelle_scheme-en.svg
http://commons.wikimedia.org/wiki/File:Liposome_scheme-en.svg
In this 3D representation
you can see that a
phospholipid bilayer is
one way that the tails
can be removed from the
water.
Phospholipid molecules
can flow past each other
laterally but can’t move
vertically
http://commons.wikimedia.org/wiki/File:Phospholipids_aqueous_solution_structures.svg
But wait! there’s more!
The plasma membrane is not just made of
phospholipids
http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg?uselang=en-gb
Proteins:
Integral proteins are permanently embedded, many go all the way
through and are polytopic (poly = many, topic = surface), integral
proteins penetrating just one surface are monotopic.
Peripheral proteins usually have a temporary association with the
membrane, they can be monotopic or attach to the surface
Some human examples on the next page
In the
membrane
Beta-secretase 1
Glycophorin (integral) carries sugar
molecules into red blood cells
Protein structure covered
in AHL 7.5
Muscle Fatty acid binding
protein (peripheral) is
involved in the transport of
fatty acids
Glycophorin A
Muscle fatty acid
binding protein
Cytoplasm
(inside)
These are just three
examples, they are
from different cell
types.
There are thousands
that have been found
so far and thousands
more are added each
year.
Extracellular
(outside)
Beta-secretase 1
(peripheral) has a role
in creating myelin
sheaths on nerve cells
http://opm.phar.umich.edu/protein.php?pdbid=2wf1
http://opm.phar.umich.edu/protein.php?pdbid=1afo
http://opm.phar.umich.edu/protein.php?pdbid=1hmt
Glycoproteins:
Are proteins with an oligosaccaride (oligo = few, saccharide = sugar)
chain attached.
They are important for cell recognition by the immune system and
as hormone receptors
Cholesterol: (It’s not all bad!)
It makes the phospholipids pack more tightly and regulates the
fluidity and flexibility of the membrane.
more later in 2.4.8
Bad analogy: imagine a room full of people wearing fluffy jumpers (sweaters).
It is crowded but they can slip past each other easily enough.
Now sprinkle the crowd with people wearing Velcro™ suits…
2.4.3 List the functions of membrane proteins
List: Give a sequence of names or other brief answers with no explanation
Proteins associated with membranes have many functions.
Can you think of any ‘jobs’ that proteins could help cells do?
Let’s look at
1
Cell Adhesion Molecules:
Enable cells to make tight
connections to one
another
They may play a
part in the immune
response.
http://www.flickr.com/photos/silveraquarius/1395277674/
2
Channel Proteins: Allow or help ions and large molecules to
pass through the membrane by diffusion
http://www.flickr.com/photos/gilderic/2728059828/
3
Protein Pumps move ions
across the membrane to
create and maintain
concentration gradients.
They require energy to carry
out this active transport
http://www.flickr.com/photos/erix/298857080/
4
Hormone Binding sites (hormone
receptors) bind to specific hormones and
start signalling processes to change the
behaviour of the cell
e.g. insulin
http://www.flickr.com/photos/18735339@N00/2926816259/
5
Cell to cell communication:
e.g. receptors for
neurotransmitters at
synapses
Outside cell
Inside membrane
Cytoplasm
Nicotinic acetylcholine
receptor, beta2 subunit
http://ifaketext.com/
http://opm.phar.umich.edu/protein.php?pdbid=2ksr
6
Enzymes on the surface of
the cell
Outer surface of membrane
Plasma platelet activating factor
acetylhydrolase
http://opm.phar.umich.edu/protein.php?pdbid=3d59
2.4.4 Define diffusion and osmosis
What is
http://www.flickr.com/photos/dpup/3338948041/
Diffusion, is the motion of all (liquid or gas) particles at
temperatures above absolute zero.
• The rate of this movement relates to temperature, viscosity of
the fluid and the size (mass) of the particles.
• Diffusion explains the net flow of molecules from a region of
higher concentration to one of lower concentration.
• The result of diffusion is a gradual mixing of material. In the
absence of other influences, the diffusion process will
eventually result in complete mixing.
Diffusive equilibrium is
reached when
the concentrations of
the diffusing substance in
the two compartments
becomes equal. (Wikipedia)
Simulate!
If you have 4 sided dice
then all the better.
