TOPIC 13. Membranous cell organelles - Lysosomes. Peroxisomes.
Lysosomes structure
Lysosomes are spherical, single membrane-bound vesicles containing around 40 different hydrolytic enzymes and are particularly abundant in cells with great phagocytic activity e.g. macrophages and neutrophils. They are slightly larger in these cells and are visible with light microscope especially after staining.
- Lysosomal hydrolases are synthesised on ribosomes of the rough ER.
- They are packaged into vesicles by the Golgi body.
- Main enzymes include proteases, nucleases, phosphatases, and sulfatases.
- The lysosomal membrane protects cytosolic components from these enzymes.
- Enzymes are only active at acidic pH, so any that leak into the cytosol become inactive and harmless.
Primary lysosome/protolysosome
Primary lysosome is a newly formed lysosome that has encountered a substrate on which their enzymes may act.
Secondary lysosomes
Secondary lysosomes are the result of the fusion of the primary lysosome with a phagosome (phagocytic vesicle) containing material to be digested.
Heterophagosome – formed by fusion of primary lysosome to phagosome which contains extracellular material. The lysosomes empty their hydrolases into the newly formed heterophagosome.
Autolysosome – formed by the fusion of lysosome with autophagosome containing partially degraded cytoplasmic components surrounded by membrane formed from the RER.
Functions
- 1. Lysosomes are the sites for intercellular digestion of molecules, cellular components, exogenous materials, dead cells and worn out organelles.
- 2. They defend against invading bacteria and viruses.
- 3. Dissolve blood clots.
- 4. Secretion of hydrolases by sperm to penetrate egg.