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Tuesday, April 21, 2015 2
All membrane shares
common features: flexibility,
self-sealing, selective
permeability, all appear
trilaminar in appearance and
are mostly 3-8 nm thick.
Overview of cell membrane
Tuesday, April 21, 2015
3
Are flexible, self-sealing, and selectively permeable to polar solutes.
Both prokaryotes and eukaryotes have same classes of chemical components.
Similar in structural organization.
Major differences in: lipids, proteins and carbohydrate but not in physiochemical interaction.
Have trilaminar appearance under electron microscope.
Plasma membrane
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6
Plasma membrane is involved in cell communication, import and export of
materials and cell growth and cell motility.
Function of cell membrane inside the cell Vs on the surface?
Role of cell inside the cell Vs outside the cell?
Membranes composition
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Membrane structure
composed of lipids, proteins and carbohydrates.
Major lipids in membranes are Phospholipids, Glycosphingolipids, and Cholesterol.
Each type of membrane have characteristic lipids and proteins.
Each type of membrane have
characteristic lipids and proteins
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Relative proportion of lipids and proteins vary with type of
membrane.
E.g.: Myelin sheath of neurons: primarily of lipids.
PM of bacteria, mitochondria, chloroplast: more protein than lipids.
Cholesterol & Cardiolipin content in Plasma membrane Vs
Inner mitochondrial membrane?
GLYCOPHORIN
So, what is the function of sugar moiety attached to
these surface glycoproteins?
Fluid mosaic model for membrane structure
according to Singer and Nicolson
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All membranes share some fundamental proper ties.
Amphipathic lipid aggregates that form
in water 10
Depending on the precise conditions and the nature of lipids
three types of lipid aggregation are formed when mixed with
water.
The lipid bilayer is 3nm (30 Å) thick
Lysophospholipids, Glycerophospholipids, sphingolipids & sterols
Proteins of cell membrane
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Form channels for movements of ions and small molecules
Serve as transporters for large molecules
Types: 1. Peripheral, 2. Integral protein, 3.
Amphitropic protein
Peripheral, Integral and amphitropic
proteins
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13
Membrane proteins can be
distinguished by the
condition required to
disassociate them from the
membrane.
Many membrane proteins span lipid
bilayer
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14
Transbilayer disposition of
glycophorin in the erythrocyte.
Types of transmembrane protein
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Type I
E.g.: LDL-
receptor
Type II
E.g.: Transferrin
Type III
E.g.: Glucose
transporter
Fluidity of the membrane
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Fluidity of the membrane depends on its lipid
composition
Longer and saturated chain less fluid
Unsaturated fatty acid chain increases fluidity
Cholesterol modifies fluidity of the membrane
Membrane transport of small
molecules
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21
Passive Active
Simple Facilitated
Via various
transporter via ion channels
In Summary
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23
The net diffusion of substance depends on the
following:
1. Its concentration gradient across the membrane.
2. The electric potential across the membrane
3. Permeability coefficient of the substance for the
membrane.
4. The hydrostatic pressure gradient across the
membrane.
5. Temperature.
Transport pathways through cell membrane,
and basic mechanism of transport
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24
Facilitated diffusion Vs Active
transport
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Similarities
Involves carrier proteins, have specificity for ions, sugars and
amino acids.
Resembles substrate-enzyme reaction.
Have specific binding site for the solute.
Carrier is saturable, so it has maximum rate of transport
(Vmax)
There is binding constant for solute, so whole system have
Km.
Structurally similar inhibitor blocks transport.
Facilitated diffusion Vs Active
transport
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Differences
Facilitated diffusion can operate bidirectionally while
active transport is usually unidirectional.
Active transport always occur against electrical and
chemical gradient and so it requires energy.
Ping-Pong model explains facilitated
diffusion 30
Facilitated diffusion is determined by:
1. Concentration gradient across membrane.
2. Amount of carrier available.
3. Rapidity of solute-carrier interaction.
4. The rapidity of the conformational change for both and loaded and the
unloaded carrier.
Active transport
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Energy is required.
Energy comes from: hydrolysis of ATP, electron movement, and light.
Pump used: Na+-K+-ATPase
Transport of glucose into myocyte by GLUT4
is regulated by insulin (Facilitated diffusion)
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