BIOB10H3 Lecture Notes - Lecture 5: Fat, Integral Membrane Protein, Facilitated Diffusion
26 May 2018
School
Department
Course
Professor
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)
components
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
group.
▪ Phosphatidylcholine
▪ 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
find more resources at oneclass.com
find more resources at oneclass.com
▪ has an insulating function
▪ myelin cells => myelin sheath
▪ enriched in cerebroside
▪ mice lacking cerebroside develop muscle tremors and
paralysis
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
microdomains
• 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
find more resources at oneclass.com
find more resources at oneclass.com
