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M&R Flashcards

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0
Q

Describe the dry composition of biological membranes

A

40% lipids
60% protein
1-10% carbohydrate

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1
Q

What are the main functions of biological membranes? (6)

A

Communication
Recognition
Signal generation in response to stimuli
Highly selective permeability barrier
Allows control of enclosed chemical environment
Energy conservation by oxidative phosphorylation

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2
Q

What percentage of a hydrated bilayer is made of water?

A

20%

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3
Q

Describe the structure of a phospholipid

A

Head group - polar e.g. choline, amines, amino acids, sugars
Fatty acid chains:
- varied length (C16 and C18 most common)

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4
Q

What is the effect of an unsaturated fatty acid side chain in the cis conformation?

A

Produces a kink in the chain, which reduces phospholipid packing thus increases membrane fluidity

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5
Q

What is sphingomyelin?

A

The only phospholipid not based on glycerol, but its conformation still resembles other phospholipids

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6
Q

What is a glycolipid?

A

Sugar-containing lipid

Where phosphate head is replaced with a sugar

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7
Q

What are the two types of glycolipid?

A

Cerebrosides - head group is a sugar monomer

Gangliosides - head group is an oligosaccharide

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8
Q

What type of structure does a fluid membrane have and which forces stabilise it?

A

Co-operative structure

Non-covalent forces, electrostatic interactions and H bonds between hydrophilic moieties

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9
Q

What are the 4 modes of mobility within a lipid bilayer?

A

Flexion - kink formation in fatty acid chain
Axial rotation
Lateral diffusion - within plane of bilayer
Flip-flop - movement of molcs from one half of bilayer to the other (1-for-1 exchange)

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10
Q

How does cholesterol contribute to membrane stability?

A

Hydrogen bonds to fatty acid chains
Abolishes endothermic phase transition
Reduces phospholipid packing (increases memb fluidity)
BUT reduces phospholipid chain motion (decreases memb fluidity)

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11
Q

Explain the functional evidence for membrane proteins

A

Facilitated diffusion
Ion gradients
Specificity of cell responses

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12
Q

Explain the biochemical evidence for membrane proteins

A

Membrane fractionation and gel electrophoresis (SDS-page)

Freeze fracture

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13
Q

Describe how integral proteins associate with the lipid bilayer

A

Deeply imbedded
Interact with hydrophobic regions
Require detergent/organic solvent to be removed (competes for non-polar interactions)

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14
Q

Describe how peripheral proteins interact with the lipid bilayer

A

Associated with surface
Interact via electrostatic interactions and hydrogen bonds
Removed by changes in pH or ionic strength

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15
Q

Name the three modes of membrane protein movement

A

Conformational change
Rotation
Lateral diffusion
CANNOT FLIP-FLOP - have large hydrophilic regions so large amounts of energy would be needed for them to pass through hydrophobic part of bilayer

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16
Q

What restricts membrane protein mobility?

A

Lipid-mediated effects - proteins tend to separate out into fluid phase or cholesterol-poor regions
Membrane protein associations
Association with peripheral/extra-membranous proteins (e.g. cytoskeleton)

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17
Q

Describe how membrane proteins are inserted into membranes (i.e the secretory pathway)

A

Protein synthesis on free ribosomes
Signal sequence produced at N-terminal
SRP recognises and binds to signal sequence
Protein synthesis stops
SRP (GTP-bound) directs protein to SRP receptors on cytosolic face of ER
SRP dissociates, GDP and Pi released
Protein synthesis continues as new polypeptide is fed into ER lumen via a pore in the membrane (peptide translocation complex)
Signal sequence cleaved by signal peptidase
Dissociation of ribosome
Protein folds spontaneously, disulphide bonds form
N-linked glycosylation via dolichol

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18
Q

What is the difference between synthesis of membrane proteins and secretory proteins?

A

Membrane proteins need to span the membrane of the vesicle rather than be contained within in

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19
Q

How is correct orientation of membrane proteins ensured?

A

Directed by membrane synthesis
Addition of a highly hydrophobic stop transfer signal (18-20aas long - distance to span a phospholipid bilayer)
When membrane protein is being translocated into ER lumen, ribosome comes across stop transfer signal
Stop transfer signal remains in ER membrane and rest of protein is translated outside of ER in cytoplasm
Therefore protein spans membrane

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20
Q

How are erythrocyte membranes prepared for analysis

A

Ghosts prepared by osmotic haemolysis
Run on gel to show >10 major proteins
Most are peripheral (released by PH or ionic change)
Found on cytosolic face of membrane

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21
Q

Which two components make up the cytoskeleton?

A

Spectrin and actin

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22
Q

Describe the structure of spectrin

A

Long rod-like molecule
Alpha and beta subunits wound together as a heterodimer
Two heterodimers form a heterotetramer

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23
Q

How is the spectrin-actin network formed?

