Transcript Membranes Dr. Imrana Ehsan

CELL MEMBRANE
DR.IMRANA EHSAN
Structure and function
of cell components
(i)
(ii)
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(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