Chapter 7 Membrane Structure and FunctionLecture Outline
:
Overview
Life at the Edge
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The plasma membrane separates the living cell from its nonliving surroundings
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,
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This thin barrier
8 nm thick
controls traffic into and out of the cell
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Like all biological membranes
the plasma membrane is selectively permeable
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allowing some substances to cross more easily than others
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Concept 7 1 Cellular membranes are fluid mosaics of lipids and proteins
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The main macromolecules in membranes are lipids and proteins
but
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carbohydrates are also important
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The most abundant lipids are phospholipids
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Phospholipids and most other membrane constituents are amphipathic
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molecules
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Amphipathic molecules have both hydrophobic regions and hydrophilic regions
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The arrangement of phospholipids and proteins in biological membranes is
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described by the fluid mosaic model
Membrane models have evolved to fit new
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data
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Models of membranes were developed long before membranes were first seen
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with electron microscopes in the 1950s
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In 1915
membranes isolated from red blood cells were chemically analyzed
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and found to be composed of lipids and proteins
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In 1925
E
Gorter and F
Grendel reasoned that cell membranes must be a
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phospholipid bilayer two molecules thick
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The molecules in the bilayer are arranged such that the hydrophobic fatty acid
tails are sheltered from water while the hydrophilic phosphate groups interact
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with water
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Actual membranes adhere more strongly to water than do artificial membranes
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composed only of phospholipids
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One suggestion was that proteins on the surface of the membrane increased
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adhesion
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In 1935
H
Davson and J
Danielli proposed a sandwich model in which the
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phospholipid bilayer lies between two layers of globular proteins
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Early images from electron microscopes seemed to support the Davson Danielli
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model
and until the 1960s
it was widely accepted as the structure of the
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plasma membrane and internal membranes
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Further investigation revealed two problems
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First
not all membranes were alike
Membranes differ in thickness
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appearance when stained
and percentage of proteins
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Membranes with different functions differ in chemical composition
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and structure
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Second
measurements showed that membrane proteins are not very
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soluble in water
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Membrane proteins are amphipathic
with hydrophobic and hydrophilic
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regions
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If membrane proteins were at the membrane surface
their hydrophobic
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regions would be in contact with water
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In 1972
S
J
Singer and G
Nicolson presented a revised model that proposed
that the membrane proteins are dispersed and individually inserted into the
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phospholipid bilayer
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In this fluid mosaic model
the hydrophilic regions of proteins and phospholipids
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are in maximum contact with water
and the hydrophobic regions are in a
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nonaqueous environment within the membrane
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A specialized preparation technique
freeze fracture
splits a membrane along
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the middle of the phospholipid bilayer
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When a freeze fracture preparation is viewed with an electron microscope
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