IV. Cell Biology | 62. Endo- and exocytosis

Question 1

Why do we need to have endocytic-exocytic cycle?

Answer 1

There is a balance between endocytosis and exocytosis, so that the PM lost by endocytosis will be added by exocytosis, and therefore there is no net change in size/volume of the cell = endocytic-exocytic cycle

Question 2

I. ENDOCYTOSIS
1. What is endocytosis?

Answer 2

Endocytosis is the way a cell takes up fluids, solids, macromolecules, PM
components etc.

Question 3

I. ENDOCYTOSIS
1. What are the features of endocytosis?

Answer 3

• Endocytosis is the way a cell takes up fluids, solids, macromolecules, PM
  components etc.
• Clathrin-coated pits are often part of the endocytic pathway, since 2% of the PM
  contain components etc.

Question 4

I. ENDOCYTOSIS
3. What are the different types of endocytosis?

Answer 4

• 3 different types of endocytosis:
  a) Pinocytosis: cell drinking, taking up extracellular fluid, occurs continuously
  b) Phagocytosis: takes up solid particles via phagosome, only in specialized cells
  c) Receptor-mediated endocytosis: forms a coated vesicle (usually clathrin) around
  a cargo receptorcomponents etc.

Question 5

I. ENDOCYTOSIS
4. What is Pinocytosis?

Answer 5

• ‘’cellular drinking’’ or ingestion of dissolved materials
• Occurs in most cell types, therefore not very specific
• Cell folds inwards (invaginates) to take in fluid containing the desired substance
• The cytoskeleton (particularly actin) in the cell cortex (beneath the PM) reorganizes
  to fold the PM and causes a vesicle to form
• Pinocytosis can also be clathrin-mediated, where the clathrin-coated pits mediate the folding of the vesiclecomponents etc.

Question 6

I. ENDOCYTOSIS
5. Describe phagocytosis?

Answer 6

Endocytosis is the way a cell takes up fluids, solids, macromolecules, PM
components etc.

Question 7

I. ENDOCYTOSIS - Receptor-mediated endocytosis
6A. Describe Receptor-mediated endocytosis

Answer 7

• Is the most specific way of taking up macromolecules and can also be regulated
• Clathrin-coated pits and vesicles provide an efficient pathway for taking up specific macromolecules from extracellular fluid
• The macromolecules bind to a transmembrane receptor, and these receptor accumulate in coated pits -> can then enter the cell as receptor-macromolecule complexes in clathrin-coated vesicles
• Example: LDL-receptor taking up LDL (cholesterol)
  +) When the cell needs cholesterol, more LDL-receptors will be synthesized
  +) The receptors will be transported to the PM, where they will diffuse to a clathrin-coated region of the PM
  => Creates a selective concentrating mechanism that increases the efficiency of internalization of particular ligands more than a hundredfold (allows uptake of large quantities of ligand, even if they are present in small amounts in the ECM)

Question 8

I. ENDOCYTOSIS - Receptor-mediated endocytosis
6B. Give the example of Receptor-mediated endocytosis

Answer 8

Example: LDL-receptor taking up LDL (cholesterol)
+) When the cell needs cholesterol, more LDL-receptors will be synthesized
+) The receptors will be transported to the PM, where they will diffuse to a clathrin-coated region of the PM

Question 9

I. ENDOCYTOSIS - Receptor-mediated endocytosis
6C. Features of receptors participate in Receptor-mediated endocytosis

Answer 9

• Many receptors are known to participate in receptor-mediated endocytosis, and they all use clathrin-dependent internalization by signals in their cytoplasmic tails that can bind adaptor proteins in the clathrin-coated pits.
• The receptors differ in whether they diffuse to the clathrin-coated pits before or after they have bound their ligands.

Question 10

I. ENDOCYTOSIS - Receptor-mediated endocytosis
6D. What are the proteins are involved in the clathrin-mediated endocytosis?

Answer 10

1. HIP1R
2. Dynamins
3. Cortactin
4. ARP2/3 complex c

Question 11

I. ENDOCYTOSIS - Receptor-mediated endocytosis
7. What are The receptor-ligand complexes?

Answer 11

The receptor-ligand complexes that are endocytosed form the early endosome.

Question 12

I. ENDOCYTOSIS - Receptor-mediated endocytosis
8. What happen during endosome maturations?

Answer 12

Question 13

I. ENDOCYTOSIS - Receptor-mediated endocytosis
9. What is the Fate of receptor proteins involved in endocytosis?

Answer 13

1) Recycling
2) Degradation
3) Transcytosis

Question 14

II. Exocytosis
1. What are the features of exocytosis?

Answer 14

• Exocytosis is the transport of materials (proteins) to be secreted out of the cell (ex: hormones)
• Usually from the Golgi to the PM (COP-I vesicles)
• Can occur through 2 pathways: Constitutive exocytosis and Regulated exocytosis

Question 15

II. Exocytosis
2. What are the features of Constitutive exocytosis?

Answer 15

Constitutive exocytosis: where newly synthesized materials, such as PM proteins or lipids are continuously transported to the membrane
-> extracellular space without regulation

Question 16

II. Exocytosis
3. What are the features of Regulated exocytosis?

Answer 16

• Regulated exocytosis: where the secretory vesicle is stored in the cytoplasm and will not be released unless stimulated by some signal (ex: hormone/neurotransmitter)
• Example: insulin vesicles in pancreatic beta cells require an increase in glucose to be secreted, in order to prevent hyperglycemia

Question 17

II. Exocytosis
4. Give an example of exocytosis

Answer 17

the insulin regulated GLUT4 trafficking
=> where the GLUT4 transporters will not be translocated to the PM before receiving a signal via the insulin receptor:

Question 18

II. Exocytosis
5. Another example of exocytosis is the insulin regulated GLUT4 trafficking
=> Explain

Answer 18

the insulin regulated GLUT4 trafficking
=> where the GLUT4 transporters will not be translocated to the PM before receiving a signal via the insulin receptor: