Session 1

Question 1

What are the five general functions of biological membranes?

Answer 1

Permeability barrier, control of enclosed chemical environment, communication, recognition, signal generation

Question 2

What is the general composition of a membrane?

Answer 2

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

20% of total weight is water as membranes are hydrated structures

Question 3

Describe the general structure of membrane phosphoglycerides

Answer 3

Glycerol centre, 2 fatty acids (C14-24, C16 and C18 most common), phosphate; forms an amphipathic molecule with a hydrophilic head and hydrophobic tail. There may be one of a range of polar molecules attached to the phosphate head

Question 4

What is Sphinogomyelin?

Answer 4

A phospholipid, the only one to not be based on glycerol.

Question 5

How are glycolipids formed from sphingomyelin?

Answer 5

The phosphocoleine moeity of sphingomyelin can be replaced with a sugar

Question 6

In which four ways can the phospholipids in the bilayer move?

Answer 6

Flexion, rotation, lateral diffusion, flip flop

Question 7

What are cis double bonds and how do they affect the bilayer?

Answer 7

Found in unsaturated fatty acids, they are double bonds in cis orientation (same side), and in phospholipids this results in a kink in the hydrophobic tail = reduced phospholipid packing

Question 8

Describe the structure of cholesterol

Answer 8

Rigid planar steroid ring structure, polar hydroxyl head group, non-polar hydrocarbon tail (more flexible)

Question 9

What is the function of cholesterol in the lipid bilayer at lower temperatures?

Answer 9

At lower temperatures cholesterol interferes with fatty acid chain associations and increases fluidity; there is reduced packaging, preventing the formation of a crystalline array

Question 10

What is the function of cholesterol in the lipid bilayer at higher temperatures?

Answer 10

At higher temperatures cholesterol tends to limit disorder and decrease fluidity; there is reduced phospholipid chain motion, due to the rigidity of cholesterol

Question 11

How is the lipid bilayer affected by increasing cholesterol concentrations?

Answer 11

The heat tolerance of the bilayer increases

Question 12

Give three examples of the functional evidence for the presence of proteins in membranes

Answer 12

Facilitated diffusion, ion gradients, specificity of cell responses (receptors)

Question 13

What is the biochemical evidence for membrane proteins?

Answer 13

Freeze fracture and gel electrophoresis

Question 14

What is freeze fracture?

Answer 14

Cell is frozen, and fractured by a knife along the lipid bilayer; this is the point of weakness. The two fracture faces produced will have different proteins and gaps, observable by electron microscopy

Question 15

What are the two fracture faces called?

Answer 15

The P fracture face (cytosol side), and the E fracture face (extracellular side)

Question 16

What are the three modes of motion of membrane proteins?

Answer 16

Conformational change (lipids flex/vibrate), rotational (circles), Lateral (across membrane), NO FLIP FLOP

Question 17

Why is flip flop of membrane proteins not possible?

Answer 17

It is not energetically possible (movement through the hydrophobic region) and would cause the fracturing of the membrane

Question 18

How is protein mobility restricted?

Answer 18

Some proteins are in aggregates, some proteins are tethered (extra or intra cellularly), and some proteins interact with other cells.

Question 19

What links signalling proteins and cholesterol?

Answer 19

Signalling proteins are found in cholesterol rich regions; other proteins tend to prefer cholesterol poor regions

Question 20

What is a peripheral protein and what bonds does it contain?

Answer 20

Peripheral proteins are membrane proteins bound to the surface of the lipid, by electrostatic interactions, hydrogen bonds, and disulphide bonds, which can be removed by changes in pH or ionic strength (salt concentration)

Question 21

What are integral proteins?

Answer 21

Integral proteins interact extensively with hydrophobic domains of the lipid bilayer; they are often, but not necessarily, transmembranous

Question 22

How are integral proteins removed from the lipid bilayer?

Answer 22

They are removed by agents that compete for non-polar interactions (e.g. detergents, organic solvents)

Question 23

Describe key features of the transmembrane domain of transmembrane proteins

Answer 23

Often alpha-helical, around 18-22 amino acids in length, and the R groups are largely hydrophobic; usually followed by a few charged amino acids

Question 24

How is hydropathy used?

Answer 24

Identifies sequences of hydrophobic amino acids likely to be transmembranous regions, hence giving an indication of the number of these in a protein

Question 25

Which two proteins of the erythrocyte membrane are integral?

Answer 25

Band 3 and band 7; they both contain covalently attached carbohydrate units and are, thus, glycoproteins

Question 26

Which two molecules make up the erythrocyte cytoskeleton?

Answer 26

Spectrin and actin

Question 27

Describe the key features of spectrin

Answer 27

Spectrin is a long, floppy rod-like molecule, α and β subunits wind together to form an antiparallel heterodimer and then two heterodimers form a head to head association to form a head to head tetramer of α2β2

Question 28

Describe the function of actin in the erythrocyte cytoskeleton

Answer 28

Short actin profilaments form cross links between spectrin tetramer rods

Question 29

How does the attachment of integral proteins to the cytoskeleton affect membrane protein movement?

Answer 29

Restricts lateral movement of the attached membrane protein

Question 30

What is hereditary spherocytosis?

Answer 30

Very rare dominant genetic disease, spectrin is depleted by 40-50%, causing erythrocytes to round up, making them less resistant to lysis; they are cleared by the spleen. Frequent transfusions are required

Question 31

What is hereditary elliptocytosis?

Answer 31

A defect in the spectrin molecule, rendering it unable to form heterotetramers, resulting in fragile, ‘rugby ball’ shaped elliptoid cells.

Question 32

How does membrane proteins biosynthesis differ from secretory protein synthesis?

Answer 32

Same pathway into ER; stop transfer signal (18-22 hydrophobic amino acids) causes the release of the protein from the translocator in the ER. Protein synthesis continues in the cytosol

Question 33

What are the three main types of membrane lipid?

Answer 33

Phospholipids, cholesterol, gycolipids

Question 34

What are the two types of membrane glycolipids? Differentiate between the two

Answer 34

Cerebrosides have a monomeric sugar head group, gangliosides have a oligosaccharide head group

Question 35

Name the two adapter proteins in the erythrocyte cell membrane; what do they link to respectively?

Answer 35

Ankyrin (band 3), and band 4.1. (glcoporphyrin A)

Question 36

How is the ribosome attached to the endoplasmic reticulum during membrane protein synthesis?

Answer 36

The ribosome is anchored to the translocation complex by riboporphyrins

Question 37

Where are mitochondrial membrane proteins synthesised?

Answer 37

Cytoplasm

Question 38

Which protein catalyses disulphide bond formation?

Answer 38

Disulphide isomerase