Chapter 10

membrane structure

• phospholipid bilayer (5 nm in height)

• encloses the cells and defines their boundaries

• maintains the essential/potential difference b/w the cytosol and the extracellular environment

• proteins embedded in the membrane→ carry out most of the specific functions

  • ex: transport, catalysis, receptors and signaling, adhesion, etc

phospholipid structure

• hydrophilic head and hydrophobic tails

• constitute 50% of membrane mass in animal cells (5 × 10⁶ lipid mol/1 um² area of the bilayer)

• generic structure consists of:

  • choline, phosphate, glycerol and fatty acid tails (one has a kink due to cis-double bond)

what are the major phospholipid types in mammalian membranes?

• phosphatidylethanolamine

• phosphatidylserine

• phosphatidylcholine

• sphingomyelin

• sphingosine

which phospholipid types are neutral in charge?

• phosphatidylethanolamine

• phosphatidylcholine

• sphingomyelin

what phospholipid types are negative in charge?

phosphatidylserine

what phospholipid types are positive in charge?

sphingosine

cholesterol structure

• polar head group

• rigid steroid ring structure

• nonpolar hydrocarbon tail

what is the ratio of cholesterol to phospholipids in the cell membrane?

1 mol:1 mol

how does cholesterol act in a lipid bilayer?

• embedded in the lipid bilayer

• cholesterol-stiffened region are tightly packed

• decreases permeability to small water soluble molecules

• makes membranes less fluid at high concentration, prevents phase transition at low temperatures

what is phase transition?

when hydrocarbon tails come together and crystallize at low temperatures; easier to break membrane

how does bilayer formation occur?

spontaneous packing; phospholipids can form micelles or bilayers

favorable and unfavorable formations of phospholipids

planar phospholipid or planar w/ edges exposed to water

• not stable

• energetically unfavorable

sealed compartment formed by phospholipid bilayer

• aka the cell

• more stable

• energetically favorable

liposomes

• present in different sizes

• lipid bilayer is a 2D fluid

mobility of phospholipids w/in the lipid bilayer

• lateral diffusion→ L to R movement

• flexion→ side to side movement on the same side

• rotation→ spins

• flip-flop→ rarely occurs, from one side to another

fluidity of the membrane depends on:

• the movement of phospholipids

• composition and temperature

  • at high temperatures, the membrane is too fluid

  • at low temperatures, phase transition/crystallinity occurs

why do cells need fluidity?

cells need to withstand mechanical forces exerted on the body such as compression and flexion

the composition of a lipid bilayer determines its ____

fluidity

unsaturated hydrocarbon chains

• contains cis-double bonds which provides kinks to the hydrocarbon tails

• loosely packed

saturated hydrocarbon chains

• no double bonds

• tightly packed together

• greater width/height than unsaturated chains

what conclusions can be derived based on the lipid compositions of different membranes?

• cell membranes of different specialized cells have varying #s and compositions

• the different cell membrane compositions are due to different environments and functions of the cell

• eukaryotes are more complex than prokaryotes

• no cholesterol present in the prokaryotic membrane

formation of lipid droplets

• triacylglycerols and cholesterol esters accumulate in the droplet from the endoplasmic reticulum

• contains a surrounding phospholipid monolayer with associated proteins embedded

• appears as a form of exocytosis

• lipids in the blood must be regulated to prevent accumulation/deposits

why is the asymmetry of the lipid bilayer functionally important?

• for the conversion of extracellular signals into intracellular ones (ex: activation of protein kinase C requires negative charge for its activity)

• protein binding → requires the presence of sugars on the ECM side

• to distinguish between live and dead cells (apoptosis)

what are some characteristics of the components and orientation of the bilayer?

• no negatively charged phospholipid heads on the extracellular space side

• negatively charged amino acids are present on the cytosol side

• glycolipids are on the extracellular space side

glycolipids are found on the ______ of all eukaryotic plasma membranes

surface

glycolipids

• sugar containing lipids

• asymmetrical distribution in the membrane→ found only on the outside

• constitute 5% of lipids in outer monolayer

• gangliosides are the most complex→ 40 types found; predominant in the nerve cells

  • ex: galactocerebroside, G_M1 ganglioside, sialic acid (NANA)

    • the G_M1 ganglioside is the most complex, commonly found in neurons

what are the functions of glycolipids?

• protect membrane from harsh conditions (pH + digestive enzymes)

• cell recognition

• electrical conduction in nerves

• cell surface receptors for bacterial toxin

different types of membrane proteins

1,2,5— transmembrane proteins (single/multipass; barrel)

3,4— exposed at only one side of the membrane

6— anchored to the cytosolic surface by an amphiphilic a-helix

7— attached via a covalent bound lipid chain (ex: prenyl group)

8— attached via an oligosaccharide linker (glycosylphosphatidylinositol, GPI anchor)

9,10— membrane associated proteins (peripheral membrane proteins)

barrel membrane proteins contain _______

B-pleated sheets

which membrane proteins are not physically bound to the membrane?

membrane associated/peripheral proteins

different types of lipid anchors

• myristoyl anchor

  • amide linkage b/w terminal amino group and myristic acid

• palmitoyl anchor

  • thioester linkage b/w cysteine and palmitic group

• farnesyl anchor

  • thioether linkage b/w cysteine and prenyl group

in most transmembrane proteins, the polypeptide chain crosses the lipid bilayer in an _______ conformation

a-helical; the protein can be single or multi-pass

what is bacteriorhodopsin?

light sensitive protein in bacterial membrane

hydropathy index

score that measures the hydrophobicity and hydrophilicity of the membrane; hydrophobic regions are positive while hydrophilic regions are negative

aquaporin

specialized integral membrane proteins that serve as channels allowing water molecules to pass; contains 6 transmembrane domains

what does the formation of a multipass transmembrane protein do following its synthesis?

