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Lecture 3

Lecture 3 Summary Made the lecture notes more readable.


Department
Biology
Course Code
BIOL 331
Professor
Bruce Reed
Lecture
3

Page:
of 14
Chapter 10 The Lipid Bilayer and Membrane Proteins
From the previous lecture...
FRET
- Protein-protein interactions are key to understanding how cells work
- Co-localization of suspected protein partners using immunofluorescence microscopy in fixed
cells is possible and popular
- Bu the resolution of the fluorescence microscope is much less than the size of a typical protein,
co-localization is often a questionable result
- FRET allows direct observation of molecular interactions (2-5 nm) in living cells
The Plasma Membrane is Crucial to Cellular Life
- The cell membrane separates the cell from its environment and creates intracellular
compartments (in eukaryotes)
- Ion gradients across the membrane are used to make ATP, to allow electrical signals in nerve
and muscle cells and to drive transmembrane movement of solutes
- All biological membranes have a common structure: a thin layer of lipid molecules with proteins
held together by non-covalent interactions
- Membranes are fluid, dynamic structures
- Lipids found as a bilayer are about 5 nm thick
- The lipid bilayer is impermeable to most water soluble molecules
- Transmembrane proteins
A) Transport specific molecules across the membrane
B) Catalyze membrane associated reactions (ATP synthesis)
C) Are structural links or cell surface receptors
- An estimated 30% of all proteins in the genome are membrane proteins
The Lipid Bilayer
- The lipid bilayer forms the basic structure of all
biological membranes
- Lipid bilayer is easily seen by TEM
- Structure of the lipid bilayer is inherent to the
properties of lipid molecules
- Lipid molecules will assemble spontaneously to form
lipid bilayers under simple artificial conditions
- Animal cell membranes by mass are half lipid/half
protein
- All lipid molecules in cell membranes are amphipathic (a hydrophobic tail and a hydrophilic
head)
- Most abundant membrane lipids are the phospholipids (a polar head
group and two hydrocarbon tails usually fatty acids 12-24 C atoms)
- One of the two tails is usually saturated, the other usually one or
more cis C=C bonds
- The main phospholipids are the phosphoglycerides (3C glycerol
backbone)
The Phosphoglycerides
E.g. phosphatidylcholine
- The four major phospholipids are half of all the membrane lipids by mass
Note: Phosphatidylserine carries a net negative charge
Cholesterol
- Also an important part of the lipid bilayer
Lipid Shape and Amphiphilic Nature
- Spontaneous assembly of bilayers or micelles
- Free edge with water is energetically unfavourable
Lipid Bilayer A 2D Fluid
- Synthetic lipid bilayers (spheres 25 nm 1 μm) liposomes, bilayers
- Each side of a bilayer is sometimes called a leaflet
- Motion of phospholipids can be measured in artificial membranes (by fluorescent labelling or by
electron spin resonance)
A) Lateral diffusion is extremely rapid (length of a bacterial cell in 1 s!)
B) Flip flop (one leaflet to the other) is extremely rare
C) Rapid rotation along long axis, flexible tails, a disordered system