Download Membranes Dr. Imrana Ehsan

CELL MEMBRANE
DR.IMRANA EHSAN
Structure and function
of cell components
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Carbohydrates
Lipids
Proteins
Nucleic Acids
Membranes
Cytoskeleton
The plasma membrane is the boundary
that separates the living cell from its
nonliving surroundings
 The plasma membrane exhibits selective
permeability, allowing some substances
to cross it more easily than others
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Many organelles contain or are enclosed
by membranes, including
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Plasma membrane
Mitochondria
Chloroplasts
Nuclear envelope
Endoplasmic reticulum
Golgi apparatus
Lysosomes
Microbodies (aka perioxisomes)
Cell membranes
Phospholipids are the most abundant
lipid in the plasma membrane
 Phospholipids are amphipathic
molecules, containing hydrophobic and
hydrophilic regions
 The fluid mosaic model states that a
membrane is a fluid structure with a
“mosaic”membranes
of various proteins
in
Cellular
are embedded
fluid
it.
mosaics
of lipids and proteins
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Structure of cell membrane
Phospholipids in the plasma membrane
can move within the bilayer
 Most of the lipids, and some proteins,
drift laterally
 Rarely does a molecule flip-flop
transversely across the membrane
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The Fluidity of Membranes
As temperatures cool, membranes switch
from a fluid state to a solid state
 The temperature at which a membrane
solidifies depends on the types of lipids
 Membranes rich in unsaturated fatty acids
are more fluid than those rich in saturated
fatty acids
 Membranes must be fluid to work
properly; they are usually about as fluid
as salad oil
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As temperatures cool, membranes switch
from a fluid state to a solid state
 The temperature at which a membrane
solidifies depends on the types of lipids
 Membranes rich in unsaturated fatty acids
are more fluid than those rich in saturated
fatty acids
 Membranes must be fluid to work
properly; they are usually about as fluid
as salad oil
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Based on the functions of the above
organelles / membranes write down some
of the roles of the membranes in the cells
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(Leave some space as we will be adding
to this list)
Role of Cell Membranes
Structure of Cell membranes
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Fluid Mosaic Model
Electron micrograph of cell
membrane
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Lipids
◦ Remember lipids have hydrophilic head and
hydrophobic tails
◦ To avoid the hydrophobic tails coming in
contact with water a continuous bilayer sphere
is formed.
◦ Therefore hydrophobic interactions hold
membranes together
Structure of Cell membranes
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Proteins
◦ Integral proteins (intrinsic)
 Proteins that are embedded in the membrane
 They are held in place by hydrophobic interactions
(integral proteins have hydrophobic groups on
their outer surface)
◦ Peripheral proteins (extrinsic)
 Proteins attached to the surface of the membrane
(often forming non-covalent bonds with integral
proteins)
Structure of Cell Membranes
Proteins cont…
◦ Phospholipid bilayer
 Forms boundary to isolate cell contents from
environment
 Restricts passage of hydrophilic substances across
the membrane
◦ Cholesterol
 Increases bilayer strength, flexibility
 Reduces membrane fluidity
 Reduces permeability to water-soluble substances
Function of membranes
macromolecules
Some proteins in the plasma membrane
can drift within the bilayer
 Proteins are much larger than lipids and
move more slowly
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Attachment
◦ Cytoskeleton
◦ Extracellular matrix
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Intercellular Junctions
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Plasmodesmata
Tight Junctions
Gap Junctions
Desmosomes
Function of Membrane Proteins
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Transport
◦ Carrier Proteins
◦ Channel Proteins
Receptors
 Enzymes
 Cell to Cell recognition
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Function of Membrane Proteins
Function of membrane protiens
Integral
(Membrane-spanning or intrinsic)
•Can span membrane several times
•Either move around or are kept in place
by cytoskeleton proteins
Allows for cell polarity
 Associated (peripheral or extrinsic)
 •Loosely bound to membrane
 •Enzymes and structural proteins
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Use all resources to find out more about
the functions of membrane proteins.
 Based on the functions of proteins, add to
your list on the role of membranes in cells
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Learning Activities