Cellular Physiology of Nerve and Muscle.doc

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Cellular Physiology of Nerve and Muscle
The Plasma Membrane (pg 63-77)
-flexible; defines the extent of the cell and separates the intracellular fluid from the
extracellular fluid (interstitial fluid)
-synonymous with cell membrane
fluid mosaic model- depicts the plasma membrane as a thin structure composed of a
bilayer of lipid molecules with protein molecules dispersed in it
-it is fluid because it is constantly changing its shape and patter
-it is a mosaic because it contains different proteins, lipid molecules etc.
-the lipid bilayer is constructed of phospholipids, with small amounts of cholesterol and
-each phospholipid molecule has a polar head that is charged and is hydrophilic (water-
loving) and an uncharged, non polar tail that is made of two fatty acid chains and is
hydrophobic (water-fearing)
-2 fatty acids attached to glycerol and phosphate group on the 3rd position
-the polar heads attach to water which is the main component of inter and extracellular
-the non polar tails avoid water and line up in the centre of the membrane
-the plasma membrane is dynamic and in constant flux
-it has a consistency like olive oil, which allows for lateral diffusion of proteins
-the inward-facing and outward-facing surfaces of the plasma membrane differ in the kinds
and amounts of lipids they contain, and these variations are important in determining local
membrane structure and function
-membrane phospholipids are mostly unsaturated, causing kinks in their tails and
increasing space between phospholipids to increase membrane fluidity
-lipids with attached sugar groups
-are found on the outer plasma membrane surface and account for 5% of the total
membrane lipid
-their sugar groups make the end of the glycolipid molecule polar
-fatty acid tails are non polar
-20% of membrane lipid is cholesterol
-cholesterol has a polar region (hydroxyl group) and a nonpolar region (ring
-it wedges its hydrocarbon rings between phospholipid tails to stabilize the
membrane, increase mobility of the phospholipid, and the fluidity of the
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-too much cholesterol interferes with cell function and fluidity
lipid rafts
-dynamic assemblies of saturated phospholipids associated with unique lipids called
sphingolipids and cholesterol making up 20% of the outer membrane surface
-more stable and less fluid than the rest of the membrane
-are assumed to be concentrating platforms for certain receptor molecules or for molecules
needed for cell signalling
Membrane Proteins
-proteins make up about half of the plasma membrane by mass and are responsible for
specialized membrane functions
-there are integral and peripheral proteins
Integral Proteins
-firmly inserted into the bilayer
-most are transmembrane proteins which span the entire width of the membrane and
protrude on both sides
-there are a few that protrude from one face only
-integral proteins have both hydrophobic and hydrophilic regions which allows them to
interact with nonpolar lipid tails in the membrane and with water inside and outside the cell
-most are involved in transport; there are some that are enzymes
-some cluster together to form channels or pores so that small, water-soluble molecules or
ions can move, allowing them to bypass the lipid part of the membrane
-some act as carriers that bind to a substance and then move it through membrane
-some are receptors for hormones and other messengers; they relay the message to the cell
Peripheral Proteins
-not embedded in the lipid
-attach loosely to integral proteins
-can be easily removed without disrupting the membrane
-peripheral proteins include a network of filaments that helps support the membrane from
its cytoplasmic side
-some are enzymes
-some are motor proteins involved in mechanical functions such as changing cell shape
during cell division and muscle contraction
-glycocalyx describes the fuzzy, sticky, carbon-rich area at the cell surface; it is enriched
by both glycolipids and glycoproteins
-the glycocalyx provides highly specific biological markers by which approaching
cells recognize each other (i.e.: a sperm recognizes an ovum by the ovum's unique
-array of proteins on the outer surface that have a sugar and protein part; it gives
the characteristics of the cell and is characteristic to each person
Membrane Junctions
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-there are 3 factors that act to bind cells together:
1) Glycoproteins in the glycocalyx act as an adhesive
2) wavy contours of the membrane of adjacent cells fit together in a "tongue-and-
groove" fashion
3) special membrane junctions are formed
-junctions are the most important factor securing cells together
Tight Junctions
-a series of integral protein molecules in the plasma membrane of adjacent cells fuse
together, forming an impermeable junction that encircles the cell
-tight junctions help prevent molecules from passing through the extracellular space
between adjacent cells
-for example, tight junctions between epithelial cells that are lining the digestive tract keep
digestive enzymes and microorganisms in the intestine from entering the bloodstream
-known as "impermeable" junctions but are known to be somewhat leaky and allow certain
types of ions to pass
-"binding bodies"
-anchoring junctions
-mechanical couplings scattered like rivets along the sides of adjoining cells that prevent
their separation
-there is a buttonlike thickening called a plaque on the cytoplasmic face of each plasma
-adjacent cells are held together by thin linker protein filaments (cadherins) that
extend from the plaques and fit together in intercellular space
-thicker keratin filaments that form part of the cytoskeleton extend from the
cytoplasmic side of the plaque and across the width of the cell to anchor to the
plaque on the cell's opposite side
-this allows desmosomes to bind to neighbouring cells, while contributing to a
continuous network of tension-reducing fibres
-desmosomes are found in tissues subjected to great mechanical stress such as skin and
heart muscle
Gap Junctions
-also known as a nexus
-is a communicating junction between adjacent cells
-at gap junctions, the adjacent plasma membranes are very close, and the cells are
connected by hollow cylinders called connexions composed of transmembrane proteins
-different types of connexions varies the selectivity
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