Session 1 - Cell Membrane

Question 1

What is the cell membrane composed of by dry weight?

Answer 1

Lipids ~ 40%
Proteins ~ 60%
Carbohydrates ~ 10%

Question 2

What proportion of the membrane is water when it is in normally cellular condition (ie. not dried out)? And why?

Answer 2

20%
Water is able to interact with the phospholipid all the way to the C=O bond on the glycerol as this section is hydrophilic.

Question 3

What affect does the presence of unsaturated acyl chains have in the membrane?

Answer 3

The presence of unsaturated acyl chains results increased membrane fluidity. This is because the 60* kink of cis C=C bond disrupts the lattice and increases the spacing between the molecules.

Question 4

Define amphipathic

Answer 4

A molecule having both hydrophilic and hydrophobic parts (ie. a phospholipid)

Question 5

A phospholipid head is constituted of what?

Answer 5

The glycerol backbone is bound to 2 acyl chains and a phosphate group which in turn is bound to a head (both of which make up the phospholipid head). The 4 major heads are choline, serine, ethanolamine and inositol. Choline is the most abundant head and makes up 50%

Question 6

What are the two main types of glycolipids?

Answer 6

Cerebrosides - primarily attached to galactose as mono or oligonucleosides. These, hence the name are predominantly found in the CNS. They also determine the major blood groups

Gangliosides - the heads consist of oligosaccharide chains which are differentiated by 1 or more Sialic acid residues

Question 7

Which glycolipid is predominantly found in the CNS?

Answer 7

Cerebrosides

Question 8

How does cholesterol effect the properties of the cell membrane?

Answer 8

Cholesterol increases fluidity and also stabilises the membrane at higher temperatures.

It stabilises the structure by forming associations between the C=O if the acyl chain to the O-H of the cholesterol by acting as an “additional mass”

It increases fluidity because the rigid steroid plates disrupt the formation of crystalline structures between neighbouring phospholipids and as such increases fluidity

Question 9

How does cholesterol aid the positioning and stabilise groups of proteins within the membrane?

Answer 9

Congregates of particular lipid types like cholesterol aggregate together and as such reduce membrane mobility. This allows specific proteins to retain there positions together.

Question 10

Name the motions of lipids within the phospholipid bilayer. Chri

Answer 10

Intracranial motion/flexion - wobble about single C-C bonds
Axial rotation - whole lipid spinning
Lateral diffusion - lipid molecules diffusing laterally (witching position) within the bilayer
Flip-flop - lipids flip between layers
Protrusion - up and down motion (bobbing)

Question 11

What factors effect the motion of lipids in the lipid bilayer?

Answer 11

Temperature, the molecular mass of the phospholipids and the interactions between other molecules

Question 12

What motion are lipids able to do but proteins aren’t in the lipid bilayer?

Answer 12

Proteins can’t flip flip as this is thermodynamically unfavourable

Question 13

What is the protein density in membranes?

Answer 13

Varies allot: 60% on average, 20% in fatty myelin, 75% in mitochondrial membranes

Question 14

Name the 4 different types of proteins involved with the cell membrane

Answer 14

Peripheral (Intracellular, extra cellular) - these are loosely bound by hydrogen and ionic bonds
Integral proteins (trans and side associated) - these have varying degrees of penetration to the membrane. Some are trans others are only side associated.

Question 15

What are lipid anchored proteins?

Answer 15

The are proteins that are covalently bonded to a glycolipid

Question 16

Give some roles of Intracellular peripheral proteins

Answer 16

• Enzymes
• regulatory subunits of receptors, ion channels and transporters
• role in organisation of the sub-cortical cytoskeleton that enhances flexibility and elasticity

Question 17

Name some roles of extracellular peripheral proteins

Answer 17

Enzymes
Antigens
Adhesive molecules

Question 18

Name some roles of integral proteins

Answer 18

Ion channels

Transport molecules

Question 19

What role does the sub cortical cytoskeleton have on the protein position?

Answer 19

It retains protein position and distance as such it optimises their function

Question 20

What are the key sub-cortical membrane proteins in all cells?

Answer 20

Spectrin
Actin
Ankyrin

Specific integral anchoring proteins vary across cell types (ie. band 3 and 4.1 in RBC)

Question 21

How can peripheral proteins be removed from the cell surface membrane

Answer 21

They are removed by changing the pH or ionic strength as they are only held in place by hydrogen or ionic bonds.

Question 22

What causes hereditary spherocytosis?

Answer 22

It is caused by a mutation in the genes for cytoskeleton proteins (Ankyrin, Spectrin, Band 3.1 or 4.1 proteins). As such it can result in (to varying degrees) a change in shape to a sphere. This can result in lower surface area to volume ratio as such takes up less O2 (enough for normal life but not strenuous activity). More importantly they’re more fragile and undergo lysis more readily as well as splenic disruption.

Question 23

Cytochalasin is an oncological drug how does it work

Answer 23

It inhibits the polymerisation of actin filaments (hence cancer cell division is inhibited).
Also, effects normal cells and prevents the correct anchoring of spectrin by actin in RBC

Question 24

Name the 3 main signalling molecule classifications and what they mean.

Answer 24

Endogenous - within the body
Exogenous 1 - Natural (plant based)
Exogenous 2 - synthetic (man made)

Question 25

What are the main extracellular signalling types?

Answer 25

Endocrine - targeting distant cells (through the blood stream)
Paracrine - targets nearby cells (same tissue and not via gap junctions)
Autocrine - Targeting itself

Question 26

What are the major types of endocrine signalling molecules?

Answer 26

Hydrophilic 1 - amines
Hydrophilic 2 - peptides and proteins
Lipophilic - steroids

Question 27

What are the main signalling molecule targets?

Answer 27

RITE K(L)ING

R- Receptors
I- Ion channels (ligand gated/v.gated/GPCR)
T-Transport proteins
E- Enzymes

Receptor sub types
K - Kinase Linked Receptors
(L)I - Ion Channel (Ligand Gated)
N - Nuclear/Intracellular 
G - G Protein Coupled Receptors