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external membrane

plasma membrane: encloses contents of entire cells


internal membranes

-nuclear envelope
-mitochondrial envelope
-chloroplast membranes
-lysosomal membrane
-endoplasmic reticulum


summary of membrane functions

1. compartmentalization
2. site of many biochemical activities
*scaffolding for many enzymes
3. provide a selectively permeable membrane
* permeable to lipids, steroids, O2 and CO2; less permeable to water and impermeable to water-soluble compounds
4.transporting solutes
*pumps needed; gradients formed b/w inside and out
5. responding to external signals
*signal transduction; interaction of receptors with ligands
6.intercellular interaction
*cell to cell attachment and communication
7.energy transduction
*electrochemical gradients; energy absorption and transfer


membrane structure

-consists of polar lipids arranged in a bilayer
-thin sectioning technique ~7.5nm wide with dark, light, dark appearance
-freeze fracture technique -smooth areas interrupted by bumps and depressions


fluid mosaic model

a)lipid bilayer is core of membrane
b)lipid molecules are present in a fluid state capable of rotating and moving laterally within membrane
c)proteins occur as mosaic of discontinuous particles
d)some proteins penetrate deeply into and even through lipid bilayer
e)membranes are dynamic structures in which components are mobile
f)components come together to engages in various transient interactions


membrane lipids

-all are amphipathic
*except for glycolipids and cholesterol, all contain phosphate group-hence, they are called phospholipids
*phospholipids have a glycerol backbone
*therefore, phospholipids are also phosphoglycerides
-different lipids in cell membrane:
*sphingolipids and glycolipids



-diglycerides-often one saturated and on unsaturated fatty acid
-3rd OH group is coupled to a phosphate plus either:
1. choline-phosphatidyl choline



-sphingosine-based lipids
-sphingosine is an amino alcohol with a long hydrocarbon chain
-sphingolipids play roles in
a) membrane structure: decreasing membrane fluidity, becoming more gel-like rather than fluid and therefore resistance to some stresses
b)signalling to other cells-usually because of the sugar groups attached to them, allow other cell receptors to bind- a subclass of sphingolipids have sugars attached at the polar head-these sphingolipids are called glycolipids


glycolipids play key roles in:

-cell membrane recognition
-electrical conduction (e.g. myelin around nerve cells)
-inter-cell communication
-binding bacterial toxins (eg. botulism) and some viruses-permitting their entry into the cell



-may constitute up to 50% of lipid molecules in plasma membrane of certain animal cells
-in absent from plasma membranes of all bacterial cells, and is quite low in plant cells
-at high temps, cholesterol tends to reduce membrane fluidity, probably by interacting with the hydrocarbon tails of the phospholipid an glycolipid molecules
-at low temps, cholesterol helps prevent membranes from freezing and thus tends to maintain membrane fluidity


Membrane fluidity

-fluidity/viscosity-a measure of freedom of movement within the membranes
-fatty acid (CH2)n chain effects fluidity
-fluidity is important to balance mobility and organization
-heterogeneity, local patches of membrane may have unique fluidity, eg. sphingolipid-rich patches or rafts- often find cholesterol in lipid rafts
*certain proteins (esp. those involved in cell signalling) are preferentially found in lipid rafts
--fluidity is dependent on type of lipid and temp-high T, more lipid--


effect of the fatty acid (CH2)n chain on fluidity:

-longer chains have less fluidity
-presence of double bond (unsaturated fat) increases "kinks" and hence fluidity
-sphingolipids are more rigid than phosphoglycerides


fluidity is important to balance mobility and organization

cell growth, cell division, inter-cellular junctions, endocytosis and vesicle traffic


cells are able to modify membrane chemistry to maintain correct fluidity

a) remodelling enzymes for short-term response
-increased #'s of double-bonds by desaturase, alters the fatty acid chains (e.g. exposure of bacteria to cold can induce desaturase activity)
-shuffling of the fatty acid chains on phospholipids
*this is accomplished by phospholipase (splits fatty acids from glycerol backbone) and acyltransferase (moves fatty acids b/w glycerol backbones)
b)over longer term, diff phosphoglycerides are made by the cell
c)remodelling occurs in prokaryotes and eukaryotes, including animals, plants depending on the temperature they live at; most mammals regulate internal temp with a few exceptions: extremities in hibernating animals often show changes in lipids


membrane carbohydrates

-all of the membrane carbohydrates face outward into extracellular space
-covalently linked to either protein or lipid
1. glycoproteins
2. glycolipids



carbohydrate is present in short, branched oligosaccharides (



may play role in certain infectious diseases
carbohydrates of glycolipids determine blood type (A, B, AB, O)


membrane carbohydrates:
carbohydrate is usually a minor component of the plasma membrane

-eg. RBC outer membrane
-52% protein
-40% lipid
-8% carbohydrate
antigens on RBCs are glycolipids- different sugar added by A or B allele


for some cells, there are large amounts of sugar associated with the outside of the plasma membrane (exterior membrane of the cell) these sugars are both ______ and _______ linked.

this is glycocalyx



-in bacteria, the glycocalyx refers to the outer coating of the cell wall
-in our gut, glycocalyx serves a protective function against digestive enzymes
-in some cells, it is involved in cell-cell recognition and bonding
-receptor/ligand interactions


membrane proteins

-100s of diff proteins associated w/ interior or plasma membrane-each cell type is diff
-highly "sided": each protein has specific internal/external orientation
*hormone receptors on outside
*signal proteins on the cytoplasmic side (G-proteins, protein kinases)


there are three classes of membrane proteins

1. integral proteins (trans-membrane proteins), lipid soluble portion, cytosolic portion and exterior portion
2. peripheral proteins, inside or outside the membrane, associated by non-covalent bonds
3. lipid-anchored proteins, inside or outside, covalently linked to a lipid


integral membrane proteins

-penetrate into lipid bilayer
-usually pass right through membrane
-have segments protruding into extracellular space
-have segments protruding into cytoplasm
-transmembrane segments
*pass through lipid bilayer
*usually consist mostly of hydrophobic or nonpolar amino acids (red dots) organized in an alpha helical conformation
*hydrophobicity plot- show which amino acids are likely to be found within membrane (above red line)
-some integral membrane proteins can form aqueous channel through lipid bilayer


Peripheral membrane proteins

-located entirely outside of lipid bilayer
-either on extracellular or cytoplasmic surface
-associated w/ membrane surface by noncovalent bonds
-weak electrostatic bonds to hydrophilic head groups of phospholipids or hydrophilic portion of integral proteins
-peripheral proteins on cytoplasmic surface function in transmembrane signal transduction


lipid anchored membrane proteins

-protein is covalently linked to lipid molecules within bilayer
-two types of lipid anchors:
a. an anchor comprised of a lipid + carbohydrate chain, usually in outer leaflet (eg. glycosyl-phosphatidylinositol linked protein)
b. an anchor comprised only of a long hydrocarbon chain, usually in inner leaflet
-often involved in cell signalling or adhesion



continuous, unbroken sheets, enclosing compartments
dynamic structures capable of fusing without losing continuity