Lecture 2: Cell Surface and Cellular Membranes

Question 1

What are the most common lipids in eukaryotes?

Answer 1

Phospholipids
Cholesterol
Glycolipids

Question 2

What are phospholipids?

Answer 2

Make up the lipid bilayer.
They have a head and a tail: head = choline, phosphate and glycerol, tail = 2 fatty acid chains
They are amphiphilic (hydrophobic tail and hydrophilic head)

Question 3

What is cholesterol?

Answer 3

Decreases membrane permeability to small water soluble molecules

Question 4

What are glycolipids?

Answer 4

sugar containing lipids which face away from the cytosol.

Question 5

What are glycolipids involved in?

Answer 5

Cell recognition processes.

Entry point for some bacterial toxins and viruses

Question 6

What are the 4 main phospholipids in mammalian cells?

Answer 6

Phophatidylethanolamine = neutral charge
Phosphatidylserine = NEGATIVE charge
Phosphatidylcholine = neutral charge
Sphingomyelin = neutral charge

Question 7

What impacts membrane fluidity?

Answer 7

Lipid composition and temperature

Question 8

Give examples of membrane proteins

Answer 8

Transporters
Linkers
Receptors
Enzymes

Question 9

What do transporters do in the membrane?

Answer 9

Move nutrients, metabolites, or ions across the cell membrane which cannot diffuse normally.
Can be involved in passive or active transport.
Channel and carrier proteins are examples

Question 10

What do linkers do in the membrane?

Answer 10

Join membranes to intra- and extra cellular macromolecules.

Can link to the cytoskeleton

Question 11

What do receptors do in the membrane?

Answer 11

ransduce signals from the environment (e.g. hormones)

Can transport ligands

Question 12

What do enzymes do in the membrane?

Answer 12

Catalyse reactions at membrane surface

E.g. membrane bound disaccharidases

Question 13

What experiments have shown that proteins can diffuse in the plane of the membrane?

Answer 13

1. Cell hybrid generation.
   - Fused a human cell with a mouse cell and they can use monoclonal antibodies that recognise a specific antigen.
   - If the protein is diffusing, the signal becomes homogenous again.
2. FRAP - Fluorescent recovery after photobleaching
   - You genetically modify the mammalian cell, and express a protein which is naturally fluorescent (e.g. GFP = green).
   - GFP domain is attached to a membrane protein that is exposed of the surface.
   - Use a laser that will destroy the GFP protein, meaning you will lose the signal locally.
   - Use very sensitive cameras to measure to what extent the signal recovers.

Question 14

How can proteins be restricted to specific domains/membranes?

Answer 14

1. There are polarised epithelial cells where the apical side faces outside of the intestine, and the basal side faces the other membrane. This causes the cell to be polarised due to the difference in protein composition on each side.
   - Tight junctions maintain the distribution.
2. Self assembly = the protein tends to accumulate by clustering together, stopping them from diffusing across the membrane.
3. Assembly of complexes = form complexes by the interaction of specific components from the matrix, by the cytoskeleton or by cell to cell interactions.

Question 15

How can proteins modulate the shape of the membrane?

Answer 15

• They can form wedges in the membrane that have a hydrophobic domain = bends membrane
• Curved proteins can also bend the membrane.
• Can combine with the lipid heads and deform the membrane

Question 16

Why is bending of the membrane by proteins important?

Answer 16

It’s essential for membrane trafficking e.g. vesicle budding

Question 17

What is the glycocalyx?

Answer 17

An extracellular protein coating that provides protection from chemical, physical, and biological damages to the cell

Question 18

What is the glycocalyx composed of?

Answer 18

glycolipids and glycoproteins

Question 19

What are the different types of glycoproteins?

Answer 19

N-glycans = sugar is attached to Asparagine
O-glycans = sugar is attached to Threonine
Proteoglycans = refers to a subset of glycoproteins where the glyco part is a GAG (Glycosaminoglycan)

Question 20

What are the functions of a glycocalyx?

Answer 20

Protection = keep unwanted interactions at a distance.
Adhesion = carbohydrate binding proteins on other cell membranes.
Recognition = cell to cell interactions.
Storage = bind and release growth factor.

Question 21

What do the glycan parts of proteins function in?

Answer 21

Cell to cell binding
Protein structure/ stability
Regulatory signalling
Protein sorting