3. Learning Objectives
1.
Describe the fluid mosaic model of membrane structure and explain the underlying
reasons for this structure.
2.
Outline the roles of phospholipids, cholesterol, glycolipids, proteins and
glycoproteins in membranes.
3.
Outline the roles of the plasma membrane, and the roles of membranes within
cells.
4.
Describe and explain how molecules can get in and out of cells (cross cell
membranes) by the processes of diffusion, facilitated diffusion, osmosis, active
transport, endocytosis and exocytosis.
5.
Describe the effects on animal and plant cells of immersion in solutions of different
water potential.
6.
Describe the features of the gaseous exchange surface of mammalian lung.
7.
Describe the features of root hairs that enable the uptake of ions by active
transport.
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4. Key words you should know
Phospholipids
Polar
Hydrophilic
Hydrophobic
Micelles
Phospholipid bilayer
Fluid mosaic model
Glycoproteins
Glycolipids
Cholesterol
Proteins
Transport proteins
Enzymes
Receptor molecules
Diffusion
Concentration gradient
Facilitated diffusion
Osmosis
Solution
Solute
Solvent
Partially permeable
Water potential
Solute Potential
Pressure Potential
Turgid
Plasmolysis
Plasmolysed
Incipient plasmolysis
Active transport
Carrier protein
Bulk transport
Endocytosis
Phagocytosis
Phagocytes
Phagocytic vacuoles
Pinocytosis
Micropinocytosis
Exocytosis
Gaseous exchange
Alveoli
Root hair
Surface area
Epidermis
Passive transport
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5. Cell membrane
All living things are surrounded by a membrane.
A cell membrane is also known as plasma membrane.
Controls
exchange of materials such as nutrients and
waste between cells and their environment.
Has
other important functions for example to enable
cells to receive hormones.
To
understand the function of anything in biology, you
must study the structure first!
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6. Cell Membranes from Opposing
Neurons (TEM x436,740).
Nerve cell
Cell membrane {
Gap between cells
}
cell membrane
7nm wide
Nerve cell
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7. Cell membranes are made of
PHOSPHOLIPIDs
HYDROPHILIC heads
(water liking)
-Attracted to the water
called POLAR
HYDROPHOBIC tails
(water fearing)
-Not attracted to the
water
called NON-POLAR
A Phospholipid
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8. Phospholipids are important structural components
of cell membranes. Phospholipids are modified so
that a phosphate group (PO4-) replaces one of the
three fatty acids normally found on a lipid. The
addition of this group makes a polar "head" and
two nonpolar "tails".
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9. HYDROPHILIC HEAD
At the other end of the phospholipid
is a phosphate group and several
double bonded oxygens. The
atoms at this end of the molecule
are not shared equally. This end
of the molecule has a charge and
is attracted to water. It is POLAR
A phospholipid
HYDROPHOBIC TAILS
The two long chains coming off of the
bottom of this molecule are made
up of carbon and hydrogen.
Because both of these elements
share their electrons evenly these
chains have no charge. They are
NON POLAR. Molecules with no
charge are not attracted to water;
as a result water molecules tend
to push them out of the way as
they are attracted to each other.
This causes molecules with no
charge not to dissolve in water.
3D model of a
Phospholipid
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10. A Phospholipid Bilayer
Phospholipids can form:
BILAYERS
-2 layers of phospholipids
with
hydrophobic tails
protected inside by the
hydrophilic heads.
The PHOSPHOLIPID
BILAYER is the basic
structure of membranes.
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12. Phospholipids
Structure of the cell membrane
Cell membranes are made mainly of
phospholipids. They have:
HYDROPHILIC heads (water liking)
-Attracted to the water POLAR
HYDROPHOBIC tails (water fearing)
-Not attracted to the water NONPOLAR
Phospholipids can form BILAYERS
-2 layers of phospholipids with
hydrophobic tails protected
inside by the hydrophilic
heads.
The PHOSPHOLIPID BILAYER is
the basic structure of membranes.
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14. Fluid mosaic model
Cell membranes also contain proteins within the phospholipid bilayer.
This ‘model’ for the structure of the membrane is called the:
FLUID MOSAIC MODEL
FLUID- because individual phospholipids and proteins can move
around freely within the layer, like it’s a liquid.
MOSAIC- because of the pattern produced by the scattered protein
molecules when the membrane is viewed from above.
