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Chapter 7

BIL 150 Chapter Notes - Chapter 7: Unsaturated Hydrocarbon, Fluid Mosaic Model, Lipid Bilayer

Course Code
BIL 150
Gaines Michael

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Chapter 7 – Membrane Structure and Function
The plasma membrane exhibits selective permeability
oIT allows some substances to cross it more easily than others
oThe ability of the cell to discriminate in its chemical exchanges with
its environment is fundamental to life
The plasma membrane is composed of phospholipids
oA phospholipid is an amphipathic molecule, meaning it has both a
hydrophilic region and a hydrophobic region
oA phospholipid bilayer can exist as a stable boundary between two
aqueous compartments because the molecular arrangement
shelters the hydrophobic tails of the phospholipids from water
exposing the hydrophilic heads to water
oFluid mosaic model – membrane is a mosaic of protein molecules
bobbing in a fluid bilayer of phospholipids
The membrane is held together primarily by hydrophobic interactions,
which are much weaker than covalent bonds
oMost of the lipids and some proteins (much larger than lipids) can
shift about laterally
A membrane remains fluid as temperature decreases until the
phospholipids settle into a closely packed arrangement and the membrane
oThe temperature which it solidifies depends on the types of lipids it
is made of
oMembrane remains fluid to a lower temperature if it is rich in
phospholipids with unsaturated hydrocarbon tails
Because of kinks in the tails where double bonds are
located, unsaturated hydrocarbon tails cannot pack together
as closely as saturated hydrocarbon tails, making the
membrane more fluid

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oCholesterol has different effects on membrane fluidity at different
Reduces membrane fluidity at moderate temperatures by
reducing phospholipid movement
At low temperature, it hinders solidification by disrupting the
regular packing of phospholipids
oMembranes must be fluid to work properly
Affects both its permeability and the ability of membrane
proteins to move to where their function is needed
oAs temperature increases, so does permeability of membrane
Like a mosaic, a membrane is a college of different proteins
oIntegral proteins – penetrate the hydrophobic interior of the lipid
oPeripheral proteins – not embedded in the lipid bilayer at all; they
are appendages loosely bound to the surface of the membrane,
often to exposed parts of integral proteins
oFunction of proteins:
Transport – transport proteins that aid in facilitated diffusion
or active transport
Enzymatic activity – a protein built into the membrane may
an enzyme with its active site exposed to substances in the
adjacent solution
Signal transduction A membrane protein (receptor) may
have a binding site with a specific shape that fits the shape
of a chemical messenger (hormone)
Cell-cell recognition A cell’s ability to distinguish one type
of neighboring cell from another
Some glycoproteins serve as identification tags that
are specifically recognized by membrane proteins of
other cells
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