Endocytosis

Question 1

Define endocytosis:

Answer 1

The cellular process of internalising substances from the extracellular environment into the cell via membrane-bound vesicles

Question 2

What are the types of endocytosis ?

Answer 2

Phagocytosis: Engulfment of large particles.
Pinocytosis: Uptake of fluids and solutes.
Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor interaction.

Question 3

What are the roles of endocytosis in cells ?

Answer 3

Nutrient Uptake - transport of essential molecules (e.g., lipids, iron via transferrin).

Receptor Regulation - downregulation or recycling of membrane receptors.

Pathogen Defense - phagocytosis of pathogens by immune cells.

Signal Transduction - regulation of signalling pathways through receptor internalisation.

Plasma Membrane Turnover - renewal and remodelling of membrane components

Question 4

Define phagocytosis and its functions;

Answer 4

Specialised form of endocytosis involving the engulfment of large particles (e.g., pathogens, cell debris) by phagocytes (e.g., macrophages, neutrophils)

Functions;

Host Defence - destruction of pathogens via lysosomal enzymes

Clearance of Debris - removal of apoptotic cells

Tissue Homeostasis - prevention of inflammation through efficient debris clearance

Question 5

Describe the endosomal pathway:

Answer 5

Early endosome;
- initial sorting of internalised materials
- recycling of proteins back into plasma membrane

Late endosome
- maturation stage, cargo destined for degradation

Lysosome;
- degradation of materials via hydrolytic enzymes

Question 6

What proteins are involved in sorting ?

Answer 6

Exogenous proteins - uptake and delivery for degradation or recycling

Endogenous proteins - sorting of membrane proteins for recycling or degradation

Question 7

What are the control mechanisms of endocytosis ?

Answer 7

Signal recognition - receptor ligand interactions trigger vesicle formation

Actin cytoskeleton - facilitates vesicle movement and plasma membrane remodelling

Membrane curvature proteins - BAR-domain proteins and dynamin aid vesicle scission

GTPases (e.g., Rab Proteins) - Regulate vesicle trafficking and fusion

Question 8

Describe the process of Receptor-Mediated Endocytosis and Clathrin-Coated Vesicles:

Answer 8

Ligand binds to specific receptors on the plasma membrane.
Receptors cluster into clathrin-coated pits.
Clathrin-coat assembles, driving vesicle formation.
Dynamin mediates vesicle scission.

Question 9

Describe the role of endocytosis in disease:

Answer 9

Hypercholesterolemia - Mutations in LDL receptors impair cholesterol uptake

Neurodegenerative Diseases - disrupted protein trafficking contributes to diseases like Alzheimer’s

Infectious diseases - viruses exploit endocytosis for entry

Question 10

Describe the process of Plasma Membrane and Neurotransmitter Recycling at the Synapse:

Answer 10

Neurotransmitters are released via exocytosis and reabsorbed via endocytosis.
Synaptic vesicles are recycled for reuse through the endosomal system.

Question 11

Describe Inhibition by Neurotoxins:

Answer 11

Botulinum Toxin: Blocks acetylcholine release by cleaving SNARE proteins.
Tetanus Toxin: Inhibits inhibitory neurotransmitter release, leading to spastic paralysis