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Lecture 5

BIOB10H3 Lecture Notes - Lecture 5: Fat, Integral Membrane Protein, Facilitated Diffusion

Biological Sciences
Course Code
Rene Harrison

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May 24, 2018
Lecture 5
Functions of the Plasma Membrane:
Compartmentalization encloses and protects contents of the cell
Scaffold for Biochemical Activities for enzyme and signalling proteins
Provide a Selectively Permeable Barrier
Transporting Solutes machinery for moving sugars, a.a.’s and ions
Responding to External Signals via receptors (integral proteins) which bind soluble
ligands or ECM
Intercellular Interaction information & materials to neighbouring cells
History of Plasma Membrane Studies
o Overton (1890’s)
o added dissolved solutes to cells
o only lipid soluble molecules could enter cells => membrane has fatty oil (lipid)
o Gorter and Grendal (1925)
o lysed red blood cells => spread lipid over water
o measured twice as much lipid as the predicted surface area => “lipid bilayer”
o Danielli and Davson (1930’s)
o proposed proteins were in lipid bilayer => globular proteins lining surfaces and
lining channels to form pores
o Electron Microscopy (1950’s)
o Confirmed proteins in the bilayer
(above hypothesis^)
o Singer and Nicolson (1972)
o membrane is fluid
o individual proteins can penetrate
bilayer - “Fluid Mosaic Model”
Composition of the Plasma Membrane:
Membrane Lipids
o Phospholipids
PC, PE, PS are types of phospholipids
Glycerol backbone & 2 fatty acids. Polar
head group is phosphate group & polar
PI (Phosphatidylinositols) => it goes from PI to PI3P by PI3 kinase.
o Sphingolipids
single fatty acid chain
attached to a sphingosine group & sugar is added (glycolipid) e.g. cerebroside
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has an insulating function
myelin cells => myelin sheath
enriched in cerebroside
mice lacking cerebroside develop muscle tremors and
o Cholesterol
less amphipathic
hydrophilic, hydroxyl group faces the membrane
surface; mostly hydrophobic rings
can form ‘rafts’ along with sphingolipids & PC
o Lipids are Not Uniform: outside you find PC&
sphingolipids; inside you find PE & PS & PI
o Function of the Lipid Bilayer:
Barrier to Water Soluble Components
Functions in Cell Signalling
Forms “Microdomains” or “rafts”
float around the PM as independent
lipoproteins accumulate in rafts
concentrated site of signalling molecules
Allows Membrane Fusion Events
Membrane Carbohydrates
o Glycocalyx are glycoproteins &glycolipids
o Composed of Glycoproteins & Glycolipids
o short branched oligosaccharides (~ 15 sugars)
o extracellular face => adhesion and interactions with ECM;
cellular identity
o Carbohydrates attached to membrane lipids/proteins on the surface of red blood cells
determine our blood groups
O antigen: has neither enzyme
A antigen: has GalNAc enzyme
B antigen: has Gal enzyme
If you’re AB: has GalNAc & Gal enzymes
Membrane Proteins
o Integral Proteins
“transmembrane” proteins
identified by freeze fracture; Freeze-fracture Electron microscopy:
rapid freezing of cells => membrane coated with heavy metals => replica made
of surface => membrane fractured with knife blade => examined by EM. See
figure ^
single or multipass
amphipathic proteins
hydrophilic channels
extracted by detergents to study proteins. We need detergents to dissolve the lipid and
protein can be examined for study.
o Peripheral Proteins
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