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Question 1

How is membrane added to the plasma membrane?

Answer 1

Via the exocytotic, secretory pathway from the Endoplasmic Reticulum (adds membrane to surface)

Question 2

What is the process by which membrane is removed from the plasma membrane?

Answer 2

Endocytosis

Question 3

What cells initiate phagocytosis?

Answer 3

Macrophages and neutrophils

Question 4

Outline the process of phagocytosis

Answer 4

• Particle binds to receptors in plasma membrane- Cell extends Pseudopods that permit further receptor interactions- Membrane invagination via a “membrane zippering” mechanism takes place- Internalised phagosomes fuse with lysosomes to form phagolysosomes in which the particulate material is degraded- Permits the clearance of damaged cellular materials and destruction of invading organisms

Question 5

What is pinocytosis?

Answer 5

he invagination of the plasma membrane to form a lipid vesicle

Question 6

What does pinocytosis allow to happen?

Answer 6

permits the uptake of impermeable extracellular solutes and retrieval of plasma membrane.

Question 7

What two forms can pinocytosis take?

Answer 7

fluid-phase and receptor mediated endocytosis.

Question 8

What is receptor mediated endocytosis?

Answer 8

Specific binding of molecules to cell surface receptors permits the selective uptake of substances into the cell.A major example is uptake of cholesterol

Question 9

What are low density lipoproteins?

Answer 9

Lipoproteins that originate in the liver and consist of a core of ccholesterol esters and TAGs, surrounded by phospholipids, cholesterol and a single protein species, apoprotein B

Question 10

What do animal cells which require cholesterol do?

Answer 10

Synthesis LDL - Receptors that recognise Apoprotein B

Question 11

Where are LDL receptors located in cells?

Answer 11

Located in clusters over Clathrin coated pits that cover approximately 2% of cell surface. These pits form spontaneously beneath LDL receptors.

Question 12

How does LDL enter cells?

Answer 12

• Bind to LDL receptors- Invaginated into cell via clathrin coated pit, form coated vesicles.- These vesicles are uncoated in a process that required ATP and fuse with larger, smooth vesicles called endosomes

Question 13

How does the LDL receptor decouple from the LDL particle in the endosome?

Answer 13

The pH of the endosome is lower than that of the cytoplasm (5.5-6.0), maintained by an ATP-dependent proton pump. At this pH, the LDL receptor has a low affinity for the LDL particle and the two dissociate.

Question 14

What is another name for the endosome?

Answer 14

Compartment for the Uncoupling of Receptor and Ligand (CURL).

Question 15

What happens to receptors after decoupling?

Answer 15

The receptors are sequestered to a domain within the endosome membrane, which buds off as a vesicle and recycles the LDL-receptor to the plasma membrane

Question 16

What happens to the LDL in the endosome once decoupled from the receptor?

Answer 16

The endosomes containing the LDL fuse with lysosomes, and the cholesterol is hydrolysed from the esters and released into the cell.

Question 17

What are three mutation which may affect the LDL receptor in hypercholesterolaemia?

Answer 17

Non-Functioning ReceptorReceptor Binding NormalReceptor Deficiency

Question 18

How does a non-functioning receptor cause hypercholesterolaemia?

Answer 18

If there is a mutation to the LDL binding site of the LDL-Receptor, it will prevent the binding and uptake of LDL.

Question 19

How can hypercholesterol aemia occur even if receptor binding is normal?

Answer 19

Mutation, causing deletion of the C-terminal cytoplasmic domain, which prevents the interaction between the receptor and the Clathrin coat. The LDL-Receptors will be distributed over the entire cell surface instead of being concentrated in 2%.

Question 20

How can a receptor defiency cause hypercholesterolaemia?

Answer 20

A deficiency caused by a mutation that prevents expression of the LDL-Receptor.

Question 21

Give another example of receptor mediated endocytosis, other than LDL

Answer 21

Transferrin

Question 22

What occurs in transferrin receptor mediated endocytosis?

