Chapter 5 Study Guide

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20 Nov 2010
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Chapter 5: The Dynamic Cell Membrane
5.1 What is the Structure of a Biological Membrane?
x Fluid Mosaic Model - a lipid pool, in which proteins and carbohydrates float
x The lipids establish the physical integrity of the membrane and create an effective barrier to the
rapid passage of hydrophilic materials
x The proteins have functions, namely: moving materials through the membrane and receiving
chemical signals from the cell's external environment
x Carbohydrates are crucial for cell recognition of molecules
Lipids
x Main: phospholipid - has both hydrophilic and hydrophobic regions
x Cholesterol is important for membrane integrity as it determines fluidity of the fatty
acids in the membrane
x The fatty acids of the phospholipids make the hydrophobic interior of the membrane
somewhat fluid
x Lateral movement is always happening; but rarely is there any flipping of phosphoplipids
x The inner and outer halves of the bilayer may be quite different in the kinds of
phospholipids they contain
x Shorter-chain fatty acids, unsaturated fatty acids, and less cholesterol = more fluidity
x Fluidity decreases at reduced temperatures; membrane functions may decline in
organisms that cannot keep their bodies warm
Proteins
x Asymmetrically distributed
x Proteins have both hydrophilic (have hydrophilic side chains - arginine, histidine, lysine,
aspartic acid, glutamic acid, serine, threonine, asparagine, glutamine, tyrosine) and both
hydrophobic (alanine, isoleucine, leucine, methionine, phenylalanine, tryptophan,
valine) regions
x Freeze fracturing - reveals proteins embedded in the bilayer
o Freeze
o Tissue is fractured via diamond/glass knife
x Two types of proteins; bond non-covalently
o Integral - span the length of the membrane with extrusions
o Peripheral - not embedded in the bilayer; polar interacting with polar regions of
the membrane
x Some proteins are covalently attached to fatty acids - resulting in a special type of
proteins
x Transmembrane proteins - integral proteins that have hydrophillic extrusions on both
sides
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x Both sides of the membrane can differ significantly
x Proteins also move freely along a membrane
x Some are anchored to a specific region of the membrane
o Cytoskeleton may have components just below the inner face of the membrane
that are attached to proteins protruding into the cytoplasm
o Lipid rafts (lipids in a semisolid state) - different composition than surrounding
phospholipids
Membranes are Dynamic
x Phospholipids are synthesized on the surface of the smooth endoplasmic reticulum and
are rapidly distributed to membranes
x Membrane proteins are inserted into the rough endoplasmic reticulum
x ER --> Golgi Apparatus --> Plasma
x Additions to the plasma membrane from fusion with vesicles derived from the Golgi
Apparatus is countered by phagocytosis
Carbohydrates
x Covalently bonded to lipids or to proteins
o Glycolipids -recognition signals for interactions between cells
o Glycoproteins - oligosaccharide chains not exceeding 15 monosaccharide units
in length - enable a cell to be recognized by other cells and proteins
x An oligosaccharide of a specific shape on one cell can bind to a mirror-image shape on
an adjacent cell - resulting in specificity of cell-cell adhesion
5.2 How is the Plasma Membrane Involved in Cell Adhesion and Recognition?
x Cell recognition - cell specifically bind to another cell of another type
x Cell adhesion - connection between two cells is strengthened
x Studies of sponges have revealed how cells associate with one another
o The cells of the sponge are connected, but can be separated mechanically by passing the
animal several times through a fine wire screen
o When shake and suspended, the cells will adhere to each other in such a fashion that it
will resemble the original sponge
x The protein the is responsible for cell adhesion and recognition is a glycoprotein (80 percent
sugar)
x Since proteins have specific shape and specific endings on its surface, the bonding to other
molecules is also specific
x The cell wall in plants also has adhesion proteins that allow cells to bind to one another
x Homotypic - same molecule sticks out of both cells and the exposed surfaces bind to each other
x Heterotypic -different chemical groups on different surfaces of proteins binding
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