Download A-PC3267 Lect 9 2007 - NUS Physics Department

Biophysics II
By
A/Prof. Xiang Yang Liu
Department of Physics,
NUS
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Outline
1.
2.
The environment in the cell
Hydrophilic vs hydrophobic and
amphiphlic molecules self
assembly :
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Cell and biomenbrane
The four essential parts of
cell:
•Nucleus,
•Cytoplasm,
•Organelles,
•Membrane
• In order to keep cells alive and function properly, the
intracellular condition is different from extra cellular conditions.
• How about biomembrane?
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Structure and Function:
The Phospholipid Bilayer
The plasma membrane is common to all cells
Separates:
Internal living cytoplasmic from
External environment of cell
Phospholipid bilayer:
External surface lined with hydrophilic polar heads
Cytoplasmic surface lined with hydrophilic polar
heads
Nonpolar, hydrophobic, fatty-acid tails sandwiched in
between
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Cell and biomenbrane (cont’d)
Biomembrane: Lipid Bilayer
Semi-ordered Liquid!
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Membrane Models
Fluid-Mosaic Model
Three components:
Basic membrane referred to as phospholipid bilayer
Protein molecules
Float around like icebergs on a sea
Membrane proteins may be peripheral or integral
Peripheral proteins are found on the inner membrane surface
Integral proteins are partially or wholly embedded
(transmembrane) in the membrane
Some have carbohydrate chains attached
Cholesterol
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Membrane Models:
Unit Membrane vs. Fluid Mosaic Model
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The Fluid Mosaic Model
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Phospholipid & Cholesterol Molecules
Charged, polar groups
Hydrophilic head: O, S, N,…
Hydrophobic tails: H, C,…
Non- polar/ weak polar groups
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Functions of Membrane Proteins
Channel Proteins:
Tubular
Allow passage of molecules through membrane
Carrier Proteins:
Combine with substance to be transported
Assist passage of molecules through membrane
Cell Recognition Proteins:
Provides unique chemical ID for cells
Help body recognize foreign substances
Receptor Proteins:
Binds with messenger molecule
Causes cell to respond to message
Enzymatic Proteins:
Carry out metabolic reactions directly
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Membrane Protein Diversity
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Self-Assembly in Cells
Questions:
How can amphiphilic molecules satisfy their
hydrophobic tails in a pure water environment?
How do the amphiphilic molecules assemble in
an aqueous solution?
How do amphiphilic molecules are packed into
different shapes of aggregates
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Hydrophobic vs hydrophilic force
Amphipathic (Amphiphilic) Molecules
Both hydrophilic and hydrophobic
A hydrophobic
part
A hydrophilic part
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Amphiphilic molecules
Two classes of amphiphiles.
(a) Structure of sodium
dodecyl sulfate (SDS), a strong
detergent. A nonpolar,
hydrophobic, tail (left) is
chemically linked to a polar,
hydrophilic head (right). In
solution, the Na+ ion
dissociates. Molecules from
this class form micelles. (b)
Structure of a generic
phosphatidylcholine, a class of
phospholipid molecule. Two
hydrophobic tails (left) are
chemically linked to a
hydrophilic head (right).
Molecules from this class form
bilayers.
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Self-assembly
Self-assembly:
appropriate molecules
gather together
spontaneously to
assemble into some
entities of certain
structures.
What is the driving
force behind it?
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Driving force for amphiphilic molecular
self-assembly
Optimal
interaction/packing for
amphiphilic
molecules:
☺
Hydrophobic region in
contact with hydrophobic
region
hydrophilic region in
contact with hydrophilic
region
hydrophilic region avoiding
hydrophilic region
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Amphiphilic molecule self-assembly at the
interface
☺
Hydrophobic-hydrophobic
Hydrophilic-hydrophilic
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Amphiphilic molecule self-assembly at the
interface
Lower surface (interfacial) tension
☺
High surface (interfacial) tension
air
water
oil
water
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Surface tensiometer to measure
the surface tension
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☺
☺
☺
Lower the surface
tension
CMC
Micelles self-assemble suddenly at a critical concentration
(Critical Micellization Concentration-CMC)
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Bilayers self-assemble from two tailed
amphiphiles
Amphipathic (Amphiphilic) Molecules
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Self -assembly of amphiphilies
Assembly of amphiphilic molecules at the
interface will reduce the interracial tension
At the water surface- reduce the surface
tension.
