Membrane Bilayer and permeability

Question 0

Describe membrane composition

Answer 0

40% lipid, 60% protein, 1-10% carbohydrate

Question 1

Give some general functions of membranes

Answer 1

Continuous, highly selective permeability barrier
Control of enclosed chemical environment
Recognition of signalling molecules, adhesion proteins, immune surveillance
Signal generation in response to stimuli

Question 2

Describe the structure of phospholipids

Answer 2

Glycerol, 2 fatty acids, phosphate-head group

Range of polar head groups eg. choline, amines, amino acids, sugars
Length of FAs is between C14 and C24, mostly C16-C18
Cis double bond gives kink in unsaturated chains to reduce phospholipid packing, increasing fluidity and reduce ability to form 2D crystals

Question 3

What movements can phospholipids undergo?

Answer 3

Flexion
Rotation
Flip flop
Lateral diffusion

Question 4

Describe the structure of glycolipids

Answer 4

Glycolipids are lipids containing a sugar (not glycerol)

In CEREBROSIDES the head group is a sugar monomer
In GANGLIOSIDES the head group is an oligosaccharide

Question 5

Describe the structure of cholesterol

Answer 5

Has a polar hydrophilic head group
Rigid planar steroid ring structure
Non-polar hydrocarbon tail

Question 6

What is the function of cholesterol in membranes?

Answer 6

It abolishes the endothermic phase transition phospholipids by forming hydrogen bonds with glycerol and altering chain motion:

It reduces fluidity at high temps (reduced chain motion)
It increases fluidity at low temps (reduced packing)

Question 7

What is the evidence for protein in membrane bilayers?

Answer 7

Functionally - facilitated diffusion, ion gradients, specificity of cell responses

Biochemically - membrane fractionation, SDS page and electrophoresis

Question 8

What movements can proteins undergo?

Answer 8

Conformational change
Rotation
Lateral diffusion
(NO FLIP FLOP)

Question 9

What restricts the movement of proteins?

Answer 9

Aggregation - slows movement
Tethering to BM or cytoskeleton - allows no movement
Interaction with other cells
Lipid mediated effects - proteins tend to separate out into the fluid phase, or cholesterol poor regions

Question 10

Describe differences between peripheral and integral proteins

Answer 10

Peripheral are bound to surface of membrane by hydrogen bonds and electrostatic interactions. Can be removed by changes in pH or ionic strength.
Integral are embedded completely in the membrane, cannot be removed pH or ionic strength, removed by detergents and organic solvents.

Question 11

Describe the general structure of an integral protein

Answer 11

R-groups of amino acids in the transmembrane domains are largely hydrophobic
Often alpha-helical
18-22 amino acids span bilayer
Can have a single or multiple transmembrane domains

Question 12

Describe the mechanism of membrane insertion of integral proteins

Answer 12

When the protein is being translated into the ER lumen the ribosome comes across a highly hydrophobic STOP TRANSFER SIGNAL.
This remains in the ER membrane and the rest of the protein is translated outside of the ER in the cytoplasm.
Proteins with multiple transmembrane domains contain multiple stop and start transfer signals.

Question 13

Why are hydropathy plots useful?

Answer 13

Can see how many transmembrane domains the protein has

Question 14

Why is membrane asymmetry important?

Answer 14

Asymmetrical orientation is important for function eg. A receptor for a hydrophilic extracellular messenger molecule must have its recognition site directed towards the extracellular space

Question 15

Describe the main features of the fluid mosaic model

Answer 15

Fluid - because of the hydrophobic integral components that can move laterally throughout the membrane, meaning it is not solid

Mosaic- because it is made up of many different parts

Question 16

Describe the structure and function of the erythrocyte cytoskeleton

Answer 16

Holds the shape of erythrocytes, and attachment of integral proteins to the cytoskeleton restricts lateral mobility of the proteins.

It is composed of SPECTRIN and ACTIN molecules, which are attached to the membrane by adapter proteins ANKYRIN (band 3) and GLYCOPHORIN (band 4.1)

Question 17

Describe Hereditary Sherocytosis

Answer 17

Spectrin depleted by 40-50%
Erythrocytes round up, become more spherical and increased lysis of the cells reduces the lifespan of RBCs
The inability of the bone marrow to compensate leads to haemolytic anaemia

Question 18

Describe Hereditary Elliptocytosis

Answer 18

Defect in spectrin means that the molecules are unable to form heterotetramers
Erythrocytes round up and become ellptical
Increased lysis and inability of bone marrow to compromise leads to haemolytic anaemia

Question 19

What properties of solutes affect their movement through the membrane?

