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what are the 5 main membrane functions?
- 1. membranes define boundaries and serve as permeability barriers
- 2. membranes are sights of specific proteins and therefore specific functions
- 3. membrane proteins regulate the transport of solutes
- 4. membrane proteins detect and transmit electrical and chemical signals
- 5. membrane proteins mediate cell adhesion and cell-to-cell communication
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what is the difference between plasma membrane and intracellular membrane?
- plasma membrane surrounds the whole cell
- intracellular membranes compartmentalize functions within the cell
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what is the function of specific enzymes associated with particular membranes?
can be used as markers to identify the membranes during isolation
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how are substances transported through the membrane?
- some substances diffuse directly across membranes
- others must be moved by specific transporters
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signal transduction
describes mechanisms by which signals are transmitted from the outer surface to the interior of the cell
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how is cell-to-cell contacts important for animals?
- critical in animal development and are often mediated by cadherins
- cadherins: have extracellular sequences of amino acids that bind calcium and promote adhesion between similar types of cells in a tissue
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tight junctions
- limit the passage of molecules and ions through the space between cells.
- most materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue.
- block the movement of integral membrane proteins
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adhesive junctions
- hold cells together
- responsible for contact inhibition
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gap junctions
- are intercellular channels that permit the free passage between the cells of ions and small molecules
- permit changes in membrane potential to pass from cell to cell.
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The fluid mosaic model is thought to be descriptive of all biological membranes. What are the 2 key features?
- a fluid lipid bilayer that is constant motion
- a mosaic of macromolecules (proteins) with diverse functions that are attached to or embedded in the bilayer
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lipids (2)
- macromolecule
- hydrophobic
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3 reasons why lipids are considered a macromolecule
- have a high molecular weight
- are important in cellular structures
- made by dehydration reaction
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dehydration reaction
- when monomers form polymers by releasing water as a product
- covalent bond formed through this chemical reaction
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what are the 6 classes of lipids?
- fatty acids
- triacylglycerol
- phospholipids
- glycolipids
- steroids
- terpenes
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Fatty acids (physical and chemical characteristics)
- are building blocks of some lipids
- is a long amphipathic, unbranched hydrocarbon chain with a carboxyl group at one end
- polar carboxyl group is the head
- non-polar hydrocarbon chain is tail
- highly reduced, upon oxidation will absorb energy?
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difference between saturated and unsaturated fatty acids.
- saturated fatty acid: each carbon atom in the chain is bonded to the maximum number of hydrogen atoms; have long straight chains that pack together well
- unsaturated fatty acid: have one more double bonds; the chains have bends that prevent packing
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Triacylglycerol (physical and chemical characteristics)
- are storage lipids
- consist of glycerol molecule attached to three fatty acids that very in length and degree of saturation
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what is glycerol and how are fatty acids linked to it?
- glycerol is a 3-carbon alcohol with a hydroxyl group on each carbon
- fatty acids linked to glycerol, one at a time, by ester bonds formed by removal of water
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phospholipids (physical and chemical characteristics)
- are amphipathic and are vital to membrane structure
- have a polar head due to a negatively charged phosphate group linked to a positively charged group
- also have 2 nonpolar hydrocarbon tails
- form the "rail road" track appearance under EM
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what are the 2 forms of phospholipids?
- phosphoglycerides
- sphingolipids
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phosphoglycerides
- predominant phospholipids in most membranes
- based on the structure of phosphatidic acid, which has 2 fatty acids (one saturated and one unsaturated) and a phosphate group attached to a glycerol
- have small hydrophilic alcohol linked to the phosphate by an ester bond indicated by the "R"
- alcohol is usually serine, ethanolamine, choline or inositol which contributes to polar nature of phospholipid head group
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sphingolipids
- based on the amine sphingosine which has long hydrocarbon chain with a single site of unsaturation near the polar end
- "R" group may or may not contain phosphate group
- can form an amide bond to a long-chain fatty acid resulting in ceramide
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glycolipids ( physical and chemical characteristics)
- specialized membrane components
- are lipids containing a carbohydrate instead of a phospholipid and are often derivatives of sphingosine and glycerol (glycosphingolipids)
- carbohydrate groups attached to glycolipids may be 1-6 sugar units of:
- D-glucose, D-galactose and/or N-acetyl-D-galactosamine
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steroids (physical and chemical characteristics)
- functions in membrane and as hormones
- are derivatives of a 4-ringed hydrocarbon skeleton, which distinguishes them from other lipids
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Terpenes (physical and chemical characteristics)
are synthesized from isoprene (5-carbon compound) and are sometimes called isoprenoids
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what are polyisoprenoids and dolichol?
