Lysosome
A lysosomes are membrane bound organelles found in Eukaryotic cells. They were first discovered by the belgian cytologist 'Christian Rene de duve' in the 1950s .[1]
Lysosomes contain up to 40 different types of hydrolytic enzymes which control the digestion of macromolecules, such as nucleic acids and proteins, old cell parts, and other foriegn pathogens. Some of the most common lysosomal enzymes include proteases, nucleases, glycosidases, lipases, phospolipases, phosphatases and sulfatases which are all acid hydrolases. [2] These enzymes optimum pH is at 4.5 - 5.5 which, compared to the cytosols pH of 7.2, is very acidic. The acidic pH of the lumen is maintained by a vacular ATPase proton pump which pumps H+ Into the lysosome. These conditions help protect the cell from possible 'leakages', as the hydrolytic enzymes would not be able to function in the neutral pH of the cytosol.
Lysosomes are linked to a number of pathways, mainly endocytosis, autophagy and phagocytosis.
Endocytosis:
In endocytosis, molecules are taken in from the outside the cell and packaged into vesicles. These vesicles containing macromolecules and other small substances fuse with organelles inside the cell called early endosomes, which mature into late endosomes by becoming more acidic (roughly pH 6) due to the V-ATPase. It is here, in the late endosomes, where the digestion of the macromolecule starts. Late endosomes are sometimes known as multivesicular bodies (MVBs) as during the mauturing stage, some molecules are sorted into smaller vesicles, forming lumenal vesicals in the endosome lumen. Before the late endosomes fuse with a lysosome, they replace the RAB5 GTPase with RAB7.[3]
Autophagy:
Autophagy or autophagocytosis, uses lysosomes to digest parts of the cell that are no longer needed. For a autophagosome to form, a double membrane forms around the unwanted molecule, which then fuses with a lysosome. The contents of which are digested by the lysosomal enzymes. Autophagy plays an important role in maintaining the basic cell contents, but also plays a role in protecting the cell against certain pathogens and also maintaining the cells nutrient levels when starvation occurs making sure the cells vital processes can continue.
References:
- ↑ http://www.britannica.com/EBchecked/topic/174733/Christian-Rene-de-Duve?anchor=ref23999
- ↑ Bruce Alberts [et al] "Molecular biology of the cell" 5th edition.
- ↑ Cell, Volume 122, Issue 5 2005, Rab Conversion as a Mechanism of Progression from Early to Late Endosomes: http://www.cell.com/retrieve/pii/S0092867405006975