Transport across biological membranes

Question 1

Describe the synthesis of phosphatidylcholine

Answer 1

1. Fatty acids synthesised in cytosol are transported to ER by FABP (fatty acid binding protein) (highly hydrophobic so need to be transported in an aqueous environment)
2. FAs embed in the membrane and in a succession of steps glycerol, phosphate and choline added (this happens in the outer cytosolic)

Question 2

Where does phospholipid synthesis occur?

Answer 2

in the outer cytosolic leaflet of the ER membrane

Question 3

How are phospholipids distributed equally between the outer and inner membranes of the ER membrane?

Answer 3

The ER enzyme scramblase catalyses a flip-flop movement (e.g trans-bilayer movement)

Question 4

What happens to newly synthesised membranes?

Answer 4

Transported to the plasma membrane and to other organelles

Question 5

What does the inner luminal leaflet of the ER become on the plasma membrane?

Answer 5

the extracellular leaflet of the plasma membrane

Question 6

How does the inner luminal leaflet of the ER become the extracellular leaflet of the plasma membrane?

Answer 6

A vesicle of membrane buds off the ER.

The vesicle fuses with the membrane in a way that the inner luminal leaflet is now on the outside

Question 7

Explain how the asymmetric distribution glycolipids occur in the plasma membrane?

Answer 7

1. Enzymes that add sugar are only found in the inner luminal leaflet of the ER
2. So when the vesicles fuse with the plasma membrane it ends up on the outside of the cell

Question 8

Where does glycosylation occur?

Answer 8

In the lumen of the ER/golgi

Question 9

Where are the glycolipids located?

Answer 9

On the extracellular leaflet of the plasma membrane

Based on sphingosine

Question 10

Where are integral membrane proteins found?

Answer 10

Embedded into the membrane

Question 11

Where are peripheral membrane proteins located?

Answer 11

On the cytoplasmic side or extracellular side when in association with integral membrane proteins

Question 12

What are the three types of transmembrane protein?

Answer 12

Single pass transmembrane protein (alpha helix)
Multi-pass transmembrane protein (multiple α-helices)
Multi-pass transmembrane protein (B-sheets rolled up into a B-barrel)

Question 13

What do soluble proteins for export contain at their N-terminal end?

Answer 13

A 15-20 amino acid hydrophobic signal peptide

Question 14

How are integral transmembrane proteins inserted into the membrane?

Answer 14

1. Signal peptide directs the growing polypeptide chain as it emerges from the ribosome to a translocator in the ER membrane
2. The growing polypeptide chain is threaded through the membrane (translocation)
3. Signal peptide is cleaved by signal peptidase and newly synthesised protein is released into ER lumen

Question 15

How does the integration of a single pass transmembrane protein occur?

Answer 15

1. Some proteins contain both a signal (start-transfer) peptide and a stop-transfer sequence
2. Signal peptide directs the growing polypeptide chain to the translocator and the growing chain is threaded through membrane
3. When translocator interacts with stop-transfer sequence translocation ceases and the translocator discharges the protein laterally into the bilayer
4. Signal peptide is cleaved

Question 16

How does the integration of a double pass transmembrane protein occur?

Answer 16

1. Double pass membrane spanning proteins have an internal start-transfer peptide and a stop-transfer sequence
2. Polypeptide synthesis is initiated in the cytoplasm, signal peptide directs the growing chain to the translocator and chain is threaded through
3. When the translocator interacts with the stop-transfer sequence translocation stops and translocator discharges protein into bilayer laterally
4. Signal peptide is not cleaved

Question 17

What is the difference between integration of double membrane pass proteins and single membrane pass proteins?

Answer 17

In double pass the signal or start-transfer sequence is internal and its not exactly at the N terminus of the polypeptode
In double signal peptidase is not involved

Question 18

Where are sugar residues localised on glycoproteins?

Answer 18

On the extracellular side of the plasma membrane

Question 19

what type of amino acids are sugar residues added to?

Answer 19

Ones that are exposed to the lumen of the ER

Question 20

Why are glycoproteins and glycolipids important for? (3 roles)

Answer 20

Cell recognition
Inflammatory response
Protection

Question 21

What is the glycocalyx?

Answer 21

A carbohydrate rich layer surrounding the cells which is composed of glycolipids and glycoproteins

Question 22

What is the role of the glycocalyx?

Answer 22

It protects cells against chemical and mechanical damage

Question 23

What is the nuclear envelope continuous with?

Answer 23

ER membrane

Question 24

What does the nuclear envelope consist of?

Answer 24

Inner and outer membrane

Question 25

What is the nuclear envelope penetrated by? And what is the role of the thing penetrating it?

Answer 25

Nuclear pores to allow bidirectional exchange between nucleoplasmic and cytoplasmic compartments

Question 26

How many nuclear pores are there per nucleus?

Answer 26

3000-4000

Question 27

Describe the nuclear pore complex

Answer 27

• Largest protein complex in the cell (50MDa)
• Greater than 400 individual protein complexes
• 30 distinct proteins (nucleoporins)
• Both integral and peripheral membrane proteins involved

Question 28

What does the enzyme flippase do?

Answer 28

Flips the Phosphatidyl Ethanolamine and Phosphotidyl Serine from the extracellular leaflet to the cytosolic leaflet creating an asymmetric bilayer.

Question 29

Describe the steps in membrane synthesis

Answer 29

1. Phospholipid synthesis occurs exclusively in the outer cytosolic leaflet of the ER membrane
2. ER enzyme Scramblase catalyses flipi-flop reaction so phospholipids distrubute equally between the membranes.
3. Newly synthesised membrane is transported to the plasma membrane and other organells.
4. the inner luminal leaflet of the ER becomes the extracellular leaflet of the plasma membrane.
5. Plasma membrane enzyme flippase flips PE and PS to the cytosolic leaflet creating the asymmetric bilayer.