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What is the function of the PM?

Defines boundaries of cell and differences between extracellular and intracellular responses


What is an organelle membrane?

Defines the boundaries of organelle and differences between intra and extra components and contents of organelle


Describe the structure of the PM

Very thin film of lipid and protein molecules
Phospholipid bilayer is common structural unit, provides basic structure
Highly dynamic, fluid structure


What is the function of the dissolved proteins in the lipid bilayer?

Mediate most of the other functions of the cell


What are the functions of the PM?

Maintain structural integrity and barrier function
Define shape
Control exchange
Site of chemical reactions (oxidative phosphorylation)
Site of ligand recognition
Cell-cell recognition
Facilitate cellular locomotion


What are the main components of the PM?

Lipids: phospholipid, cholesterol, glycolipid
Proteins: transmembrane, peripheral, integral


What is meant by amphipathic?

Has both polar (hydrophilic) and non-polar (hydrophobic) regions i.e. phospholipids


What determines if phospholipids will for a micelle of bilayer?

Number of tails
1 tail: form circular micelle
2 tails: lipid bilayer


Describe the movement of lipids within the bilayer

Bilayer is highly dynamic
Lipids can:
move, diffuse freely within bilayer
readily exchange place with neighbour in monolayer
rotate around long axis
hydrocarbon chain flexion
RARELY swap side of monolayer


What determines the fluidity of the bilayer?

Composition and temperature


What 2 lipids are usually highly expressed in bilayer?

Cholesterol and glycolipids


Describe the structure of cholesterol

Rigid molecule of 4 hydrophobic steroid rings interacts with fatty acyl chains of membrane phospholipids


What is the function of cholesterol?

Highly expressed: up to 1 for every phospholipid to v important
Enhances permeability-barrier properties of the bilayer - tightly bound to phospholipid making membrane less soluble to v small water-soluble molecules


Describe the effect of temp on cholesterol

At physiological temp. cholesterol limits fluidity of membrane
At lower temp. prevent membrane becoming less fluid by preventing hydrocarbon chains binding each other


What are lipid rafts and what are their functions?

Micro-domain of PM rich in cholesterol and sphingolipids
Help organise proteins for transport in small vesicles or enable to function together


Explain the asymmetry of the bilayer

Outer layer of RBC have choline group
Inner layer have terminal primary amino group


What is the importance of the difference in symmetry?

Cytosolic proteins bind to specific lipid head groups thus different membranes won't bind same proteins


What are glycolipids?

Sugar-containing lipids, found only in outer layer
Thought to partition into lipid rafts, self associate into micro-aggregates by forming H bonds with each other


What is the importance of sugar group exposure?

On surface important for interactions of cell with surroundings


Describe the 3 main types of membrane protein

Transmembrane: extend across bilayer, domains exposed both intra and extracellularly
Integral: exposed only on 1 side, usually tightly associated with membrane by lipid group
Peripheral: linked via non-covalent bonds with MP, easily released


Give examples of transmembrane, peripheral proteins

Transmembrane multipass: band 3 in RBC
Single pass: glycophorin
Peripheral: spectrin 1/4 of MP mass


What are glycoproteins?

Oligosaccharide chains bound to MPs (glycoproteins) and lipids (glycolipids)
Can also occur as polysaccharides bound to protein core forming proteoglycan (lots of sugar, little protein)


Describe the carb layer on the bilayer

Carbohydrate chains of glycolipids, glycoproteins and proteoglycans surround cell in thick coat allowing for protein binding


Why is the PM selectively permeable?

Retain barrier to EC environment
Ensure essential molecules: lipid, glucose, AAs enter cells
Maintain ionic gradient
Intracellular organelles can also have selectively permeable membrane


What is simple diffusion?

Diffusion of small molecules down conc. gradient without aid of MPs


What is the importance of membrane transport proteins?

Allow passage of polar molecules into cell
Each transport only transports particular class of molecule, usually certain molecular species


What are carrier and channel proteins?

Carrier: bind specific solute, undergo conformational change to transfer solute across membrane
Channel: weakly associated, form aqueous pores when open allow specific solutes to cross membrane


What is passive diffusion?

Movement of molecules down conc. gradient without using energy usually just the conc. gradient dictates
If charged solute, both MP and conc. gradient (electrochemical gradient) will influence diffusion


What is active transport?

Movement of solutes up conc. gradient requiring energy
Carrier proteins can be active or passive, active tightly coupled to source of metabolic energy (ATP hydrolysis, ion gradient) and is directional


Define uniport, symport, antiport

Uniport: passive, 1 solute in 1 direction
Symport: active, 2 solutes in same direction
Antiport: active, 2 solutes in opposite direction (uses energy of 1 going down gradient to push other up)