- types of terpenes
- polyisopernoids are found in membranes of archaea
- dolichol and some electron carriers are isoprene derivatives found in organelle membranes
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The hydrophobic effect
- hydrophobic molecules force water to form a cage-like structure
- because cage is highly organized, it costs energy to form
- energy cost minimized if the hydrophobic molecules cluster together instead and limit the contact of water to the smallest possible number of molecules
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the hydrophobic effect is important for 4 reasons:
- the formation of lipid bilayer
- the insertions of membrane proteins into the nonpolar lipid environment
- protein folding
- protein-small molecule interactions
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Heterogeneous lipids all share the following 3 characteristics:
- hydrophobic nature, thus little affinity for water (hydrophobic effect)
- readily soluble in nonpolar solvents
- relatively few polar groups, but some are amphipathic, having polar and nonpolar regions
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3 lipids form the basic structure of the lipid bilayer, what are they?
- phospholipids
- glycolipids
- sterols
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what is Thin-layer chromatography (TLC) and why is important?
- TLC separates different kinds of lipids based on their relative polarity
- important technique for separating lipids and analyzing the lipid composition of membranes
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Because membrane components are amphipathic (having both polar and non-polar) what 5 things occur?
- component spontaneously form bi-layers
- hydrophilic regions face the water sides while hydrophobic regions make up the central core
- they never have a free end due to cohesion
- spontaneously reseal, able to self-heal
- fuse with other membrane structures
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Fatty acids of phospholipids are essential to membrane structure and function, how?
- the sizes of fatty acids range between 12-20 carbons long, which is optimal for bilayer formation and dictates the usual thickness of membranes
- long hydrocarbon tails provide a barrier to the diffusion of polar solutes
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The membranes of most eukaryotes contain significant amounts of sterols. what is the main sterols found in animal, plant and fungal cell membranes?
- animal cells: cholesterol, needed to stabilize and maintain membrane
- plant cell: cholesterol and phytosterols
- fungal cells: ergosterol, similar to cholesterol
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what are the effects of sterols on membranes?
- decrease the permeability of membranes to ions and small polar molecules
- the fill spaces between the hydrocarbon chains of phospholipids which blocks the routes
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what are 3 ways that phospholipids move within the membrane?
- can rotate about their axis
- can move within the monolayer via lateral diffusion
- exchange from one leaflet to another (Transverse diffusion-rare)
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Transverse diffusion occurs with assistant, how?
some membranes, in particular the Smooth ER membrane, have proteins called phospholipid translocators or flippases that catalyze the flip-flop of membrane lipids
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what are the 2 things that membrane fluidity depends on?
- temperature: membrane fluidity decreases as temperature falls and vice versa
- Membrane composition: temperature at which the membrane changes fluidity is dependent on the types of lipids that go into composition (length of fatty acid and degree of saturation)
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Every lipid bi-layer has a characteristic transition temperature, Tm what is it and the effect on the membrane?
- the temperature at which it becomes fluid
- change of state is called a phase transition (solid to liquid)
- below the Tm, any function that rely on the membrane fluidity will be disrupted
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The level of fatty acid saturation impacts the temperature at which the membrane changes fluidity, which fatty acid is more fluid and why?
- saturated fatty acids pack together well in the membrane
- unsaturated fatty acids have one or more double bonds that have bends in the chains that prevent proper packing
- therefore unsaturated fatty acids are more fluid and have lower Tms
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How does the length of fatty acid chains also influence the fluidity of the membrane?
- longer the chains, the more neighbouring chains interact with one another
- this decreases fluidity and increases rigidity
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