15.3 Endocytosis

Question 1

allows cell to take up macromolecules, fluids, and large particles such as bacteria

Answer 1

endocytosis

Question 2

general principle of endocytosis

Answer 2

taken up material is surrounded by an area of PM, which buds off inside the cell to form a vesicle containing the ingested material

Question 3

2 main activities in endocytosis

Answer 3

1. phagocytosis
2. macropinocytosis

Question 4

ingestion of large particles → cell eating

Answer 4

phagocytosis

Question 5

give an overview of phagocytosis

Answer 5

1. binding of particle to receptors on the cell surface triggers the extension of pseudopodia
2. pseudopodia surround the particle and fuse to form a large vesicle called a phagosome
3. phagosomes fuse with lysosomes to form phagolysosomes, in which material is digested by lysosomal acid hydrolases

Question 6

in phagocytosis, pseudopodia surround the particle and fuse to form a large vesicle called a ()

Answer 6

phagosome

Question 7

in phagocytosis, phagosomes fuse with lysosomes to form (), in which material is digested by lysosomal acid hydrolases

Answer 7

phagolysosomes

Question 8

function/application of phagocytosis

Answer 8

• many amoebas use phagocytosis to capture food particles (e.g. bacteria)
• in multicellular animals, phagocytosis is used as a defense mechanism against invading microorganisms and to eliminate aged or damaged cells

Question 9

in mammals, macrophages, neutrophils, and dendritic cells (WBCs) are ()

Answer 9

professional phagocytes

Question 10

in mammals, () are professional phagocytes

Answer 10

macrophages, neutrophils, and dendritic cells (WBCs)

Question 11

() in particular eliminate aged or dead cells from tissues throughout the body

Answer 11

macrophages

Question 12

uptake of extracellular fluids in large vesicles

Answer 12

macropinocytosis

Question 13

give an overview of macropinocytosis

Answer 13

• lamellipodia (sheet-like projections of PM) curve into open cups
• membrane then fuses to form large intracellular vesicles

Question 14

() provides a mechanism for the selective uptake of specific macromolecules

Answer 14

clathrin-mediated endocytosis

Question 15

overview of clathrin-mediated endocytosis

Answer 15

1. macromolecules bind to cell surface receptors in specialized regions called clathrin-coated pits
2. internalization signals on the receptors bind to cytosolic adaptor proteins, which in turn bind clathrin on the cytosolic side of the membrane
3. clathrin then assembles into a basket-like structure that distorts the membrane and forms invaginated pits
4. pits bud from the membrane with the help of dynamin driven by GTP hydrolysis to form small clathrin-coated vesicles
5. clathrin-coated vesicles then fuse with early endosomes

Question 16

in clathrin-mediated endocytosis, macromolecules bind to cell surface receptors in specialized regions called ()

Answer 16

clathrin-coated pits

Question 17

in clathrin-mediated endocytosis, pits bud from the membrane with the help of (1) driven by (2) to form (3)

Answer 17

1. dynamin
2. GTP hydrolysis
3. small clathrin-coated vesicles

Question 18

() provided a key model of understanding clathrin-mediated endocytosis at the molecular level

Answer 18

uptake of cholesterol by mammalian cells

Question 19

cholesterol is transported through the bloodstream in the form of lipoprotein particles, most commonly ()

Answer 19

low-density lipoprotein, LDL

Question 20

clathrin-mediated endocytosis was first studied in patients with ()

Answer 20

familial cholesterolemia (FH)

Question 21

cells of familial cholesterolemia (FH) patients were unable to internalize (1) from extracellular fluids due to (2)

Answer 21

1. LDL
2. inability to bind to LDL

Question 22

why were familial cholesterolemia (FH) cells unable to bind to LDL?

Answer 22

• mutations prevented LDL receptor from concentrating in the coated pits within the cytoplasmic tail of the receptor
• mutations are commonly the replacement of tyrosine with cystein in the internalization signals of the LDL receptor

Question 23

cells also possess several pathways of (), which does not appear to involve selection of specific plasma membrane receptors or the formation of coated vesicles

Answer 23

clathrin-independent endocytosis

Question 24

following their internalization, clathrin-coated vesicles rapidly shed their coats and fuse with ()

Answer 24

early endosomes

Question 25

important feature of early endosomes is that they () as the results of a membrane H+ pump

Answer 25

maintain acidic internal pH (about 6.0 to 6.2)

Question 26

why is it important for early endosome to maintain acidic internal pH?

Answer 26

acidic pH leads to dissociation of many ligands from their receptors → ligands can be transported to different intracellular destinations

Question 27

() is a major fate of membrane proteins taken up by clathrin-mediated endocytosis

Answer 27

recycling in the plasma membrane