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16. TEM of freeze-fractured cell membrane.
The fracture occurs
between the two
phospholipid
layers.
You can clearly see
the exposed
proteins sticking
out of the two
layers.
Individual
phospholipids are
too small to see.
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18. Features of the fluid mosaic model
Double layer – BILAYER of phospholipids which can move
about by ………………………… in their own
……………………….
Phospholipid tails point inwards forming a ……. ……….
………………………… interior. The phospholipid heads point
outwards facing the aqueous (water containing) medium
surrounding the membrane.
Some phospholipids fatty acid tails are ………………………..
– straight so fit together tightly. Some are
………………………… – bent so fit together
……………………. The more unsaturated tails there are the
more ……………… the membrane becomes. The lower the
temp, the ……………….fluid.
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19. Features of the fluid mosaic model
Most protein molecules …………………. like icebergs in the
layers, some are fixed to ………………………. inside the
cell and don’t float.
Some proteins are embedded in the outer layer, some in
the inner layer and some ………………… the two layers.
Hydrophobic and Hyrdophilic parts of the protein molecules
sit next to the …………………………… and
………………………….. portions of the
……………………………… of the membrane. This ensures
the proteins stay in the membrane.
The membrane is ……nm thick on average.
Some phospholipids have carbohydrates attached to them
called- ………………………..
Some of the proteins have carbohydrates attached to them
called– ……………………
The membrane also contains molecules of
………………………………..
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20. Features of the fluid mosaic model
Double layer – BILAYER of phospholipids which can
move about by DIFFUSION in their own
MONOLAYER
Phospholipid tails point inwards forming a NONPOLAR HYDROPHOBIC interior. The phospholipid
heads point outwards facing the aqueous (water
containing) medium surrounding the membrane.
Some phospholipids fatty acid tails are SATURATED
– straight so fit together tightly. Some are
UNSATURATED – bent so fit together loosely. The
more unsaturated tails there are the more ‘fluid’ the
membrane becomes. The lower the temp, the less
fluid.
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21. Features of the fluid mosaic model
Most protein molecules float like icebergs in the layers,
some are fixed to structures inside the cell and don’t float.
Some proteins are embedded in the outer layer, some in
the inner layer and some span the two layers. Hydrophobic
and Hyrdophilic parts of the protein molecules sit next to
the Hydrophobic and Hydrophilic portions of the
phospholids of the membrane. This ensures the proteins
stay in the membrane.
The membrane is 7nm thick on average.
Some phospholipids have carbohydrates attached to them
– GLYCOLIPIDS
Some of the proteins have carbohydrates attached to them
– GLYCOPROTEINS
The membrane also contains molecules of
CHOLESTEROL
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22. Roles of components of cell membranes
Using the following headings produce a table on A4 to summarise roles of t he
different types of molecules found in the cell membrane. Try not to copy, pick
out the relevant information and write it in note form You may want to use
bullet points, different coloured pens etc basically whatever helps you to
remember them.
There will be a short test on this next lesson! Use pages 53-54.
Component
1.
Phospholipids
2.
Cholesterol
3.
Proteins
4.
Function
Glycolipids and Glycoproteins
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23. Summary
Cell membranes have a basic structure composed of a PHOSPHOLIPID BILAYER.
Phospholipds have HYDROPHOBIC (non-polar) tails and HYDROPHILIC (polar) heads.
The best model of the cell membrane is called the FLUID MOSAIC MODEL
The average thickness of the membrane is 7nm.
The fatty acid tails of phospholipids can be SATURATED (straight) or UNSATURATED (bent)
Proteins can float or be fixed and also have hydrophobic and hydrophilic portions.
Some proteins and phospholipids have carbohydrates attached to them to form
GLYCOPROTEINS AND GLYCOLIPIDS.
Phospholipids form the bilayer, act as barrier to most water soluble substances
Cholesterol regulates the fluidity of the membrane, gives mechanical stability and help to
prevent ions from passing through the membrane.
Proteins act as transport proteins to act as channels for substances to move into or out of the
cell. Some act as membrane enzymes and some have important roles in membranes of
organelles.
Glycolipids and Glycoproteins help to stabilise membrane structure, some act as receptor
molecules eg for hormones and neurotransmitters or as antigens for other cells to recognise
them.
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