Vesicular Transport

Question 1

How do SMALL molecules travel across the membrane?

Answer 1

Water and small molecules enter and leave the cell by passing through the lipid bilayer of the cell membrane. They pass through by means of:
1. Passive Transport
* Passive diffusion
* Facilitated diffusion
2. Active Transport

Question 2

What protein is involved in
facilitated diffusion?

Answer 2

Channel proteins

Question 3

What protein is involved in
active transport?

Answer 3

Carrier proteins

Question 4

How do LARGE molecules travel across the membrane?

Answer 4

Proteins and polysaccharides and larger particles cross the membrane using VESICLES.
The membranes of organelles communicate with each other through fusion of the vesicles.

Question 5

Key processes of vesicular transport

Answer 5

EXOCYTOSIS – secretions of macromolecules through fusion of vesicles with the plasma membrane.
* ENDOCYTOSIS – cell takes in macromolecules by forming new vesicles from the plasma membrane.

Question 6

Budding & Fusion

Answer 6

• Budding from donor.
• Fusion with target.
• Membrane is transferred.
• Proteins retain original configuration.
• Soluble components transferred.

Question 7

Vesicles

Answer 7

• Form part of the endomembrane system.
• Small membrane-bound sacs.
• Transport or store substances.
• Membrane is made of two layers = lipid bilayer.
• Can fuse with organelles to release their contents within the cell.
• Can also fuse with the cell/plasma membrane and release their contents outside of the cell.

Question 8

Types of vesicles

Answer 8

• Vacuoles
• Lysosomes
• Peroxisomes
• Endosomes
• Transport vesicles
• Secretory vesicles

Question 9

Exocytosis

Answer 9

• A transport vesicle buds from the golgi apparatus and moves to the plasma membrane.
• Vesicular membrane and plasma membrane make contact.
• Bilayers re-arrange to allow fusion.
• Cargo in the vesicle released into the extracellular fluid.
• Vesicular membrane becomes part of the plasma membrane.

Question 10

Types of Exocytosis

Answer 10

1. Constitutive exocytosis
2. Regulated exocytosis

Question 11

Constitutive exocytosis

Answer 11

• Steady stream of transport vesicles from trans Golgi to plasma membrane.
• New lipids and proteins are continuously supplied to the plasma membrane for membrane growth, rejuvenation and remodelling.

Question 12

Regulated exocytosis

Answer 12

• Functions only in cells specialised for secretion.
• Lots of secretory vesicles found in specialised secretory cells- hormones, mucous, digestive enzymes.
• Extracellular signal will stimulate their fusion with the plasma membrane and release into the extracellular fluid.

Question 13

Endocytosis

Answer 13

• New vesicles are formed by the plasma membrane.
• It is the reverse process of exocytosis, using different proteins.
• Plasma membrane pinches in to form a vesicle containing extracellular material.
• Three types of endocytosis.

Question 14

Types of Endocytosis

Answer 14

1. Phagocytosis
2. Pinocytosis
3. Receptor-mediated Endocytosis

Question 15

Phagocytosis

Answer 15

• A cell engulfs a particle, wrapping pseudopodia around it and packaging it into a membrane enclosed sac large enough to be classified as a vacuole.
• The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.
• Protozoa use phagocytosis to feed.
• Macrophages defend against microorganisms.
• Macrophages/cleaner cells –engulf old RBCs.
• Known as Cellular eating!

Question 16

Pinocytosis

Answer 16

• engulfment of extracellular fluid by wrapping cell membrane around it to form a vacuole/taken into small vesicles.
• Example: Droplets of extracellular fluid enters the cells via small vesicles.
• Pinocytic vesicles are returned to the cell surface after ingestion.
• Macrophages swallow 25% of their own volume/hour.
• Known as Cellular drinking!

Question 17

Receptor-mediated Endocytosis

Answer 17

• Selective uptake of specific macromolecules from extracellular fluid.
• Receptor proteins in the membrane exposed to the extracellular fluid.
• Extracellular substances (ligands) bind to the receptor sites on receptor proteins.
• Receptor proteins are clustered in parts of the membrane called ‘coated pits’.
• The cytoplasmic side of this area of membrane is coated with a special protein.
• Example: cholesterol travels in blood in particles called low-density lipoproteins (LDLs) which bind to LDL receptors on the membrane.

Question 18

Types of coated vesicles

Answer 18

1. CLATHRIN
2. COP I
3. COPII

Question 19

Clathrin

Answer 19

• protein discovered in 1975 and is a ‘triskelion’ comprised of 3 light chains and 3 heavy chains.
• coated vesicles traffic between the golgi network and the lysosomes; and between the plasma membrane and the endosomes.
• Triskelions form a framework of hexagons and pentagons to form coated pits on the membrane surface.

Question 20

Formation of a clathrin coated vesicle

Answer 20

1. Molecules bind to surface receptors of the plasma membrane proteins located in areas of clathrin-coated pits.
2. Pits bud to form clathrin-coated vesicles.
3. Fusion with endosomes or lysosomes.

Question 21

Endosomes

Answer 21

• appear as a complex set of connected membrane tubes and larger vesicles.
• Two populations of endosomes:
  
  • Early endosomes (beneath plasma membrane) (pH6.0-6.2).
  • Late endosomes (near nucleus) (pH5.5-6.0).
• Endosome – main sorting station in the endocytic pathway.
• Acidic environment promotes cargo release.