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Nursing (RLE70) Lecture 2 Membrane Structure

John Marsh
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Capitol University

Nursing (RLE70)

Lecture 2 Membrane Structure and

Membrane Structure and Function
 The plasma membrane is the boundary
that separates the living cell from its
 The plasma membrane exhibits
selective permeability, allowing some
substances to cross it more easily than
Cellular membranes are fluid mosaics of lipids
and proteins
 Phospholipids are the most abundant
lipid in the plasma membrane
 Phospholipids are amphipathic
molecules, containing hydrophobic and
hydrophilic regions
 The fluid mosaic model states that a
membrane is a fluid structure with a
“mosaic” of various proteins embedded
in it.
Membrane Models: Scientific Inquiry
 Membranes have been chemically
analyzed and found to be made of
proteins and lipids
 Scientists studying the plasma
membrane reasoned that it must be a
phospholipid bilayer
 two Dutch scientists, E. Gorter and F
 Dehydration Process joining of two
molecules with removal of water
 If phospholipid is placed in a water
solution: micelle is formed.
 In 1935, Hugh Davson and James
Danielli proposed a sandwich model in
which the phospholipid bilayer lies
between two layers of globular proteins
 Later studies found problems with this
model, particularly the placement of
membrane proteins, which have
hydrophilic and hydrophobic regions
 In 1972, S. J. Singer and G. Nicolson
proposed that the membrane is a
mosaic of proteins dispersed within the
bilayer, with only the hydrophilic
regions exposed to water
 Freeze-fracture studies of the plasma
membrane supported the fluid mosaic
 Freeze-fracture is a specialized
preparation technique that splits a
membrane along the middle of the
phospholipid bilayer
The Fluidity of Membranes
 Phospholipids in the plasma membrane
can move within the bilayer
 Most of the lipids, and some proteins,
drift laterally (10^7 times per second)
 Rarely does a molecule flip-flop
transversely across the membrane
(once per month)
 As temperatures cool, membranes
switch from a fluid state to a solid state
 The temperature at which a membrane
solidifies depends on the types of lipids
 Membranes rich in unsaturated fatty
acids are more fluid than those rich in
saturated fatty acids
 Membranes must be fluid to work
properly; they are usually about as fluid
as salad oil
 The steroid cholesterol has different
effects on membrane fluidity at
different temperatures
 At warm temperatures (such as 37°C),
cholesterol restrains movement of
 At cool temperatures, it maintains
fluidity by preventing tight packing



Nursing (RLE70) Lecture 2 Membrane Structure

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