Membrane Structure
Chapter 11
Questions in this chapter you should be able to answer:
Chapter 11- #s1 - 19
Membrane Structure
1
Membranes are described as
a “2-dimensional liquid”. Why?
Jmol membrane model
Laser Tweezers
Membrane Fluidity
Membrane Structure
2
How can we measure membrane fluidity?
FRAP: ‘Fluorescence Recovery After Photobleaching’
P 378
GFP: Green fluorescent protein
Cell fusion also
shows fluidity
FRAP
See Figure 11-30
Membrane Structure
3
What factors determine how fluid a membrane is?
Properties of P-lipids:
Chain length
Saturation
Properties of membranes:
Cholesterol content
Cytoskeleton association
Membrane Structure
4
What are the principal
membrane lipids?
Phospholipids
Glycolipids
Other membrane lipids
(not phospholipids)
Cholesterol
Cerebrosides
Sphingolipids
Ceramide
Membrane Structure
5
How are P-lipids distributed in lipid bilayers?
P-lipids are synthesized on the ER
membrane….
Scramblase
vs
Flippase
How do they get to other side of
membrane?
How is asymmetry achieved?
Membrane Structure
6
What are the principal functions of membrane proteins?
How are membrane proteins connected to the membrane?
Transmembrane (integral)
Single-pass Multi-pass
Membrane associated (peripheral)
Covalent vs Noncovalent
Membrane Structure
7
Why do transmembrane proteins occur as alpha helices
and beta-pleated sheets??
Jmol Transmembrane proteins
Membrane Structure
8
How can membrane proteins be purified and studied?
-- detergents ‘mimic’ P-lipid structure around proteins
Question 11-5
Why is red part hydrophilic
and blue part hydrophobic?
Membrane Structure
9
What do we know about
the structure of bacteriorhodopsin?
Function?
Structure?
Mechanism?
Bacteriorhodopsin
Membrane Structure
10
How is the cell membrane structurally reinforced?
-- Cell cytoskeleton
-- also influences fluidity
Membrane Structure
11
Why are carbohydrates particularly abundant on the cell surface?
Functions:
Surface protectant
Cell recognition
Cell adhesion
-- extracellular matrix
Membrane Structure
12
How can protein movement in cell membrane be restricted?
Consider challenge of intestinal epithelium…
Fig 12-16 p 395
Membrane Structure
13
You have isolated two mutants of a normally pear-shaped microorganism that have lost
their distinctive shape and are now round. One of the mutants has a defect in a protein
you call A and the other has a defect in a protein you call B. You grind up mutant and
normal cells separately and separate the plasma membranes from the cytoplasm by
centrifugation. You then wash the membrane fraction with a low concentration of urea (
which disrupts their ability to interact with other proteins) and centrifuge the mixture.
The membranes and their constituent proteins form a pellet while the proteins liberated
by the urea wash remain in the supernatant. When you check each of the fractions for
the presence of A or B, you obtain the results given below.
First cell extract
After urea wash and
centrifugation
Membrane
Cytosol
Membrane
Supernatant
A and B
no A or B
B
A
Mutant A
B
A
B
no A or B
Mutant B
B
A
B
no A or B
Normal cells
Answer the following statements about your results?
(a) Which is an integral and which is a peripheral membrane-associated
protein. The results for which cell-type shows this?
(b) How does the mutation to protein-A alter its properties?
(c) How does the mutation to protein-B alter its properties?
(d) Which result(s) most indicate an interaction between A and B?
Explain.
Membrane Structure
14