Download Copyright © 2008 by John Wiley & Sons, Inc.

Cell and Molecular Biology
Fifth Edition
Chapter 4
Membrane structure and function
Copyright © 2008 by John Wiley & Sons, Inc.
4.1 An overview of membrane
functions
1.
2.
3.
4.
5.
6.
7.
Compartmentalization
Biochemical activities
Providing a selectively permeable barrier
Transport solutes
Responding to external signals
Intercellular interaction
Energy transduction
4.2 A brief history of studies on
plasma membrane structure
1. E. Oberton 1890s: more lipid soluble the
compound, the more rapidly it would enter the
root hair cells
 2. E. Gorter and F. Grendel 1925: extracted
the lipid from human red blood cells and
measured the amount of surface area the lipid
would cover when spread over the surface of
water------lipid bilayer

Surface tension is much lower than the
pure lipid
3. Davson and Danielli 1930s : a protein
film over an artificial lipid bilayer the
lipid bilayer was also penetrated by
protein-lined pores.
Singer and Nicholson 1972: the
fluid mosaic model
The dynamic properties of
membrane
4.3 The chemical composition
of membrane
1. membrane lipids
 2. membrane carbohydrates
 3. membrane proteins

1.
The structures of membrane lipids
phosphoglycerides (磷酸甘油酯)
sphingolipids (神經鞘酯類)
cholesterol
Sphingolipids (神經鞘酯類)
 Derivatives
of sphingosine, an amino
alcohol that contains a long
hydrocarbon chain
 Sphingolipid (ceramide) consists of
sphingosine and fatty acid.
Cerebroside(腦苷酯類), ganglioside (神經
節苷酯)play a crucial function in
cellular functions
1.muscular tumor and paralysis
2.fungus toxin (fumonisins) inhibits
glycolipid synthesis and results in
poor cell division and cell-cell interaction
3.cholera toxin, influ virus bind to GS
5.Tay-Sachs disease is a fatal
inherited condition that results from
the build-up of a particular lipid (a
ganglioside) in cells of the brain.
6. Membrane lipid also provide the
precursors for highly active chemical
messengers that regulate cellular
function.
The nature and importance of the lipid
bilayer
Membranes are always continuous and are never
seen to have a free edge due to flexibility of lipid
bilayer.
Influence the membrane protein activity
determine the physical states of the membrane
play a role in health and disease
The dynamic properties of the
plasma membrane
Self assembly
Membrane carbohydrates
Short branched oligosaccharides fewer than
15 sugars per chain
 Mediate the interactions of cell with other
cells as well as its nonliving environment

4.4 Membrane proteins
May Contain from 12 to more than 50
different proteins
 Membrane sideness
1. Integral proteins
2. Peripheral proteins (noncovalent bond)
3. Lipid-anchord proteins (covalent bond
with lipid molecule within the bilayer)

Studying the structure and properties of
integrated membrane proteins


Ionic detergents
Nonionic detergents
Identify transmembrane domain
Hydropathy plot
 Hydrophobicity is measured by the free
energy required to transfer each segment of
the polypeptide from a nonpolar solvent to
an aqueous medium.

Determining spatial relationships
within a integral membrane
protein by site-directed crosslinking
 Lactose
permease
Determine spatial relationships
(distance) between amino acid in a
membrane protein
Study dynamic events that occurs as a
protein carries out its function
 Introduce a chemical group which are
sensitive to the distance that separate them.
 Mutate glycine to cystine
 NO· attach to SH group
 Electron paramagnetic resonance (EPR)
spectroscopy

Gerald Karp
Cell and Molecular Biology
Fifth Edition
CHAPTER 4 Part 2
The Structure and Function of the
Plasma Membrane
Copyright © 2008 by John Wiley & Sons, Inc.
4.5 Membrane lipids and
membrane fluidity
1. The importance of membrane
fluidity
2. Maintaining membrane fluidity
3. The asymmetry of membrane lipids
4. Lipid rafts
The importance of membrane
fluidity
1. Supporting structure
2. Intercellular junction
3. Newly synthesized component easy to get
in
4. Cell growth, movement, division,
secretion,
5. Endocytosis
Lipid rafts
Microdomains:
 Nonsoluble in nonionic detergents, such as
Triton X-100
 Consisted cholesterol and sphingolipids
 Atomic force microscope: measure the
height of various parts of the specimen at
the molecular level.

