Membrane Bilayer

Question 1

How does the membrane bilayer respond to stimuli?

Answer 1

It generates chemical or electrical signals.

Question 2

Name the constituents of the plasma membrane.

Answer 2

40% lipid and 60% protein. It can also be 1-10% carbohydrate but this varies.

Question 3

Where in the bilayer is water found?

Answer 3

Water molecules are hydrogen bonded to the surface of the bilayer. It is a hydrated structure where 20% of the weight is water.

Question 4

What is a phospholipid?

Answer 4

It is a glycerol molecule with two fatty acid molecules attached and a phosphate group.

Question 5

Name molecules which can be the head group of a phospholipid.

Answer 5

Amines, amino acids, choline and sugars.

Question 6

State some properties of the fatty acid chains in a phospholipid.

Answer 6

They are usually C14-C24 and predominantly C16-18. This means that the bilayer has a fairly constant thickness. Cis double bonds introduce a kink into the tail.

Question 7

Name a phospholipid not based on a glycerol backbone.

Answer 7

Sphingomyelin.

Question 8

What is a glycolipid?

Answer 8

This is where we replace the phosphocholine moiety with a sugar.

Question 9

What is the difference between a cerebroside and a ganglioside?

Answer 9

These are both glycolipid a but a cerebroside only has a single sugar residue whilst a Ganglioside has multiple sugar residues.

Question 10

What movements can phospholipids undergo within a phospholipid bilayer?

Answer 10

Flip flop (rare) as has to move through hydrophobic domain
Lateral diffusion
Rotation
Flexion

Question 11

What do double bonds in a fatty acid chain do to the membrane?

Answer 11

They limit the close packing of the molecules and therefore cause increased fluidity of the membrane.

Question 12

Describe the structure of Cholestrol.

Answer 12

It is a rigid molecule with a polar head group.

Question 13

State how Cholestrol can increase membrane fluidity.

Answer 13

It reduces phospholipid packing

Question 14

State the effect that cholestrol has on phospholipid chain motion

Answer 14

It reduces it, therefore reducing membrane fluidity.

Question 15

What method can we use to find out about the proteins present in a membrane?

Answer 15

Freeze fracture, then gel electrophoresis.

Question 16

Explain the principle of freeze fracture.

Answer 16

This is where the membrane is surrounded by ice. It is then broken. It will break at the weakest point and this is between the two lamellae.

Question 17

State the functional properties of the proteins in a membrane.

Answer 17

Facilitated diffusion, ion channels and gradients, ensure specificity of cellular responses.

Question 18

How can proteins in the membrane move?

Answer 18

Conformational change, lateral diffusion and rotation.

Question 19

Why can’t proteins flip flop?

Answer 19

They are too large molecules and there is not sufficient energy to move the, through the hydrophobic domain in the membrane.

Question 20

What can prevent protein movement within a membrane?

Answer 20

Aggregations, tethering (to basement membrane or to internal structure), interactions with other cells.

Question 21

What are the lipid mediated effects on proteins in the membrane?

Answer 21

This is the fact that proteins in the membrane will tend to spread out to regions where there is poor Cholestrol.

Question 22

How can peripheral proteins be removed from a membrane bilayer?

Answer 22

Peripheral proteins are only bound by H bonds and electrostatic forces. This means that they are affected by changes in pH and ionic strength and can be removed.

Question 23

What type of proteins interact with the hydrophobic domain of the membrane bilayer?

Answer 23

Integral proteins. These can only be removed by organic solvents or detergents.

Question 24

State two properties of transmembrane peptides.

Answer 24

They tend to be 18-20aa in length and predominantly a helix with hydrophobic r groups.

Question 25

What does a hydropathy plot show?

Answer 25

Areas of hydrophobia and hydrophilia

Question 26

What is membrane protein topology?

Answer 26

This is the importance of the proteins beings in the correct orientation in the membrane to ensure that they are functional.

Question 27

What is the erythrocyte cytoskeleton made up from?

Answer 27

A network of spectrin and actin molecules which can form a heterotetramer.

Question 28

In erythrocytes what affects the lateral motility of membrane proteins?

Answer 28

Integral proteins to the cytoskeleton.

Question 29

What is a haemolytic anaemia?

Answer 29

These are caused by mutations, affecting the cytoskeleton of erythrocytes. The cells change shape and are therefore cleared by the spleen.

Question 30

What disorder is a result of 50% loss of spectrin in erythrocytes?

Answer 30

Hereditary spherocytosis. This means that the RBC are round and more resistant to lysis.

Question 31

What is hereditary ellipocytosis?

Answer 31

This is when heterotetramers are not formed and this leads to rugby ball shaped cells which are fragile.

Question 32

Describe the process of membrane protein synthesis.

Answer 32

This begins in the cytoplasm and is stopped by SRP until in contact with the membrane. (Signal is a group of hydrophobic amino acids). When in contact with membrane, translation continues until hydrophobic domain is in membrane - cannot be pushed through and so synthesis continues in the cytoplasm. The signal is cleaved. This process ensures correct positioning of the protein within the membrane.

Question 33

How are multiple transmembrane domains folded into the membrane?

Answer 33

It is thought that the first is inserted into the membrane and then the rest are synthesised in pairs and then folded into the membrane.

Question 34

When vesicles bud off, this can lead to inside out membrane. Why is this important?

Answer 34

This will tell us which substances the vesicles will interact with.

Question 35

What are the roles of the membrane bilayer?

Answer 35

It is a highly selective barrier. It has roles in both communication and recognition.