Lecture 11 - Membrane structure and function

Question 1

Membrane lipids (7)

Answer 1

Lipid bilayer.
Majority of the lipids are phospholipids.
Hydrophobic head and hydrophilic tail.
Phospholipid and glycolipids are amphiphatic.
Readily form biomolecular sheets aqueous media.
Form lipid bilayer (liposomes)/micelles.
Non-covalent assemblies.

Question 2

Membrane asymmetry (6)

Answer 2

SM = Spingomyelin.
PC = Phosphatidylcholine.
PS = Phosphatidylserine.
PE = Phosphatidylethanolamine.
PI = Phosphatidylinositol.
CI = Cholesterol.

Question 3

Fluid mosaic model (3)

Answer 3

1972 Singer and Nicolson.
Proposed a model that allowed the passage of moleules through the membranes.
Mosaic patterns.

Question 4

Intrinsic /Integral proteins (6)

Answer 4

Transmembrane proteins, on both side of the membrane.
Have amino acids with hydrophobic R groups.
Strong non-covalent bonds.
Trans-membrane domain - a helix.
External surfaces interact with hydrophobic core f the membrane - keeping them in place.
Channel/Carrier proteins.

Question 5

Intrinsic/Integral proteins - Channel proteins (2)

Answer 5

Provides a hydrophilic channel that allows passive movement of polar molecules/ions down a conc gradient through membranes.
Held in position between the hydrophobic core of the membrane and hydrophobic R-groups on the outside of the proteins.

Question 6

Intrinsic/Integral proteins - Carrier proteins (2)

Answer 6

An important role in passive and active transport into cells.
Shape of protein changes.

Question 7

Extrinsic/peripheral proteins

Answer 7

Only on one side of the bilayer - located on the extracellular/cytosolic membrane.
Associated by non-covalent bonds.
Hydrophilic R-groups on their outer surfaces and interact with polar heads of the phospholipids/intrinsic proteins.
Glycoproteins and enzymes.

Question 8

Extrinsic/peripheral proteins - Glycoproteins (5)

Answer 8

Contain oligosaccharide chains (glycans) covalently bonded to AA side chains. The carbohydrate is attached in a cotranslational/posttranslational modification. This process is known as glycosylation.
Secreted extracellular proteins are often glycosylated.
Embedded in cell surface membrane with attached carbohydrate chains (varying in length and shape).
Role in cell adhesion.
Role as receptors for chemical signal.
Cell communication/signalling (when chemical binds to a receptor and this elicits a response- direct or sets of a cascade of events).

Question 9

Extrinsic/peripheral proteins - Glycoproteins Examples (3)

Answer 9

Receptors for neurotransmitters e.g. acetylcholine at nerve cell synapses. The binding of the neurotransmitter triggers or prevents an impulse in the next neuron.

Receptors for peptide hormones, including insulin and glucagon, which affects the uptake and storage of glucose by cells.

Some drugs bind to cell receptors  Beta-blockers are used to reduce the response of the heart to stress.

Question 10

Extrinsic/peripheral proteins - Glycolipids (2)

Answer 10

Lipids attached with a carbohydrate (sugar) chains.

Cell markers/ antigens and can be recognised by the cells of the immune system as self or non-self.

Question 11

Lipid anchored membrane proteins (2)

Answer 11

Covalently linked to a lipid molecule such as glycero-phosphatidylinositol.

Question 12

Cholesterol (4)

Answer 12

A lipid with a hydrophilic head and a hydrophobic tail.
Regulates fluidity of membranes.
Positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the tails, putting them together. It adds stability to membranes without making them too rigid.
Prevents the membrane become solid by stopping the phospholipid molecules from grouping too closely and crystallising.

Question 13

Membrane carbohydrates (5)

Answer 13

Carbohydrates are associated with both membrane lipids and proteins.
Form 2-10% of the membrane weight.
Carbohydrate on all membranes faces away from the cytosol.
They are often involved in cell-cell interactions or cellular recognition.
In RBCs 8% of the weight is carbohydrate.

Question 14

Membrane fluidity (2)

Answer 14

Lateral movement of lipids in the membrane is rapid.

Transverse movement is slow and requires the action of three enzymes.

Question 15

Membranes synthesis lipids (4)

Answer 15

Enzymes responsible for the synthesis of membrane phospholipids are associated largely with the cytosolic side of the ER. Newly synthesized lipids are then move to the luminal side by the flippases.
Synthesis ER lipids may be altered as the membranes flow from the ER to golgi to vesicles. Exceptions sphingomyelin and glycolipids started in ER and finished in golgi.

Question 16

Membranes synthesis lipids - Enzymes (6)

Answer 16

Floppase - moves phospholipids from the inner to the outer leaflet. Requires ATP.
Flippase (flipase) – moves phospholipids from the outer to the inner leaflet. Requires ATP.
Scamblase – bidirectional movement.

Question 17

Apoptosis (4)

Answer 17

Programmed death.
Doesn’t release contents.
Doesn’t cause inflammatory.
Phosphatidylserine (blue) on outside of                        
cell: ‘Eat me signal’ .

Question 18

Factors that affect fluidity (6)

Answer 18

Temperature decreases.
Temperature increases.
Fatty acid composition.
Chain length.
Degree and extent of saturation.
Cholesterol content.

Question 19

Factors that affect fluidity -Temperature decreases (2)

Answer 19

Saturated fatty acids become compressed, kinks in the tail push phospholipid molecules away- maintaining membrane fluidity.
Cholesterol buffers the effect of lowered temperatures, to prevent a reduction in the membranes fluidity. It prevents the phospholipids from packing together too closely by inserting itself between them.

Question 20

Factors that affect fluidity -Temperature increases (2)

Answer 20

Phospholipids acquire more KE and move around randomly, increasing membrane fluidity and permeability.
Affects the way membrane-embedded proteins are positioned and may function.
Enzymes in the membrane may drift and his could alter the rate of reactions they catalyse.
Cholesterol molecules buffer increasing heat as it reduces the increase of membrane fluidity, makes it more rigid (holds it together).

Question 21

Factors that affect fluidity - Chain length (1)

Answer 21

Increased length –> increased rigidity (more interactions) so less fluid.

Question 22

Factors that affect fluidity - Degree and extent of saturation (1)

Answer 22

More saturated (straight chains) -->
increased rigidity so less fluid.

Question 23

Factors that affect fluidity - Cholesterol content (2)

Answer 23

OH (hydroxyl group) forms H bonds between the phospholipids hydrophobic tail.
Disrupts regular interaction between fatty acid chains.

Question 24

Spur cell anemia (7)

Answer 24

Cholesterol content increased by 25-65%.
Decreased membrane fluidity, more rigid.
Less able to work as a RBC.
Spur cell anaemia is found in end-stage alcoholic liver disease.
The morphologic changes of spur cells are caused by accumulation of excess membrane cholesterol, leading to an impaired deformability and consequently a reduced red cell survival.