Membrane Bilayer and permeability Flashcards Preview

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Flashcards in Membrane Bilayer and permeability Deck (40)
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0
Q

Describe membrane composition

A

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

1
Q

Give some general functions of membranes

A

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

2
Q

Describe the structure of phospholipids

A

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

3
Q

What movements can phospholipids undergo?

A

Flexion
Rotation
Flip flop
Lateral diffusion

4
Q

Describe the structure of glycolipids

A

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

5
Q

Describe the structure of cholesterol

A

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

6
Q

What is the function of cholesterol in membranes?

A

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)

7
Q

What is the evidence for protein in membrane bilayers?

A

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

Biochemically - membrane fractionation, SDS page and electrophoresis

8
Q

What movements can proteins undergo?

A

Conformational change
Rotation
Lateral diffusion
(NO FLIP FLOP)

9
Q

What restricts the movement of proteins?

A

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

10
Q

Describe differences between peripheral and integral proteins

A

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.

11
Q

Describe the general structure of an integral protein

A

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

12
Q

Describe the mechanism of membrane insertion of integral proteins

A

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.

13
Q

Why are hydropathy plots useful?

A

Can see how many transmembrane domains the protein has

14
Q

Why is membrane asymmetry important?

A

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

15
Q

Describe the main features of the fluid mosaic model

A

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

16
Q

Describe the structure and function of the erythrocyte cytoskeleton

A

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)

17
Q

Describe Hereditary Sherocytosis

A

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

18
Q

Describe Hereditary Elliptocytosis

A

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

19
Q

What properties of solutes affect their movement through the membrane?

A

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

20
Q

Distinguish passive diffusion, facilitated diffusion and active transport

A

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,

21
Q

Describe features of channel proteins

A

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
22
Q

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

A

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

23
Q

NaK-ATPase

A

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

24
Q

K Channels

A

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

25
Q

Ca-ATPases

A

Control resting Ca concentration

Uses ATP to pump ions (active transport)

26
Q

What are the 2 types of Ca-ATPases?

A

PMCA = Plasma Membrane Ca-ATPase

SERCA = Sarco/Endoplasmic Reticulum Ca-ATPase

27
Q

PMCA

A

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

28
Q

SERCA

A

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

29
Q

NaCa-exchanger (NCX)

A

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

30
Q

Describe the action of NCX in Ischaemia

A

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

31
Q

Which ion transporters contribute to regulating pH?

A

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

32
Q

NHE

A

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

33
Q

NBC

A

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

34
Q

Anion Exchanger (AE)

A

Cl/HCO3-exchanger

Removes base from cell, decreases pH, regulates cell volume

35
Q

Outline how ion transport regulates cell volume

A

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

36
Q

Outline bicarbonate reabsorption by the proximal tubule

A

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

37
Q

How are transporters affected in cystic fibrosis?

A

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.

38
Q

How are transporters affected in diarrhoea?

A

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

39
Q

Outline renal anti-hypertensive therapy

A

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)