Study These Flashcards
A

Spectrin rods cross linked into networks by short actin profilaments, band 4.1 and adducin molcs

24
How is the spectrin-actin network attached to the membrane?
Attached to membrane proteins via adapter proteins Ankyrin binds to Band 3 Band 4.1 binds to glycophorin A
25
Which membrane protein movement does attachment to the skeleton restrict?
Lateral
26
Explain hereditary spherocytosis
``` Spectrin levels 40-50% depleted Erythrocytes become spherical Cells less resistant to lysis (more subject to shearing forces in capillary beds) Shortened life span Bone marrow cannot compensate ```
27
Explain hereditary elliptocytosis
Defect in spectrin molcs - unable to form heterotetramers Results in fragile elliptoid cells Shortened life span Bone marrow cannot compensate Treated with cytochalasin drugs to cap growing end of polymerising actin filaments
28
Which types of molecules CAN diffuse across membranes?
Hydrophobic - O2, CO2, N2, benzene | Small uncharged polar molcs - water, urea, glycerol, ethanol
29
Which types of molcs CANNOT diffuse across membranes?
Large uncharged polar molcs - glucose, sucrose | Ions - Na+, K+, HCO3-, H+
30
Why do membranes act as a permeability barrier to large hydrophilic molecules and ions?
A large free energy change would be required for them to cross the hydrophobic core of the membrane
31
Describe passive diffusion
Molecules move down their concentration gradient Dependent on permeability and concentration gradient Rate increases linearly with increasing concentration gradient
32
Describe facilitated diffusion
Type of passive transport Molecule moves from one side of memb to the other with help of a specific carrier molc ('ping-pong' transport) or protein/ion channel incorporated into bilayer Increases rate and capacity for transport Saturable (finite no. receptors in memb)
33
Describe active transport
Allows transport of molcs and ions against an unfavourable chemical/electrical gradient Requires energy from hydrolysis of ATP (can be direct or indirect)
34
What determines whether energy is required for transport?
Free energy change (deltaG) of transported species | This is determined by conc gradient and electrical potential across memb (if species is charged)
35
What is a uniport?
Transports a single ion/molecule
36
What is a symport?
Transports 2 ions/molcs in the same direction
37
What is an antiport?
Transports 2 ions/molcs in opposite directions
38
Describe the sodium-potassium-ATPase pump
3Na+ in/2K+ out Assoc with plasma membrane Active - energy provided by ATP hydrolysis P-type ATPase Present in all cells 2 subunits: - alpha carries out activity by providing binding sites - beta is a glycoprotein that targets pump to membrane surface
39
Describe the functions of the sodium-potassium-ATPase
``` Forms Na+ and K+ gradients necessary for electrical excitability Drives secondary active transport: - pH control - ion homeostasis - regulation of cell volume - nutrient uptake - regulation of Ca2+ concentration ```
40
What inhibits the sodium-potassium pump?
Ouabain binds to alpha subunit
41
Describe the plasma membrane Ca2+ ATPase (PMCA)
Ca2+ out, H+ in Active - uses energy from ATP Antiport High affinity, low capacity - removes residual calcium
42
Describe the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA)
``` Ca2+ into SR/ER, H+ out into cytoplasm Builds intracellular store of calcium Uses ATP Antiport High affinity, low capacity - removes residual calcium ```
43
Describe the sodium-calcium exchange (NCX)
3Na+ in, Ca2+ out Secondary active transport - uses Na+ conc gradient set up by Na/K+-ATPase Antiport Low affinity, high capacity - removes majority of calcium Electrogenic (changes potential of cell) - current flows in direction of N-+ gradient Activity is dependent on membrane potential - direction reversed by depolarisation
44
Explain the behaviour of NCX in ischaemia
``` ATP depleted - Na/K-ATPase inhibited Na+ accumulates in cell NCX reverses Na+ moves out, Ca2+ moves in High Ca2+ is toxic to cell ```
45
Which transporters are acid extruders?
NHE | NBC
46
Which transporter is a base extruders?
AE
47
Describe the sodium-hydrogen exchange (NHE)
``` Na+ in, H+ out Electroneutral Secondary - uses Na+ conc gradient set up by Na/K+-ATPase Alkalinises cell (acid extruder) Regulates cell volume Activated by growth factors Inhibited by amiloride ```
48
Describe the NBC
``` Na+ and HCO3- in H+ and Cl- out Alkalinises cell (acid extruder) Secondary - uses Na+ conc gradient set up by Na/K-ATPase Helps regulate cell volume ```
49
Describe the anion exchange (AE)
Cl- in HCO3- out Acidifies cell (base extruder) Helps regulate cell volume
50
What is the membrane potential?
Electrical potential difference across the membrane
51
How is the resting membrane potential measured?
Using a microelectrode that penetrates the cell membrane | Expresses inside relative to outside
52
What is the resting potential of a nerve cell?
-70mV
53
What is the resting membrane potential of cardiac/skeletal muscle?
-80 to -90mV
54
How is the resting potential set up?
Selective permeability to potassium K+ channels are open at rest, creating an: - Outward chemical diffusion gradient - inward electrical gradient When these gradients are equal and opposite there is no net movement of ions (but negative membrane potential)
55
Define equilibrium potential
Membrane potential at which there is no net movement of that ion across the membrane (conc gradient = electrical gradient)
56
What is the Nernst equation?
Used to calculate equilibrium potential At 37C E= (61/z) log10(conc out/conc in)
57
What is depolarisation and what causes it?
Decrease in size of membrane potential Inside of the cell becomes less negative Caused by opening of Na+ or Ca2+ channels
58
What is hyperpolarisation?
Increase in size of membrane potential Inside of the cell becomes more negative Caused by opening K+ or Cl- channels

ESA2 (5 decks)

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