Lecture 6: The endomembrane system 2

Question 1

What is exocytosis?

Answer 1

Secretory proteins being released

Question 2

What are the two routes of exocytosis?

Answer 2

Unregulated/constitutive
Regulated

Question 3

What is unregulated/constitutive exocytosis?

Answer 3

• A constant stream of transport vesicles that bud from trans-golgi network and fuse with plasma membrane
• Doesn’t need signal
• Default pathway

Question 4

What are the functions of unregulated/constitutive exocytosis?

Answer 4

Supply membrane proteins and lipid for plasma membrane growth
Allow protein secretion

Question 5

What is regulated exocytosis?

Answer 5

Proteins are sorted into secretory vesicles and stored until a specific signal is received

Question 6

What are some features of regulated exocytosis?

Answer 6

Only happens in specialised secretory cells
Rapid process

Question 7

What is an example of regulated exocytosis?

Answer 7

Insulin
Secreted by pancreatic beta cells
Happens in response to increase in blood glucose

Question 8

What is endocytosis?

Answer 8

Fluid and molecules enter the cell from the external environment

Question 9

What is the overall process of endocytosis?

Answer 9

1. Material enclosed by plasma membrane
2. Pinches off to from endocytic vesicle
3. Delivered to endosomes for sorting

Question 10

What are the three main types of endocytosis?

Answer 10

Phagocytosis
Pinocytosis
Receptor mediated endocytosis

Question 11

What is phagocytosis?

Answer 11

Uptake of large particles
Phagocytic cell extends pseudopods to engulf
This forms a phagosome which fuses with lysosome
Only in specialised cells

Question 12

What is pinocytosis?

Answer 12

Uptake of fluid and macromolecules
Non-selective
Mediated by clathrin coated vesicles

Question 13

What is receptor mediated endocytosis?

Answer 13

Receptor mediated endocytosis
Cell surface receptors capture cargo to increase efficiency

Question 14

What is the process of endocytosis of low density lipoproteins?

Answer 14

1. Cholesterol transported in blood as low density lipoproteins
2. Binds to LDL receptors on cell surface
3. LDL internalised by clathrin coated vesicles
4. Fuses with endosome
5. LDL dissociated from receptor

Question 15

What are endosomes?

Answer 15

Main sorting station of endocytosed macromolecules
Have an acidic pH (low)

Question 16

What is an endosome compartment?

Answer 16

Cluster of connected tubules and vesicles

Question 17

What are the three different fates of receptor proteins after endocytosis?

Answer 17

Recycled
Degraded
Transcytosis

Question 18

What is the role of lysosomes?

Answer 18

Site of degradation of macromolecules

Question 19

What is the structure of lysosomes?

Answer 19

Contain 40 hydrolytic enzymes to degrade substrates into useful products
Optimal activity at pH 5

Question 20

Why is it important that lysosomal enzymes are optimal at pH5

Answer 20

If they leak into the cytosol where the pH is higher (7.2) they will be inactive and not start degrading the entire cell

Question 21

How is the acidic pH maintained in lysosomes?

Answer 21

By an ATP dependent proton pump

Question 22

How are enzymes sorted to lysosomes?

Answer 22

1. Lysosomal enzymes are made at ER
2. Follow secretory pathway to the golgi
3. Modified with mannose-6-phospate
4. Mannose-6-phospate receptor in trans golgi sorts and packages them into transport vesicles
5. Delivered to lysosomes

Question 23

What is autophagy?

Answer 23

1. Damaged organelles engulfed by a double membrane that forms in cytosol
2. Autophagosome forms
3. Fuses with lysosome to deliver its contents