4.6 The dynamic nature of the
plasma membrane
a. cell fusion
b. fluorescence recovery after
photobleaching (FRAP)
Membrane domains and cell polarity
The red blood cells: an example of
plasma membrane structure

4.7 The movement of substances
across cell membrane
Diffusion of substances through
membranes
Partition coefficient: ratio of solubility in a
nonpolar solvent
 Size, smaller uncharged, larger polar
 1. The diffusion of water through
membranes
 2. The diffusion of ions through membranes

Gerald Karp
Cell and Molecular Biology
Fifth Edition
CHAPTER 4 Part 3
The Structure and Function of the
Plasma Membrane
Copyright © 2008 by John Wiley & Sons, Inc.
The diffusion of water through membrane
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Through diffusion
Pater Agre and colleagues at Johns Hopkins isolate
and purify the membrane proteins responsible for
the Rh antigen on the surface of RBC
During this pursuit, they identified a protein
They engineered frog oocytes to incoporated the
newly discovered protein into their plasma
membranes
placed oocytes in a hypotonic medium, the oocytes
swelled as predicated
Aquaporins
Aquaporins (a four subunits protein)




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Contains a central channel (hydrophobic aa)
Highly specific for water molecules
A billion water molecules through each channel
every second
H+ ions are not able to penetrate the open pores
X-ray crystallographic studies the protein structure
and the computer-based simulations

Water channel movie
Very near its narrowest
point, the wall contains a
pair of precisely positioned
positive charged(N203,
N68) residues that attract
oxygen atom of each water
molecule, prevent form the
H-bond with neighboring
water molecules.
Prominent in cells such as kidney tubules
or plant root hairs
 Vasopressin stimulates water retention in
collecting ducts of kidney acts by AQP2

The diffusion of ions through
membranes

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Nerve impulse
Muscle contraction
Secretion of calcium ions
Exocytosis
Regulation of cell volume opening the
stomatal pore
1955, Alan Hodgkin, Bernard
Katz, Andrew Huxley used giant
squid nerve cell to discover the ion
channel.
1991 Erwin Neher, Bert Sakmann
developed “patch and clamp”
technique to study the signal
channel.
1998, R. MacKinnon et al. at
Rockefeller University provided
the first atomic-resolution image
of an ion channel: Three
dimensional structure of the
bacterial KcsA channel and the
selection of K+ ions.
2003 :Chemistry Nobel prize
Four subunits, two
are shown here
P (pore) segment,
the ions pass
Carbonyl groups of
aa residues project
into the channel
and interact with
K+ ions selectively
Each ring contains
four O atoms, and
each ring is just
large enough so
that 8 O atoms can
coordinate a single
K+ ions, replacing
its normal water of
hydration
3 A in diameter
Closed
conformation
Eukaryotic voltage-gated K+ (Kv)
channels have been isolated and
the molecular anatomy of their
proteins scrutinized
a pore domain (S5, S6, P segment)
a voltage-sensor domain (S 4
segment also is called Drosophila
K+ Shaker ion channel)

A single cell (human, nematode or plants)
is likely to possess a variety of different K+
channels that open or close in response to
different voltages.
Facilitated diffusion
Bacteriorhodopsin: a light-driven
proton pump
A seven transmembrane domians and a
retinal group which serves as the lightabsorbing element (chromophore).
 2. Protons move from the cytoplasm to the
cell exterior through a central channel in
the potein

Co-transport coupling active transport to
existing ion gradients