Membrane Structure

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Membrane Structure

• Creation of the first cell and membrane
• The two most important elements of cellular
  lifethe boundary that separates the inside and
  the outside of the cellthe membrane system
  and the inheritance moleculeone that can
  duplicate itself and passed on to the next
  generation (what?).
• The evolution of the membranefrom the single
  cell membrane in the first cell and the primitive
  cells like bacteria (prokaryotes) to more
  complicated membrane systemsnot only the outside
  boundary but intracellular membranes that
  surrounds each compartment or organelle. Plasma
  membrane (PM), chloroplast envelopes, ER, Golgi,
  Mitochondria, Nucleus,
• Functional diversitycomposition hold the key--
  What makes the difference? Proteins! PM has
  transporters, receptorsNuclear proteins
  chloroplast and photosynthetic complexes

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Lipid bilayer
Ancestor cell
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• Lipid bilayer
• 1) composition phospholipids (50 or more) and
  other lipids why choosing these materials?
  Chemical properties perfect for the purpose.
  Minors Cholesterols and others.

Amphapathic/ Amphaphilic molecules
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2) Formation of bilayers It is an
automatic/spontenous process. How so? The
chemical nature of phospholipids and other
amphapathic molecules (such as detergents) allows
interaction with water and formation of two
structures in water solution the micelles and
the bilayer vesicles
water
This is like a tiny oil droplet (w/ polar head
facing the water and the non-polar tails facing
each other and away from water)
water
This is like a pita-bread w/ water both inside
and outside (more stable than the micelle)
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• 3) Dynamics of lipid bilayer
• Mobility children blow bubbles in the airif you
  look carefully the surface of the bubbles
  moves. The molecules in the lipid bilayer move
  just like those detergent molecules in the
  bubbles. This property is called mobility of
  the bilayer.
• There are several types of movements lateral
  diffusion (lateral movement in the same layer),
  tail flexion (hydrophobic tails kick around
  inside the layer), rotation (the molecule turns
  around inside the same layer) vertical bobbing
  and exchange between the two layers
  (flip-floprequires enzymes to occur).

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B) Fluidity, melting point, and phase transition
of lipid bilayers The mobility of molecules in
the lipid bilayer determines the fluidity. Like
other fluid/liquid, lipid bilayers also change
between liquid and solid statephase transition.
The temperature at which the transition occurs is
the Melting point of a particular lipid bilayer.
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What determines melting point of a lipid bilayer?
This Tm, like Tm of water and other things is a
constant for a particular matter. It is
determined only by its chemical composition. As a
lipid bilayer consists of many molecules, a
number of factors affect its Tm. The composition
of the lipids the length and the saturation of
the fatty acid chain/tails, the content of
cholesterols are major factors---the composition
of the cell membranes are dynamic and regulated
based on the developmental and environmental
responses An example on how plant cells
regulate its membrane Tm During winter time or
when temperature suddenly drops, some plants can
change the composition of their membranes. One
way is to decrease the saturation of the membrane
fatty acidsto decrease Tm so that the membrane
can be fluid at even lower temperature. This is
done by increasing he level of enzyme activity of
desaturase. Low temperature---activation of genes
encoding desaturase---more mRNAmore proteinmore
activity---more unsaturated fatty acid---Tm
drops.
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How to measure membrane property such as fluidity
and function of a particular membrane
component? Model systems liposome (bilayer
sphere) used for measuring mobility of a lipid
molecule, deliver drugs and DNA into the cells
etc planar membrane (bilayer within a plastic
support) study permeability and transport
activity of the membrane and a membrane protein
(channel or transporter)
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C) Asymmetry of lipid bilayers the compositions
of the two layers (leaves) of the bilayer are
differentasymmetrical. This also refer to other
components such as proteins in the membrane.
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• 4) Membrane proteinswhat determine the function
  of the membrane
• Peripheral and integral proteins also called
  extrinsic and intrinsic membrane proteins
  referring to the way a protein is associated with
  the lipid bilayer--- defined based on
  experimental procedures to extract the
  proteinssome can be extracted by high salt and
  extreme pH conditions and are peripheral/extrinsic
  proteins (they were loosely bound to the surface
  of the membrane through salt or ionic
  interactions) those that can be exacted only by
  breaking the membrane apart using detergents are
  integral/intrinsic proteins (bound tightly and
  often span membrane bilayer)----see the above
  picture
• Molecular and covalent interactions Non-covalent
  bonds such as H-bond, ionic interaction, and van
  der Waals. The hydrophobic/van der Waals
  interaction is quite strong and is the major
  force for the integral proteins to interaction
  with the hydrophobic tails (interior of the
  membrane lipid bilayer), the H-bond and ionic
  interactions are weaker and usually used for
  peripheral proteins to associate with the surface
  of the membranes.

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Covalent association proteins may be modified by
lipid or hydrocarbon chains so that he protein
can be inserted into the membrane through this
hydrophobic chain/tag.
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3) Proteins form complexes in the membrane Ion
channels, electron carriers in the
mitochondria/chloroplast, the nuclear pore
complexes etc Multiple subunits and complicated
assembly processes poorly understood. 4)
Proteins move along with the membrane
lipidsfluidity/mobility 5) Asymmetry as
mentioned earlier.