ch7&8 - Chapter 7 Membrane Structure and...

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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, , . 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
. allowing some substances to cross more easily than others
. 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
. 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
. 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
. described by the fluid mosaic model Membrane models have evolved to fit new
. data
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Models of membranes were developed long before membranes were first seen
. with electron microscopes in the 1950s
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, In 1915 membranes isolated from red blood cells were chemically analyzed
. 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
. 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
. with water
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Actual membranes adhere more strongly to water than do artificial membranes
. composed only of phospholipids
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One suggestion was that proteins on the surface of the membrane increased
. adhesion
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, . . In 1935 H Davson and J Danielli proposed a sandwich model in which the
. phospholipid bilayer lies between two layers of globular proteins
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- Early images from electron microscopes seemed to support the Davson Danielli
, , model and until the 1960s it was widely accepted as the structure of the
. 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
, . appearance when stained and percentage of proteins
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Membranes with different functions differ in chemical composition
. and structure
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, Second measurements showed that membrane proteins are not very
. soluble in water

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, Membrane proteins are amphipathic with hydrophobic and hydrophilic
. regions
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, If membrane proteins were at the membrane surface their hydrophobic
. 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
. phospholipid bilayer
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, In this fluid mosaic model the hydrophilic regions of proteins and phospholipids
, are in maximum contact with water and the hydrophobic regions are in a
. nonaqueous environment within the membrane
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, - , A specialized preparation technique freeze fracture splits a membrane along
. 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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