the cell

fluid mosaic model

membranes are semi-fluid, not static

calpain

a protease activated by calcium, its secretion leads to inflammation and its activation can promote apoptosis

asymmetry of lipid bilayers

two layers have different lipid composition

transverse diffusion (flip-flop)

when a phospholipid moves from one layer to the other

phospholipids translocators - flipases:

• catalyse the flip-flop process

• flipases help maintain unequal distribution = membrane asymmetry

significance of ER in membrane synthesis

asymmetric distribution of lipids, proteins and carbon determined by membrane’s synthesis and modification by the ER

lipids of the membrane

phospholipids - phosphoglycerides and sphingolipids

glycolipids

cholesterol

fuel for metabolism in membranes

triacylglycerides (1 glycerol, 3 fatty acids) - store a large amount of energy efficiently

signalling molecules for membranes

steroid hormones (sex hormones + cortisol)

eicosanoids (involved in pain, inflammation)

difference between triacylglycerides and phosphoglycerides

triacylglycerides - glycerol + 3 fatty acids

phosphoglycerides - glycerol + 2 fatty acids + phosphate + hydroxyl group

phospholipids structure

amphiphilic/ amphipathic ( = has both properties)

head - hydrophilic/ water loving

fatty acid chains - hydrophobic/ water fearing

fatty acid structure

long hydrocarbons of 14-24 carbons

saturated/ unsaturated

• unsaturated = double bonds trans (almost straight chain), cis (kink in chain)

melting point increases with length

melting point decreases with increasing number of double bonds

C=C introduces a kink in the chain SO irregular packing

irregular packing = lower melting point, more fluidity

arachidonic acid

synthesised from linoleum acid

functions as part of phospholipids in membrane

precursor for eicosanoids - plays important role in inflammation

eicosanoids in role of pain and inflammation

inflammatory signal > membrane releases arachidonic acid > enzymes convert it to eicosanoids and prostaglandins > prostaglandins and eicosanoids trigger inflammation and pain responses

aspirin and ibuprofen inhibit the production of prostaglandins and eiconsanoids

common head groups in phospholipids

fatty acids + glycerol + phosphate + head group

serine

~choline

~ethanolamine

~linositol

~glycerol

functions of head groups

phosphatidylcholine + phosphatidylinositol - head groups can be cleaved

• inositol and choline = important signalling molecules

sphingolipids

based on the amino alcohol sphingosine

important for:

• membrane structure

• cell signalling

• nerve insulation

• cell recognition

sphingomyelin

a type of sphingolipid found in cell membrane - especially nervous system

part of myelin sheath

significance of sphingomyelin

important in signal transduction and apoptosis

myelin sheath increases speed of electrical impulse

demyelination = disease (multiple sclerosis)

glycolipids

sugar instead of phosphate group

animal cells; derived from sphingosine not glycerol

functions: immune responses, cell-cell recognition, attachment

cholesterol

sterol - modified steroid (type of lipid made of four fused carbon rings)

effects membrane fluidity

planar structure

glycans

recognition and signalling

proteins - glycoproteins

lipids - glycolipids

importance of membrane fluidity

lipids diffuse laterally

proteins not involved in anchoring also diffuse

transport across by diffusion/ via transporter

vesicles need to bud off and fuse

how to measure rate of lateral diffusion (FRAP)

FRAP - fluorescence recovery after photobleaching

membrane with flourophores

intense light bleaches flourophores (destroying fluorescence there)

unbleached fluorescent molecules from nearby areas move into bleached spot by lateral diffusion

rate of diffusion of flourophores can be measured

lipid movement in the membrane

rotation + flexion

lateral diffusion

transverse diffusion (flip-flop)

membrane fluidity

temperature DECREASE, fluidity DECREASE

• lipid molecules move slowly

• membrane becomes less permeable

temperature INCREASE, fluidity INCREASE

• lipid molecules move faster

• membrane becomes more permeable

too fluid = membrane is disordered, too much permeability

too solid = gel slows down movement too much

how does lipid composition affect membrane fluidity

increases fluidity:

• unsaturated lipids giving kinks

• short chains allowing fewer interactions between lipids

decreases fluidity:

• saturated chains

• long chains

how do bacteria/plants regulate their lipid composition

bacteria = temperature sensitive proteins that help adapt membrane composition

plants = sensors in plasma membrane that detect changes in fluidity, fluidity increases indicates temperature is increasing - allows plant to prepare for heat stress

cholesterol structure

sterol lipid

• 4 fused hydrocarbon rings - rigid, non-polar

• hydroxyl group - polar, hydrophilic

• short hydrocarbon tail - more fluid, non polar, hydrophobic

increases fluidity in middle of membrane (flexible tail)

decreases fluidity at edge of membrane (steroid ring)

ethanol affect on membrane fluidity

increases membrane fluidity

membrane proteins

integral membrane proteins traverse (cross) membrane

peripheral membrane proteins associated with membrane face

proteins bind to surface of integral proteins

proteins covalently anchored to the membrane

integral membrane proteins

1. single span hydrophobic a-helix

2. multi-spanning containing a-helixes

3. B-barrel protein forming a pore

how is membrane topology maintained

by hydrophobic and electrostatic interactions

• pos charged AA interact with neg charged lipid head groups

glycosylation of integral membrane proteins

glycosylation = attachment of carbohydrate chains (glycans) to proteins

integral membrane proteins = usually glycosylated, carbohydrate chains found on the extracellular side of the membrane

ICAM

involved in cell-cell adhesion

expressed on cells of the immune system and endothelial cells

unregulated during inflammation

bacteriorhodopsin

light causes conformation change in retinal

pumps protons from th ecytso to extracellular space

proton gradient used for photosynthesis

how can hydrophobicity predict secondary structure

hydrophobic amino acids, placed where they can interact with hydrophobic environments (lipid membranes) - transmembrane a-helix

hydrophilic amino acids prefer water regions

porins

barrel shaped structure with pore in centre

• allows bacteria to take up sucrose

peripheral membrane proteins

• don’t transverse the membrane

• proteins - cytoplasmic or exoplasmic

• interact with lipid head groups and integral membrane proteins

• non-covalent interactions = electrostatic interactions, H-bands, vow bonds

• proteins anchored to the membrane through hydrocarbon groups

• protein is covalently attached to a hydrocarbon group

• hydrophobic hydrocarbon group inserts into the lipid bilayer

palmitoylation

lipid modification where a fatty acid (palmitic acid) is covalently attached to a protein

ankyrin

spectrin

spectrin = cytoskelotn proteins creating a scaffold on intracellular side of membrane

ankyrin = binds to several integral membrane proteins AND binds to spectrin

maintains plasma membrane integrity via the spectrinf-actin based cytoskeletal structure

cells covered in carbohdyrates

• found on the exoplasmic side of membranes

• attached to both glycolipids and glycoproteins

  • glycocalyx = network of glycoproteins with mucus like consistency

• carbohydrates also acts as a physical barrier (mechanosensing - ability to respond to mechanical forces )

membrane carbohydrates: structure

carbohydrate units exist as oligosaccharide chains or single sugar residues

• glycoproteins = oligosaccharide chains

• glycolipids = single sugar residues

membrane carbohydrates: function

cell-cel recognition, communication, adhesion

important in immune response: i