These are membrane bounded organelles that are a part of the endomembrane system in that their proteins are synthesized on the RER, travel to and are sorted in the Golgi. Primary lysosomes bud from the trans golgi network.
A primary lysosome is a membrane bounded sac that buds from the golgi apparatus.
A primary lysosome contains many enzymes (collectively called acid hydrolases) that are synthesized on the RER and sorted in the golgi.
The membrane of the lysosome has intergral proteins called hydrogen ion ATPases (proton pumps). These protein are involved in an active transport process whereby protons are transported into the lumen of lysosome from the cytosol of the cell. Since protons are being transported against their concentration gradient the proton pump is an ATPase. The relatively high concentraction of protons in the lumen of the lysosome results in a pH of about 5.0 in the lumen of the lysosome.
The "acid hydrolases" in the lysosome work best at this acidic (5.0) pH.
There are many differet enzymes in lysosomes. However they all function best in an acidic pH (5.0). In the table below some of the enzymes and catagories of enzyme are listed.
enzymes |
normal substrate |
disease of |
DNase and RNase |
RNA and DNA |
|
many different acid proteases |
proteins |
` |
polysaccharide and mucopolysaccharide degrading enzymes |
disaccharides, polysaccharides, glycolipids, glycoproteins, mucopolysaccharides |
Krabbe's disease (beta galactosidase deficient) Pompe's diseaase (alpha glucosidase deficient) Wollman's disease (acid lipase deficient) |
lipid degrading enzymes |
fatty acid esters, phospholipids |
Tay-Sach's disease (hexoseaminidase A deficient) Farber's disease (ceramidase deficient) |
The term auto (self) phage (to eat) literally refers to a process whereby the cell digests its own contents. This process is most often refered to as " cellular turnover." Whatever it is called, it refers to a very important, normal cellular process. Many important cellular constituents , such as proteins, organelles and membranes, are in a constant state of flux. That is they are constantly being synthesized and degraded. Indeed, an interuption of this process can lead to a disease condition such as Tay Sach's disease.
In the diagram below a cellular constituent such as a mitochondria fuses with a primary lysosome. The enzymes of the lysosome digest the macromolecules and the digestion products can be resused by the cell or ejected by and exocytotic process.
The term hetero (different) phagy (to eat) refers to a process whereby lysosmes aid in the intracellular digestion of material gathered from outside of the cell by some kind of endocytotic mechanism. The endocytotically ingested material is sequested in a membrane bounded vesicle.
In the diagram above bacteria are ingested by phagocytosis. The ingested bacteruia are packaged in a phagocytic vesicle. This vesicle fuses with a primary lysosome forming a secondary lysosome. The enzymes of the lysosome digest the bacterial macromolecules.