Bulk Transport

Notes

BULK TRANSPORT

Endocytosis

Exocytosis

Delivery of intracellular material to the plasma membrane or extracellular space

Both processes work together to maintain the fluid balance and membrane balance of the cell

EXOCYTOSIS

1) Proteins synthesized in the endoplasmic reticulum and trafficked to the Golgi apparatus

2) From the Golgi, the proteins are sorted and can follow different paths:

3a) Constitutive secretory pathway

Also called default pathway as it does not require a signal for cargo to enter this pathway
Can transport soluble proteins or plasma membrane proteins
Transport vesicles fuse with the plasma membrane and release their contents (plasma membrane proteins become accessible to the extracellular space)

3b) Regulated secretory pathway

Found mainly in specialized cells that release molecules like hormones, neurotransmitters, etc.
Secretory vesicles are blocked from fusing with the plasma membrane without a proper signal
An extracellular ligand binds a cellular receptor which causes an intracellular signal that releases the block on the secretory vesicles
Secretory vesicles then fuse with the plasma membrane and release their cargo

ENDOCYTOSIS

Membrane invagination has solutes in it
Vesicle formed from this invagination traffics to the early endosome
Next the solutes are trafficked to the late endosome
Solutes are then delivered to the lysosome where they are broken down for use by the cell

Early Endosome vs Late Endosome

Three types of endocytosis:

1) Pinocytosis

2) Phagocytosis

3) Receptor-mediated endocytosis

Transcytosis

Clinical Correlation:

Antibodies in breast milk are absorbed by the intestinal epithelial cells, transcytosed, and ejected into the bloodstream.