• displaces some phospholipids in the membrane

• hydrophilic amino acid regions assimilate together in the membrane

B-barrels

• exist in prokaryotes and eukaryotes

• forms large channels

• the larger the barrel, the wider it is and the more space it occupies

different types of B-barrels

• OmpA: receptor for a phage

• OMPLA: lipase

• porin: water transport

• Fep A: transports iron ions across the bacterial outer membrane

single pass glycosylated transmembrane protein

• contains inter and intrachain disulfide bonds, oligosaccharides, and sulfhydryl groups in the cytosol region

• single pass transmembrane a helix

what is the purpose of the carbohydrate layer (glycocalyx) surrounding a plasma membrane?

protects the cell from harsh environments

what is the carbohydrate layer of the membrane composed of?

• transmembrane glycoprotein

• adsorbed glycoprotein

• glycolipids

• transmembrane proteoglycan

detergents ______ and ______ membrane proteins

solubilize; purify

what does it mean to be solubilized?

to be soluble in solution (removed from membrane)

critical (detergent) micelle concentration — CMC

minimum concentration of detergent in a solution at which molecules begin to self-assemble into micelles

• below the CMC, detergent molecules dissolve independently or pack tightly on the liquid’s surface

• above the CMC, surfactant requires a higher concentration of molecules in solution before micelles can be formed

  • indicates that the detergent is easier to remove from the system

what does a detergent like sodium dodecyl sulfate (SDS) appear like?

a fatty acid

what are the 3 common types of detergents?

• sodium dodecyl sulfate (SDS)

• triton X-100

• B-octylglucoside

why are detergents important?

they can break membranes apart; hydrophobic ends of detergents bind to hydrophobic regions of membrane proteins and displace lipid molecules

1. detergent monomers w/ hydrophobic tails and hydrophilic heads can form detergent micelles

2. separates proteins from phospholipids in the bilayer by creating water-soluble protein-lipid-detergent complexes and micelles

how is the Na⁺/K⁺ pump isolated?

reconstituting functional membrane protein systems

1. detergent micelles and monomers are added to lipid bilayer

2. forms solubilized membrane proteins, lipid-detergent micelles

3. Na⁺/K⁺ pump is purified by adding phospholipids (mixed w/ detergent) and removing the detergent from it

4. bilayer is recreated w/ just the Na⁺/K⁺ pump incorporated into phospholipid vesicle

   • helps researchers study the function of the pump

many membrane proteins _____ in the plane of the membrane

diffuse;

• in the presence of a human/mouse hybrid cell there is diffusion of plasma membrane proteins w/ time

• lateral movement of mouse cell and human cell proteins

how is the rate of lateral diffusion of membrane fluorescent protein measured after photobleaching?

• the fluorescence goes away in the bleached area

• over recovery time, the fluorescence comes back due to protein movement

• some intensity is reached, not the same as before bleach

cells can confine proteins and lipids to specific ______ w/in a membrane

domains

protein and lipid domains of membrane

• localizes membrane proteins to 1 side; limiting its function

• in a tight junction, 2 cells are attached and nothing (proteins) can go through them

protein domains in sperm

• the components of a sperm are:

  • anterior head

  • posterior head

  • tail

• each segment of the sperm was stained for a different type of protein

• labelled w/ antibodies for specific proteins of a region

  • ex: egg penetrating proteins on the head, motor proteins on tail for mobility

four ways of restricting membrane protein movement

1. protein self-assembly into large aggregates

   • proteins are clumped together in specific parts of the cell

2. tethered by interactions w/ assemblies of macromolecules outside the cell

   • ex: bound to extracellular matrix protein such as collagen or fibroskeleton

3. tethered by interactions w/ assemblies of macromolecules inside the cell

   • ex: membrane protein is bound to a cytoskeleton protein

4. interact w/ proteins on the surface of another cell

   • protein complexes are stuck to one another on adjacent cells

what does the cytoskeleton provide to the membrane?

• mechanical strength; resistance to compressive forces

• restricts membrane protein diffusion

glycophorin

transmembrane protein that contains a junctional complex consisting of:

• actin

• spectrin/spectrin dimer

• adducin

• tropomyosin

3 ways in which membrane-bending proteins shape membranes

1. hydrophobic region of the protein can insert as a wedge into a lipid monolayer

2. curved surface of a protein can bind to lipid head groups and deform the membrane or stabilize curvature

3. protein binding causing lipid clustering→ large head groups→ thereby bending the membrane