Cell Membrane

Question 1

What are the general functions of biological membranes?

Answer 1

1. Continuous, highly selective permeable barrier
2. Control of the enclosed chemical environment
3. Communication
4. Recognition: signalling molecules, adhesion proteins, immune surveillance.
5. Signal generation in response to stimuli (electrical, chemical)

Question 2

What is the general composition of the membrane?

Answer 2

It varies with the source of the membrane but generally, the dry weight of the membrane contain approximately:
40% lipid
60% protein
1-10% carbohydrate

Although, membranes are hydrated structures so 20% of total weight is water.

Question 3

What is the name for a molecule that contains but a hydrophilic and a hydrophobic moiety?

Answer 3

Amphipathic

Question 4

Talk about phospholipids.

Answer 4

They have a range of polar head groups. Eg choline, amines, amino acids, sugars.
The fatty acid chain varies in length between C14-C24 (although C16 and C18 are most prevalent).
The cis double bind induces a kink.
Phospholipids get their name from from head group. Eg phosphatidylcholine.

Question 5

What is inositol?

Answer 5

A phospholipid head group. It is a substrate for signalling molecules in response to message. It releases molecules into the cell.
It plays a role in lipid signalling, cell signalling and membrane trafficking.

Question 6

What is a glycolipid?

Answer 6

Glycolipids are lipids with a carbohydrate attached by a glycosidic bond. Their role is to maintain stability of the membrane and facilitate cellular recognition.
(The phosphate head has been replaced with a sugar)

Question 7

What are the two types of glycolipid?

Answer 7

Cerebroside - the head group is a sugar monomer

Ganglioside - the head group is a oligosaccharide (sugar multimers)

Question 8

Name the formations that phospholipids get into

Answer 8

Micelles
Liposome
Lipid bilayer

Question 9

Name the motions or phospholipids (4)

Answer 9

Flexion
Rotation
Lateral diffusion
Flip flop

Question 10

What effect does a double bond in a phospholipid have on the membrane?

Answer 10

A cis double bond induces a kink in the chain. This reduces phospholipid packing and increases the possibility for movement.
You need unsaturated fatty acids in the diet to keep the membrane dynamic.

Question 11

What is the structure of cholesterol?

Answer 11

It has a polar head group;a rigid, planar steroid ring structure. (It has 4 rings); and a non-polar hydrocarbon tail.

Question 12

Why is cholesterol described as having a ‘paradoxical’ effect?

Answer 12

Because it prevents phospholipids forming crystals at low temperatures and prevents lots of movement at high temperatures.
It homogenises phase transition from crystalline to fluid by preserving a fluid environment.
It does both of these things by forming hydrogen bonds with the carbonyl of the phospholipids.

Question 13

What is the evidence for membrane proteins?

Answer 13

Functional:
Facilitated diffusion, ion gradients, specificity of cel responses.
Biochemical: membrane fractionation and gel electrophoresis, freeze fractures.

Question 14

Freeze fracture

Answer 14

Freeze cells then put a knife on them and push. If lucky, it will fracture through the middle of the bilayer. Some proteins will stay in the membrane and others will leave a whole where they should be.
You then preserve with Osmium and view under an electron microscope.

Question 15

How do membrane proteins move?

Answer 15

Conformational change (flicker between stable conformations),
Rotational,
Lateral ,

NO FLIP FLOP

(Think of it like ships in a sea of lipids)

Question 16

How can membrane protein movement be restricted?

Answer 16

Aggregation (when one protein joins with another to form a raft),
Tethering (adhesion proteins p- Use proteins to join cells to basement membrane or to anchor it to a structure in the cell),
Interactions with other cells (this fixes proteins because they are used to join the two cells together),
Lipid mediated effects,
Membrane protein associations,
Association with extra-membranous proteins (peripheral proteins like the cytoskeleton)

Question 17

Where do proteins tend to go?

Answer 17

Cholesterol poor regions

Question 18

What types of proteins do not go to cholesterol poor regions?

Answer 18

Signal proteins

Question 19

What are the two types of membrane protein?

Answer 19

Peripheral and integral

Question 20

Where are peripheral proteins located and how can they be removed?

