chapter 11 - membrane structure

membranes are _ barriers

selective barriers

molecular composition of a cell is same/different from its environment

different

the plasma membrane is involved in

• cell communication

• import and export of molecules

• cell growth

• motility

internal membranes form different _ in eukaryotic cell

form different compartments 

membrane composition

lipids

• phospholipids

• glycolipids

• sterols

• amphipathic

proteins

• integral

• peripheral

lipids - phospholipids

most abundant component

lipids - glycolipids

sugar part of head group

lipids - sterols

e.g. cholestrol

proteins - integral

directly attached to membrane

proteins - peripheral

loosely associated with membrane

fluid mosaic model of cell membrane

amphiphatic phospholipids for a lipid _

lipid bilayer

_ is the most common phospholipid in cell membranes

phosphatidylcholine

different types of membrane lipids are _

amphiphatic

a phospholipid bilayer with spontaneously rearrange to eliminate _

• eliminate free edges

• helps repair damage

• helps with formation of a closed compartment - liposome

liposomes ( and vesicles) have a _ bilayer and an _ interior

• have a phospholipid bilayer and aqueous interior

liposome vs micelle

membranes behave as a _ fluid

• behave as a two dimensional fluid

• molecules move 

the fluidity of a cell membrane

• the ease with which its lipid molecules move within the plane of the bilayer

• its important for membrane function and has to be maintained within certain limits

fluidity is/is not the same as flexibility (bending)

fluidity is not the same as flexibility (bending)

factors affecting membrane fluidity

• phospholipid composition

• sterols

phospholipid composition and fluidity

• fatty acid chain length - short chains increase fluidity

• fatty acid chain saturation - double bonds increase fluidity

• chain length and level of saturation determines the properties of fats

sterols and fluidity

• cholestrol - more cholestrol decreases fluidity

  • adds rigidity

  • but cholestrol can increase fluidity if most fatty acids are saturated

_ fills spaces in the bilayer

• cholestrol fills spaces

• makes the bilayer more rigid

• amphipathic

membranes are symmetrical/asymmetrical

• asymmetrical

• the distribution of phospholipids and glycolipids creates different surfaces

membrane assembly beings in the _

• begins in the ER

• flippases have specificity for different types of phospholipids

• newly synthesized phospholipids are added to the cytosolic side of the ER membrane and then redistributed by transporters that transfer them from one half of the lipid bilayer to the other

  • biosynthetic enzymes bound to the cystosolic monolayer of the ER membrane produce new phospholipids from free fatty acids and insert them into the cytosolic monolayer

  • transporters called scramblases then randomly transfer phospholipid molecules from one monolayer to the other, allowing the membrane to grow as a bilayer in which the two leaflets even out continuously in size and lipid composition

cell molecular structure

certain phospholipids are confined to one/multiple side of the membrane

• confined to one side of the membrane

• flippases help to establish and maintain the asymmetric distribution of phospholipids characteristics of animal cell membranes

  • when membranes leave the ER and are incorporated in the golgi, they encounter a different set of transporters called flippases, which selectively remove phosphatidylserine and phosphatidylethanolamine from the noncytosolic monolayer and flip them to the cytosolic side

  • this transfer leaves phosphatidylcholine and sphingomyelin concentrated in the noncytosolic monolayer

  • the resulting curvature of the membrane may help drive subsequent vesivle budding 

membranes retain their orientation during transfer between _

• transfer between cell compartments

• membranes are transported by a process of vesicle budding and fusing

• a vesicle buds from the golgi and fuses with the plasma membrane

• the orientation of both membrane lipids and proteins are preserved during the process

  • the original cytosolic surface of the lipid bilayer remains facing the cytosol

  • the noncytosolic surface continues to face away from the cytosol, toward the lumen of the golgi and the transport vesicle or towards the extracellular fluid

• the glycoprotein remains the same orientation with its attached sugar facing the noncytosolic side 

glycolipids obtain their sugar from _

• from the inside of the golgi

• when these membranes are transferred to the plasma membrane, the sugar groups remain oriented on the no cytosolic face

membrane proteins carry out

carry out most membrane functions

different ways in which membrane proteins can associate with a lipid bilayer

• integral

  • transmembrane

  • membrane associated

  • lipid-linked

• peripheral

  • protein -ttached

integral membrane proteins can be removed from membranes only by _

removed only using detergents

peripheral membrane proteins can be removed by _

removed by more gentle extraction methods

an alpha helix containing many _ can span a membrane bilayer

• many nonpolar amino acids

• the hydrophilic polypeptide backbone (stabilized by h bonds) is shielded from the hydrophobic lipid carbons

multiple amphiphatic alpha helices can associate to form a _

• form a hydrophilic core

• individual helices would not be stable in the membrane due to their hydrophilic side chains

a large. curved beta sheet can form a _ that spans the membrane

• can form a beta barrel

• the amino acid side chains alternate between hydrophilic and hydrophobic to create a water filled interior and hydrophobic exterior that facets the bilayer core

• a single beta strand cannot shield its hydrophilic backbone regardless of the side chain composition

detergents are used to separate

• separate integral membrane proteins from the lipid bilayer

• disrupt hydrophobic associations

• solubilization of integral membrane proteins

properties of detergents

• amphiphatic

• single hydrophobic chain

• do not form bilayers

carbohydrate layer of cell surface - composition

• glycoprotein

• proteoglycan

• glycolipid

carbohydrate layer - glycoprotein

protein with short, covalently attached oligosaccharide

carbohydrate layer - proteoglycan

protein with one or more long oligosaccharide

carbohydrate layer -glycolipid

lipid with covalently attached oligosaccharide

function of carbohydrate layer

• protection from chemical and mechanical damage - lubrication

• cell-cell recognition

  • lectins - proteins that bind oligosaccharide chains

lectin binding is responsible for recruitment of _  to sites of _

recruitment of white blood cells to sites of infection

mobility of membrane proteins

some, but not all, proteins are freely diffusible in the membrane

not all proteins are mobile, some are restricted to _

membrane domains

the cell cortex is

• is a framework of proteins

  • largely spectrin

• attaches to membrane proteins, restricting their mobility

• a given membrane protein will exist as a mixed population of anchored and unanchored proteins

tight junctions in the cell cortex preserve the _

• preserve the asymmetric distribution of membrane proteins

asymmetric distribution of transport proteins