AS Biology
FOUNDATION
Chapter 4
CELL MEMBRANES
and
TRANSPORT
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1
The Cell
AS Biology. Foundation. Cell
membranes and Transport
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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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membranes and Transport
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Key words you should know
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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
AS Biology. Foundation. Cell
membranes and Transport
Pinocytosis
Micropinocytosis
Exocytosis
Gaseous exchange
Alveoli
Root hair
Surface area
Epidermis
Passive transport
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Cell membrane
All living things are surrounded by a membrane.
A cell membrane is also known as plasma membrane.
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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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membranes and Transport
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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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membranes and Transport
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Cell membranes are made of
PHOSPHOLIPIDs
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HYDROPHILIC heads
(water liking)
-Attracted to the
water
called POLAR
HYDROPHOBIC tails
(water fearing)
-Not attracted to the
water
called NON-POLAR
AS Biology. Foundation. Cell
membranes and Transport
A Phospholipid
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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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membranes and Transport
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A phospholipid
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
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.
AS Biology. Foundation. Cell
membranes and Transport
3D model of a
Phospholipid
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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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membranes and Transport
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membranes and Transport
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Structure of the cell membrane
Phospholipids
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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membranes and Transport
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Diagram representing the cell membrane
Remember the membrane is 7nm wide
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membranes and Transport
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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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membranes and Transport
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Diagram of a cell membrane
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membranes and Transport
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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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membranes and Transport
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Cell Membranes from Opposing
Neurons (TEM x436,740).
} Phospholipid Bilayer
7nm wide
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membranes and Transport
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Features of the fluid mosaic model
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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.
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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Features of the fluid mosaic model
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Double layer – BILAYER of phospholipids which can move about by DIFFUSION
in their own MONOLAYER
Phospholipid tails point inwards forming a NON-POLAR 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.
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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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
Function
1.
Phospholipids
2.
Cholesterol
3.
Proteins
4.
Glycolipids and Glycoproteins
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Summary
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Cell membranes have a basic structure composed of a PHOSPHOLIPID BILAYER.
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Phospholipds have HYDROPHOBIC (non-polar) tails and HYDROPHILIC (polar) heads.
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The best model of the cell membrane is called the FLUID MOSAIC MODEL
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The average thickness of the membrane is 7nm.
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The fatty acid tails of phospholipids can be SATURATED (straight) or UNSATURATED
(bent)
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Proteins can float or be fixed and also have hydrophobic and hydrophilic portions.
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Some proteins and phospholipids have carbohydrates attached to them to form
GLYCOPROTEINS AND GLYCOLIPIDS.
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Phospholipids form the bilayer, act as barrier to most water soluble substances
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Cholesterol regulates the fluidity of the membrane, gives mechanical stability and help to
prevent ions from passing through the membrane.
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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.
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Glycolipids and Glycoproteins help to stabilise membrane structure, some act as receptor
molecules eg for hormones and neurotransmitters
orCell
as antigens for other cells to
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recognise them.
Movement of selected molecules
across the cell membrane
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membranes and Transport
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 Moving
amoeba
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membranes and Transport
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