Answer 20

Peripheral proteins are not in the bilayer but are attached to the cell surface.
They are attached by electrostatic and hydrogen bind interactions so can easily be removed by changes in pH or ionic strength (salt)

Question 21

Where are integral proteins located in the membrane and how can they be removed?

Answer 21

Integral proteins interact extensively with hydrophobic domains of the lipid bilayer. This means that they cannot be removed by manipulation of pH or ionic strength (salt).
This means that you have to use agents this compete for non polar interactions and so dissolve the bilayer. For detergents and organic solvents.

Question 22

Summarise how a transmembrane protein might fold

Answer 22

Transmembrane domains are often alpha helical and the R groups of the amino acid residues are largely hydrophobic.
The hydrophobic amino acids are usually in the centre of the me,brand while the polar ones end up on the outside. The small amino acids (eg glycine) have very little effect on the membrane shape.

Question 23

What parts of the protein are usually on the outside of the cell?

Answer 23

Disulphide bonds and carbohydrates always face outwards (NOT into cytoplasm).

The C terminal is usually on the outside (and N terminal on the inside) but not always!

Question 24

What is spectin?

Answer 24

A large protein found in the membrane of erythrocytes

They are rod like molecules that join end to end through the inside surface of the erythrocyte membrane

Question 25

What is the general structure of a cytoskeleton?

Answer 25

The cytoskeleton is Rod like proteins glued onto the membrane. It is also a complex structure of interlinking filaments and tubules that’s extend through the cytoplasm from the nucleus to the plasma membrane.
They exist so that the cell can bend and change shape but maintain its integrity. It is what causes the biconcave shape of an erythrocyte

Question 26

Name two types of Haemolytic anaemia and what causes them.

Answer 26

Hereditary Spherocytosis:
The amount of Spectrin made is depleted by 40-50%. This is because only one of the alleles are working so only half the RNA is made so only half of the protein (Spectrin) can be made.
This means that the cytoskeleton is weaker so the erythrocytes round up.
This makes them less resistant to lysis (as more likely to break when in capillaries). So people become anaemic.
This is then cleared up by the spleen.

Hereditary Elliptocytosis:
Spectrin is deformed so it is unable to form heterotetramers (form rugby balls instead of rods)
This leads to fragile elliptoid cells

Question 27

How do you treat haemolytic anaemias?

Answer 27

Patients will need blood transfusions forever. This is a condition that will affect them forever because of mutations. They are rare.

Question 28

Describe the process of secretory protein biosynthesis

Answer 28

The signal sequence(hydrophobic sequence) is the first thing the ribosome translates
This is recognised by the signal recognition particle (SRP)
SRP binds to RNA and ribosome therefore preventing transcription from continuing in the cytoplasm.
SRP then binds to the docking protein in the endoplasmic reticulum (ER).
At ER, the signal sequence is released and is instead bound by a signal sequence receptor in the bilayer.
This then feeds the signal through a protein translocator complex to the lumen of the ER
So, once synthesis is finished, the protein is present in the lumen of the ER and it is already packaged.

Also, as the signal sequence arrived in the ER, there is a signal peptidase that cleaves off the hydrophobic chain

Question 30

How do you produce right side out and inside out vesicles?

Answer 30

To make a right side out vesicle, use exocytosis. If stuff is leaving then membrane will fuse around it and just produce right side out vesicles.

To make an inside out vesicle, use endocytosis. These will be smaller than right side out vesicles.

You can separate the two types of vesicle through centrifugation.

Question 31

Describe the differences between secretory and membrane protein biosynthesis

Answer 31

It is the same sequence up to a point but, in membrane protein synthesis, the hydrophobic sequence locks proteins into the membrane so there is no secretory pathway.
This is called a STOP TRANSFER SEQUENCE
The ribosome then lifts off the ER and makes the rest of the protein in the cytoplasm. This means that it end with a protein in the membrane with the N terminal facing the lumen and C terminal facing the cytoplasm.

Question 34

REMEMBER TO LOOK AT SLIDES FOR STRUCTURE OF MEMBRANE

Answer 34

Membrane structure (densely packed with many different types of proteins - learn about different type but need picture so look at slides)