What happens after CMC?
Assembly into different shapes of micelles
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At C > CMC
☺
Micelle
Self-assembly
Micelle solution
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Aggregates results from molecular self
assembly
How can these
aggregates be built into
different shapes?
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Hydrophobic forces:
Van der Waals
Steric
Configurational…
U(r)
Hydrophilic forces:
Electrostatic
Polar-polar
Hydrogen bond…
Virtual Diameter
stail shead
0
stail > shead
r
stail = shead
stail < shead
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The optimal intermolecular
interactions correspond to
optimal “packing” of these
molecules, which leads to the
self assembly of molecules into
different shapes.
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l
s
v
P = stail / shead
Due to different physical
forces, amphiphilic
molecules will self
assemble into aggregates
with different shapes
stail = v/l
P<
1
3
Spherical
1
3
≤
P
≤
1
2
P ~1
Perfect balance
of hydrophobic
and hydrophilic
forces
Rod-like
Disk-like
Bilayer
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Emulsion form when amphiphlic molecules
reduce the oil-water interfacial tension
(a) An oil–water interface stabilized by the addition of a small amount of
surfactant. Some surfactant molecules are dissolved in the bulk oil or water
regions, but most
migrate to the boundary as shown in the inset.
(b) An oil–water emulsion stabilized by surfactant:
The situation is the same as (a), but for a finite droplet of oil.
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Micellization- A special type of
phase transition
N monomers ⇔ One aggregate (N-mer)
CN/C1N = Keq
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Self-Assembly in cells
To form micelles, the volume NVtail occupied by the tails of N
surfactants must be compatible with the surface area
Nahead occupied by the heads for some N.
Suppose that N amphiphiles pack into a spherical micelle
of radius R. Find two relations between ahead, Vtail, R,
and N. Combine these into a single relation between
ahead, Vtail, and R.
Suppose instead that amphiphiles pack into a planar
bilayer of thickness 2d. Find a relation between ahead,
Vtail, and d.
Suppose instead that amphiphiles pack into a planar
bilayer of thickness 2d. Find a relation between ahead,
Vtail, and d.
Why are one-tail amphiphiles likely to form micelles,
whereas two-tail amphiphiles are likely to form bilayers?
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Why phospholipid?
Why Nature has chosen the phospholipid bilayer
membrane as the most ubiquitous architectural
component of cells:
The self-assembly of two-chain phospholipids (like PC)
into bilayers is even more avid than that of one-chain
surfactants (like SDS) into micelles.
Chemical drive for self-assembly: This free energy cost ε
enters the equilibrium constant and hence the CMC. A
big difference between e-ε/kT (single chain) and e-2ε/kT
(double chain). -The CMC for phospholipid formation is
tiny. Membranes resist dissolving even in environments
with extremely low phospholipid concentration.
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Why phospholipid?
Similarly, phospholipid membranes automatically form bilayer
vesicles which , can be almost unlimited in extent; it is
straightforward to make “giant” vesicles of radius 10 μm, the size of
eukaryotic cells.
Phospholipids are not particularly exotic or complex molecules.
They are relatively easy for a cell to synthesize.
Unlike, say, a sandwich wrapper, bilayer membranes are fluid. No
specific chemical bond connects any phospholipid molecule to any
other, just the generic dislike of water for the hydrophobic tails. This
fluidity makes it possible for membrane-bound cells to change their
shape.
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Membrane-Assisted Transport:
Exocytosis
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Membrane-Assisted Transport:
Three Types of Endocytosis
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Why phospholipid?
Because of the nonspecific nature of the hydrophobic
interaction, membranes readily accept embedded
objects; hence they can serve as the doorways to.
Solubilization of integral membrane proteins (black blobs)
by detergent (objects with shaded heads and one tail).
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Review
Self assembly of amphiphilc molecules
CMC
Packing parameter and shape
Key characteristics of phospholipid bilayer
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Reference
Chapter 8 in Biological physics
Chapter 1, D. Fennell Evans, Hakan
Wennerstrom, The Colloidal Domain
where physics, chemistry, biology, and
Technology Meet, VCH, 1994
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