Lecture 6 - Exam 4: Endocytosis and Mitochondria

Question 1

What is endocytosis?

Answer 1

Type of active transport that moves particles, such as large molecules, obsolete cells, nutrients and even fluids into a cell (internalization of material).

Question 2

What are the types of endocytosis?

Answer 2

Phagocytosis, Pinocytosis, and Receptor mediated endocytosis.

Question 3

What is phagocytosis?

Answer 3

(cell eating) engulfment of large particles (500-20000nm) such as bacteria, cell debris or intact cells. Characteristic of professional phagocytes:
Macrophages, neutrophils, and dendritic cells.

Question 4

What is pinocytosis?

Answer 4

(cell drinking) small particles suspended in extracellular fluid (200-1000nm). Present in all cell types. Non specific receptors – general endocytosis.

Question 5

What are the types of receptor mediated endocytosis?

Answer 5

-Clathrin-mediated endocytosis
-Caveolae-mediated endocytosis
-Clathrin-and caveolae-independent endocytosis
-Macropinocytosis

Question 6

Describe the process of phagocytosis.

Answer 6

1. Binding of a solid particle (>500nm) to the cell (bacteria, cell debris or intact cells).
   -Surface binding stimulates the extension of a pseudopodium, which eventually engulfs the solid particle.
2. Fusion of the pseudopodium membranes results in formation of a large intracellular vesicles called phagosome.
3. The phagosome fuses with the lysosomes to form a phagolysosome within which the solid particle is digested.

Question 7

What is endosomal recycling?

Answer 7

Recycle cellular components for reuse at the plasma membrane (energy efficiency)

Question 8

What is the endosomal recycling Rab?

Answer 8

Rab11

Question 9

Describe Clathrin-mediated endocytosis.

Answer 9

-The most studied form of endocytosis.
1. The macromolecules to be internalized bind to specific cell surface receptors that are concentrated in specialized regions of the plasma called clathrin-coated pits (CCP).
2. Adaptor proteins bind both to Clathrin and receptors.
3. Dynamin assembles around neck of CCP and undergoes GTP hydrolysis to drive membrane scission. (dynamin constricts when GTP is hydrolyzed)

Question 10

What are the experimental methods for clathrin-coated vesicle formation?

Answer 10

TEM imagining and mutational analysis of the genes involved.

Question 11

How much of the cell surface (at any given time) is clathrin coated pits?

Answer 11

1-2%

Question 12

What is the life time of clathrin coated pits? What does this mean for the rest of the cell membrane?

Answer 12

1-2 minutes. So, the entire cell membrane is internalized every 2 hours!!!!
But only 5% of the membrane is newly synthesized.

Question 13

What does the updated fluid mosaic model show?

Answer 13

There are distinct domains that play important structural and functional roles. The mobility of membrane proteins is limited by as a result of the association with the cytoskeleton or with specialized lipid domains.

Question 14

What are lipid rafts?

Answer 14

Limits free diffusion of some membrane proteins by their association with specialized lipid domains, or lipid rafts.
-Are small (10-200nm) transient structures that act as platforms in which specific proteins can be concentrated to facilitate their interaction.

Question 15

What are lipid rafts enriched with?

Answer 15

GPI-anchored proteins and transmembrane proteins.

Question 16

What is caveolae-mediated endocytosis characterized by? What does it also interact with?

Answer 16

Characterized by the presence of the membrane protein caveolin dimer, essential to initiate the biogenesis of the caveolae.
Also interacts with the cytoplasmic protein Cavin and cholesterol.

Question 17

Due to the caveolin dimer, caveolae assume what shape?

Answer 17

Caveolae assume their hallmark flask shaped structure, engulfing molecules which bind to caveolae surface

Question 18

Caveolin are part of what?

Answer 18

Lipid membranes.

Question 19

In caveolae-mediated endocytosis, dynamin is present and does what?

Answer 19

Drives membrane scission (breakage of chemical interactions between phospholipids).

Question 20

Describe Clathrin and caveolae - independent endocytosis.

Answer 20

-Cellular entry occurs in cells devoid of both clathrin and caveolae.
-Inhibitors of clathrin and caveolae demonstrated the existence of other receptors.

Question 21

Describe macropinocytosis.

Answer 21

-Special case of clathrin-caveolae-independent endocytosis.
Initiated by the transient activation of cellular receptors (tyrosin kinases and growth factors). This activation mediates a signal cascade that trigger the formation of membrane “ruffles.”
Ruffles surround and engulf the cargo and fuses with “macropinosome.”
-This pathway is possible for virtually any cell type with few exceptions such as macrophages and brain micro vessel endothelial cells.
-Cargo: bacteria, apoptotic bodies, necrotic cells and viruses.