Vesicular Transport

Question 1

Vesicular transport

Answer 1

Sometimes cells need to transport unusual or large substances through the membrane for which there isn’t a specific carrier protein. Or it may need to store a substance prior to release to the ECF. To do this it needs a different sort of transport mechanism

Question 2

VT 2

Answer 2

Materials are transferred between ECF and ICF
or between structures within cell in vesicles, small membranous sacs that form at or fuse with the cell membrane
Requires energy from ATP

Question 3

Endocytosis:

Answer 3

vesicular transport into the cell

Question 4

Exocytosis:

Answer 4

vesicular transport out of the cell

Question 5

Vesicle formation:

Answer 5

1. Trigger (ligand, presence of object, etc)
2. Indentation of the membrane
3. “Pouch” formed on membrane
4. Neck of pouch sealed off
5. Vesicle detached from the membrane

Question 6

Endocytosis: Receptor-mediated endocytosis

Answer 6

Uptake of specific substances bound to membrane surface receptors
…..collection of specific molecules
1. Substances attach to
membrane receptors. 
2. Membrane pockets 
inward.
3. Pocket pinches off 
as endocytic vesicle 
containing target molecule.

• Cell keeps membrane recycling and reuse receptors

Question 7

Endocytosis: Pinocytosis

Answer 7

• “Cell drinking”- vesicle forms filled with ECF
• Similar process to receptor-mediated endocytosis but not as selective and not initiated by ligand binding
• Target seems to be general ECF contents
• Performed by most cells

Question 8

Endocytosis: Phagocytosis

Answer 8

• Engulfs solid objects - may be as large as the cell itself
• Performed by specialised cells eg. macrophages

Vesicle (called phagosome)
fuses with lysosome>Lysosomal enzymes digest contents > Remains may be exocytosed

Scanning electron micrograph of white blood cell phagocytosing old red blood cell

Question 9

Exocytosis

Answer 9

Functional reverse of endocytosis

• Vesicles are created in Endoplasmic Reticulum (ER) or Golgi apparatus within cell cytoplasm
• Basically four types of vesicles produced, only some of which are directly involved in exocytosis

Question 10

Exocytosis

Four forms of vesicles:

Answer 10

Transfer vesicles - transfer enzymes between ER and Golgi apparatus and between saccules of Golgi apparatus

• Lysosomes - contain intracellular enzymes
• Secretory vesicles - allows cell to secrete products such as hormones
• Membrane renewal vesicles - contain new/recycled membrane components, e.g. phospholipids, carrier proteins, receptors

Question 11

Endoplasmic reticulum

Answer 11

Produces transfer vesicles

• Network of intracellular fluid-filled tubules and flattened sacs
• Synthesises and stores proteins, carbohydrates and lipids
• Detoxifies drugs or toxins Transfers products to the golgi bodies (via transfer vesicles) for further processing

Question 12

Golgi apparatus

Answer 12

Produces lysosomes, membrane renewal vesicles, secretory vesicles and transfer vesicles

• Sets of stacked, flattened membraneous discs called “saccules”
• Processes material from ER into final form
• Synthesises & packages enzymes for use within the cell - Synthesises & packages secretions, such as hormones etc, for release through exocytosis
• Produces new membrane and membrane structures

Question 13

Lysosomes

Answer 13

Produced by the Golgi 
apparatus, contain digestive 
enzymes. 
Bind to vesicles to form 
secondary lysosomes.
Digest or modify the contents
of other vesicles, which may 
be released into the cytosol 
or be exocytosed
Eg. Phagocytosis

Question 14

Membrane renewal vesicles

Answer 14

Add new lipids and proteins to cell membrane
- Allows cell to change number and types of carrier proteins or receptors in cell membrane thus changing sensitivity of cell

Question 15

Secretory vesicles

Answer 15

Produced by Golgi apparatus of cells specialised for secretion of hormones or enzymes

• Vesicles bud off from the Golgi apparatus and then fuse with cell surface membrane to release products to ECF
• Secretory vesicles may release products constitutively (unregulated) or may remain in the cell until release is triggered (regulated)

Question 16

Secretory vesicles can be either:

Answer 16

Constitutive
…..continuous, unregulated output of material
Or regulated e.g. neurons, exocytosis regulated at a synapse:
Action potential opens voltage-gated Ca++ channels, Ca++ triggers the exocytosis of neurotransmitter

Question 17

Endocytosis and Exocytosis:

Answer 17

For the cell to maintain its size and membrane surface area there must be a balance between the intake of cell membrane during endocytosis and the incorporation of membrane during exocytosis.