Answer 22

Two Fe3+ ions bind to Apoptransferrin to form Transferrin in the circulation.Transferrin, but not Apoptransferrin, binds to the Transferrin Receptor at neutral pH and is internalised in a similar way to LDL

Question 23

What occurs when transferrin reaches the acidic endosome?

Answer 23

The Fe3+ ions are released

Question 24

What happens to Apoptransferring after Fe3+ dissociation at endosome?

Answer 24

• At acidic pH of endosomethe Apoptransferrin remains associated with the transferrin receptor. - The complex is sorted in the CURL for recycling back to the plasma membrane, where at pH 7.4 the Apoptransferrin dissociates from the receptor again.

Question 25

How are insulin receptors different to both LDL and Transferrin receptors?

Answer 25

insulin receptors only congregate over Clathrin coated pits when their agonist is bound. Insulin binding. Induces a conformationalchange in the receptor that allows it to be recognised by the pit.

Question 26

What happens to insulin in the endosome?

Answer 26

In the endosome Insulin remains bound to the receptor and the complex is targeted to the lysosomes for degradation.

Question 27

What does the degradation of the insulin-receptor complex allow for?

Answer 27

the reduction in the number of insulin receptors on the membrane surface, desensitising the cell to a continued presence of high circulating insulin concentration.

Question 28

What can happen to some ligands which remain bound to their receptors?

Answer 28

They may be transported across the cell in the process of transcytosis

Question 29

Give an example of transcytosis

Answer 29

Transfer of immunoglobulin A (IgA) from the circulation to bile in the liver or maternal immunoglobulins to the foetus via the placenta.

Question 30

What occurs during transport of IgA in transcytosis to bile in the liver?

Answer 30

the receptor is cleaved, resulting in the release of immunoglobulin with a bound ‘secretory component’ derived from the receptor.

Question 31

Whatis common to all proteins that undergoe endocytosis?

Answer 31

Ligands enter the cell via the same Clathrin coated pits and the pathway from coated pits to the endosome

Question 32

What differs between proteins which undergo endocytosis?

Answer 32

destination of the internalised receptor and ligand

Question 33

How are receptors targeted to different cellular destinations?

Answer 33

By short amino acid motiffs.These are recognised and and sorted within the CURL and sent to discrete regions of membrane, which they get to via transport vesicles.

Question 34

What happens in mode 1 of receptor mediated endocytosis (LDL) is an example?

Answer 34

Receptor is recycled, Ligand is degraded. Function is metabolite uptake.

Question 35

What happens in mode 2 of receptor mediated endocytosis? (example is transferrin)

Answer 35

Receptor is recycled, ligand is recycled. Function is metabolite uptake

Question 36

What happens in mode 3 endocytosis? (insulin and immune complexes)

Answer 36

Receptor degraded, Ligand degraded. Function is receptor down regulation or, in the case of immune complexes, removale from circulation of foreign anti-gen.

Question 37

What happens in mode 4 endocytosis? (examples include IgA and Maternal IgG

Answer 37

Receptor transported, ligand transported. Function is transfer of large molecules across cell.(Maternal IgG provides maternal immunity to foetus via placenta, whereas IgA is transferred from circulation to bile)

Question 38

For the following modes of transport, gives exapmplesMode 1 Mode 2Mode 3Mode 4

Answer 38

1 - LDL2 - Transferrin3 - Insulin, immune complexes4 - Maternal IgG, IGA

Question 39

What pathogens can exploit process endocytosis?

Answer 39

Membrane enveloped viruses and some toxins

Question 40

What is the process by which membrane enveloped viruses enter cell?

Answer 40

- adventitious binding to receptors in the plasma membrane- Once in the endosome, the acidic pH allows the viral membrane to fuse with the endosomal membrane,- releases the viral RNA into the cell where it can be translated and replicated by the host cell’s machinery to form new viral particles.

Question 41

What two toxins can enter cell?

Answer 41

Cholera toxin, Diptheria toxin