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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
what is the difference between plasma membrane and intracellular membrane?
- plasma membrane surrounds the whole cell
- intracellular membranes compartmentalize functions within the cell
what is the function of specific enzymes associated with particular membranes?
can be used as markers to identify the membranes during isolation
how are substances transported through the membrane?
- some substances diffuse directly across membranes
- others must be moved by specific transporters
describes mechanisms by which signals are transmitted from the outer surface to the interior of the cell
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
- 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
- hold cells together
- responsible for contact inhibition
- 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.
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
3 reasons why lipids are considered a macromolecule
- have a high molecular weight
- are important in cellular structures
- made by dehydration reaction
- when monomers form polymers by releasing water as a product
- covalent bond formed through this chemical reaction
what are the 6 classes of lipids?
- fatty acids
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?
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
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
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
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
what are the 2 forms of phospholipids?
- 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
- 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
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
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
Terpenes (physical and chemical characteristics)
are synthesized from isoprene (5-carbon compound) and are sometimes called isoprenoids
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
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
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
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
3 lipids form the basic structure of the lipid bilayer, what are they?
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
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
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
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
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
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)
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
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)
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
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
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