Answer 19

Hydrophobic molecules can pass through
Small uncharged polar molecules can pass through
Large uncharged polar molecules cannot pass through
Ions cannot pass through

Question 20

Distinguish passive diffusion, facilitated diffusion and active transport

Answer 20

Passive: dependent on permeability and concentration/electric potential gradient to provide energy, rate increases linearly with increasing concentration gradient.
Facilitated: permeability for substance increases by incorporating specific protein into bilayer, may be carrier molecules or protein channels
Active transport: movement agains conc/electric gradient requiring energy,

Question 21

Describe features of channel proteins

Answer 21

May be gated and open and close in response to stimulus.
More than one type of molecule maybe transported: Co-transporters

Uniport= single transported molecule, one direction
Symport= two transported molecules, one direction
Antiport= two transported molecules, opposing directions

Question 22

What determines whether movement across the membrane is passive or active?

Answer 22

Whether or not energy is required is determined by the free energy change of the transported species, this is determined by the concentration gradient and electrical gradient across the membrane

Question 23

NaK-ATPase

Answer 23

Uses ATP to pump 2K in and 3Na out (Antiport) forming the Na and K gradients that are necessary for excitation.
P-type ATPase as ATP phosphorylates aspartate producing a phosphoenzyme intermediate
Alpha subunit- binding sites for ions
Beta subunit- directs pump onto the surface
Drives secondary active transport for control of pH, cell volume, Ca concentration, Na absorption in epithelia, nutrient uptake

Question 24

K Channels

Answer 24

Na pump creates high intracellular K, and K diffusion through channels down this concentration gradient is responsible for the membrane potential

Question 25

Ca-ATPases

Answer 25

Control resting Ca concentration

Uses ATP to pump ions (active transport)

Question 26

What are the 2 types of Ca-ATPases?

Answer 26

PMCA = Plasma Membrane Ca-ATPase

SERCA = Sarco/Endoplasmic Reticulum Ca-ATPase

Question 27

PMCA

Answer 27

Uses ATP to expel Ca from the cell in exchange for H (Antiport)
High affinity, low capacity
Removes residual Ca

Question 28

SERCA

Answer 28

Uses ATP to transport Ca into the sarco/endoplasmic reticulum in exchange for H (antiport)
High affinity, low capacity
Removes residual Ca

Question 29

NaCa-exchanger (NCX)

Answer 29

Expels I Ca from the cell in exchange for 3Na (antiport)
Uses the Na conc set up by the NaK-ATPase (secondary active transport)
Low affinity, high capacity. Expels Ca during cellular recovery ie. after contraction
Electrogenic
Activity is membrane potential dependent

Question 30

Describe the action of NCX in Ischaemia

Answer 30

ATP is depleted in Ischaemia so Na pump is inhibited and Na accumulates in the cell
Cell becomes depolarised so NCX reverses: Na moves out and Ca moves in
Intracellular Ca increases = toxic

Question 31

Which ion transporters contribute to regulating pH?

Answer 31

Acid Extruders: Na-H-Exchangers (NHE), Sodium Bicarbonate Co-transporter (NBC)
Base Extruders: Anion Exchanger

Question 32

NHE

Answer 32

Exchanges extracellular Na for intracellular H
Uses Na conc set up by NaK ATPase (secondary active transport)
Raises intracellular pH, regulates cell volume
Activated by growth factors, inhibited by amiloride

Question 33

NBC

Answer 33

Na-dependent-Cl/HCO3-exchanger
Acid out, base in
Uses Na gradient set up (secondary active transport)
Raises intracellular pH and regulates cell volume

Question 34

Anion Exchanger (AE)

Answer 34

Cl/HCO3-exchanger

Removes base from cell, decreases pH, regulates cell volume

Question 35

Outline how ion transport regulates cell volume

Answer 35

Osmotically active ions ( K, Na,Cl, amino acids) are transported in/out of cell and water follows causing swelling or shrinkage.
No standard method, different cell types use different transporters to achieve the regulation they need

If cell is swelling extrude ions, if cell is shrinking influx ions

Question 36

Outline bicarbonate reabsorption by the proximal tubule

Answer 36

Na/K transporter keeps intracellular Na low to drive other channels
NHE pumps H into the proximal tubule lumen
H picks up bicarbonate and brings it back into the cell

Question 37

How are transporters affected in cystic fibrosis?

Answer 37

Transport of Na by Na/K exchanger allows for the symport of 2Cl into the cell with Na and K
In CF, faulty CFTR protein leads to high intracellular Cl, low extracellular Cl. Water moves in via osmosis giving thick, viscous mucus in the lumen.

Question 38

How are transporters affected in diarrhoea?

Answer 38

CFTR is overly active once phosphorylated by protein kinase A.
Excessive transport of Cl into lumen, water follows giving symptoms of diarrhoea

Question 39

Outline renal anti-hypertensive therapy

Answer 39

The goal is to reduce the re uptake of Na so less water is re absorbed by osmosis, blood volume and blood pressure decreases.

Loop diuretics- block Na reuptake in the thick ascending limb
Amiloride- acts in the DCT and PCT to prevent Na reuptake
Spironolactone- is a glucocorticoid receptor agonist used to treat high aldosterone (as aldosterone up regulates Na transporters)