Endocytosis, Exocytosis and Epithelial Transport

Question 1

state the (4) types of endocytosis that occur in cells

Answer 1

1. pinocytosis
2. phagocytosis
3. receptor mediated endocytosis
4. potocytosis (special type of receptor mediated endocytosis)

Question 2

explain pinocytosis (2 things)

Answer 2

1. in pinocytosis (fluid endocytosis) an endocytotic vesicle encloses a small volume of extracellular fluid
2. process is non-specific as engulfs any extracellular fluid and whatever solutes it contains (excluding macromolecules)

Question 3

explain phagocytosis (4 things)

Answer 3

1. cells engulf bacteria or debris
2. pseudopodia (large extensions of cell membranes) fold around surface of a particle entirely engulfing it
3. pseudopodia fuse into phagosomes
4. phagosomes migrate and fuse with lysosomes and content is hydrolysed by lysozymes

Question 4

explain receptor-mediated endocytosis (5 things)

Answer 4

1. molecules in extracellular fluid bind to receptors on CSM
2. receptor undergoes conformational shape change
3. clathrin binds to L-R compels via adapter proteins
4. forms a gage like structure which leads to the formation of a clathrin-coated pit which pinches of CM and forms a clathrin-coated vesicle
5. when pinched off, clathrin coat and clathrin proteins are recycled

Question 5

state 3 functions of receptor mediated endocytosis

Answer 5

1. fuse with the membranes of intracellular organelles
2. fuse with the opposite side of the membrane and undergo exocytosis
3. fuse with endosomes which modifies and processes ligands
4. fuse with lysosomes and exposes content to lysozymes
5. potocytosis

Question 6

explain how potocytosis differs to regulated receptor-mediated endocytosis (2 things)

Answer 6

1. in potocytosis, ligands are restricted to low molecular weight (eg - vitamins)
2. in potocytosis, vesicles are caveolae and caveolae deliver contents directly to cell cytosol only

Question 7

state 3 functions of caveolae

Answer 7

1. cell signalling
2. transcellular transport
3. cholesterol homeostasis

Question 8

state what it is meant by the key term ‘compensatory endocytosis’

Answer 8

compensatory endocytosis is where cells are actively undergoing exocytosis recovers bits of the plasma membrane

Question 9

state what the three functions of exocytosis is in cells

Answer 9

1. proved a way to replace plasma membrane that endocytosis has removed
2. add new components to the plasma membrane
3. enables membrane impermeable substances to exit the cell (eg - hormones)

Question 10

state 3 facts about exocytosis

Answer 10

1. exocytosis is stimulated by stimuli leading to an increase in cystolic calcium concentration
2. stimuli opens calcium ion channels in cell membrane and/or organelle membranes
3. increase in cytosolic calcium ion concentration activates the proteins required for vesicles to fuse with the cell membrane

Question 11

state what it is meant by the key term ‘apical membrane’

Answer 11

the apical membrane (luminal/mucosal membrane) is one surface of an epithelial cell facing a hollow/fluid filled chamber

Question 12

state what it is meant by the key term ‘basolateral membrane’

Answer 12

the basolateral membrane (serosal membrane) is a membrane of an epithelial cell adjacent to an network of blood vessels

Question 13

state and explain the two ways in which substances can cross the epithelial membrane

Answer 13

1. the paracellular pathway in which diffusion occurs between adjacent cells in the epithelium
2. the transcellular pathway in which a substance moves into an epithelial cell through the cytosol and exists across the basolateral membrane

Question 14

state 2 facts about diffusion in the paracellular pathway

Answer 14

1. diffusion through paracellular pathway is limited due ti presence of tight junctions between adjacent cells as junctions form a seal around apical end of epithelial cells
2. amount of paracellular diffusion is limited to the tightness of the junctional seals and relatively small surface area for diffusion

Question 15

state a fact about transcellular movement

Answer 15

during transcellular movement, the movement of molecules occurs via diffusion and mediated transport

Question 16

state and explain why transport and permeability of the apical and basolateral membranes are not the same

Answer 16

1. contain different channel proteins and different transporters
2. differences allow a net movement of substances from low to high concentration on one side of the epithelium to the other side of the epithelium