This simulation is known
as a random walk.
How could you simulate
an increase of heat
energy?
1. Grab your classmates or a group
of friends.
2. Take one die each.
3. Stand together in a clump; in a
room or outside.
4. Everybody rolls their die in their
hand
5. For a 1, step forward; for a 2,
step to the right; for a 3, step
backwards and for a 4, step left
6. If you roll 5 or 6, or if you can’t
move because a person or
object is in the way, roll again
Do this for 3 minutes, you should
have dispersed, some will have got
further than others.
http://www.flickr.com/photos/stevon/3145375973/
What is
osmosis?
http://www.flickr.com/photos/luchilu/399970490/
Osmosis may occur when there is a partially
permeable membrane, such as a cell
membrane.
When a cell is submerged in water, the water
molecules pass through the cell membrane
from an area of low solute concentration
(outside the cell) to one of high solute
concentration (inside the cell) (Wikipedia)
Aquaporin is an integral protein that, as
it’s name suggests, acts as a pore in the
membrane that speeds the movement
of water molecules
http://opm.phar.umich.edu/protein.php?pdbid=1sor
The
importance
of osmotic
control
http://commons.wikimedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svg
http://commons.wikimedia.org/wiki/File:Turgor_pressure_on_plant_cells_diagram.svg
2.4.5 Explain passive transport across membranes in terms of simple diffusion and facilitated
diffusion.
Simple
Diffusion
http://commons.wikimedia.org/wiki/File:Scheme_simple_diffusion_in_cell_membrane-en.svg
Facilitated Diffusion:
Large and polar molecules can’t get across the membrane via
simple diffusion
Transmembrane (polytopic) proteins recognise a particular
molecule and help it to move across the membrane. The
direction it moves is dependent on the concentration gradient.
Watch the animation
http://commons.wikimedia.org/wiki/File:Scheme_facilitated_diffusion_in_cell_membrane-en.svg
2.4.6 Explain the role of protein pumps and ATP in active transport across membranes
Primary active transport requires ATP.
Integral protein pumps use the energy from the
hydrolysis of ATP to move ions or large
molecules across the cell membrane.
Molecules are moved against their
concentration gradient
http://commons.wikimedia.org/wiki/File:Scheme_sodium-potassium_pump-en.svg
In secondary active transport, the required energy is
derived from energy stored in the form of concentration
differences in a second solute.
http://commons.wikimedia.org/wiki/File:Scheme_secundary_active_transport-en.svg
Typically, the
concentration
gradient of the
second solute was
created by primary
active transport,
and the diffusion of
the second solute
across the
membrane drives
secondary active
transport.
2.4.7 Explain how vesicles are used to transport materials within a cell between the rough
endoplasmic reticulum, Golgi apparatus and plasma membrane.
Vesicles are small
spheroidal
packages that bud
off of the RER and
the Golgi apparatus
They carry proteins
produced by
ribosomes on the
RER to the Golgi
apparatus, where
they are prepared
for export from the
cell via another
vesicle
2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and reform
during endocytosis and exocytosis.
Endocytosis: The taking in
of external substances by an
inward pouching of the
plasma membrane, forming
a vesicle
Exocytosis: The release of
substances from a cell
(secretion) when a vesicle
joins with the cell plasma
membrane.
Diagrams on following slides
Constitutive
secretion
occurs
continuously in
cells,
depending on
their function
Regulated
secretion is in
response to a
trigger e.g. the
release of
neurotransmitters
http://commons.wikimedia.org/wiki/File:Exocytosis_types.svg
“Cell eating”
“Cell drinking”
http://commons.wikimedia.org/wiki/File:Endocytosis_types.svg
As mentioned, phospholipids can move freely past each other
laterally but very rarely do they move vertically or flip
The plasma membrane is embedded with proteins
Cholesterol molecules stiffen and stabilise the plasma
membrane
It is because of the lateral movement and the presence of
other molecules studding it’s surface that our understanding
of the plasma membrane is referred to as the
fluid mosaic model
Further information:
Most of the excellent biology images in this slideshow are by graphic designer
Mariana Ruiz Villarreal (LadyofHats) who has graciously released them to the
public domain.
Three of the best sites for
IB-specific Biology
information. The top link
takes you to the PPT by
Stephen Taylor