internalization of material and subsequent lysosomal digestion:
Active transport
Movement across a membrane from an area of low concentration to high concentration.
Since carriers are involved, it demonstrates MichaelisMenten kinetics.
It is a unidirectional transport that requires energy (ATP).
a. Endocytosis—vacuole formation utilizing plasma membrane and released inside the cell.
May result in degradation.
If vacuole is fused with lysozymes, results in degradation.
(1) Pinocytosis—
internalization of interstitial fluid.
(2) Phagocytosis—
internalization of material and subsequent lysosomal digestion.
(3) Receptor mediated endocytosis—
selective internalization of receptor/ligand complex. Receptor complex may return to surface.
b. Exocytosis—secretory vesicle membrane from within the cell fuses with the plasma membrane, and the contents of the vesicle are released outside the cell.
c. Primary active transport—energy derived from ATP.
Specific carriers are utilized, which are saturable.
(1) Ca2+-ATPase pumps are important active transports in sarcoplasmic reticulum membranes.
(2) Na+K+-ATPase pump is present in all plasma membranes.
- Establishes and maintains a Na+ and K+ gradient by transporting Na+ out and K+ into the cell.
- The Na+ concentration gradient is the force for secondary active transport mechanisms.
d. Secondary active transport—ATP establishes an ionic gradient, which is the driving force (secondary transport).
(1) Na+ gradient established by membrane pumps drive this system.
(2) Protein carriers bind both the substance transported and Na+.
(3) Cotransport systems—Na+ and substance transported move in the same direction.
Examples include:
- Glucose and galactose across GI mucosa and resorption in renal tubule.
- Amino acid transport in GI mucosa and renal tubules.
(4) Countertransport—
- Na+ and transported substance move in opposite directions.
Examples include:
- Na+/H+ exchange (H+ out and Na+ into cells).
- Na+/Ca2+ exchange (Ca2+ out and Na+ into cells)
Pinocytosis
Phagocytosis
Receptor mediated endocytosis
Exocytosis