Lysosome

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Lysosomes are membrane bound organelles found in <a href="Eukaryotic cells">Eukaryotic cells</a>. They were first discovered by the belgian cytologist '<a href="Christian Rene de Duve">Christian Rene de duve</a>' in the 1950s [1]. They are around 0.5-1.0 µm in diameter[2].

Lysosomes, the digestive system of the cell, break down substances from outside the cell as well as digesting material from inside the cell. The organelle, which is mosly sperical in shape, varies in size and shape, depending on the material that is being broken down [3]. Lysosomes contain up to 40 different types of <a href="Hydrolytic enzymes">hydrolytic enzymes</a> which control the digestion of macromolecules, such as <a href="Nucleic acids">nucleic acids</a> and <a href="Proteins">proteins</a>, old cell parts, and other foriegn pathogens. Some of the most common lysosomal enzymes include <a href="Proteases">proteases</a>, <a href="Nucleases">nucleases</a>, <a href="Glycosidases">glycosidases</a>, <a href="Lipases">lipases</a>, <a href="Phospholipases">phospholipases</a>, <a href="Phosphatases">phosphatases</a> and <a href="Sulfatases">sulfatases</a> which are all <a href="Acid hydrolases">acid hydrolases</a> [4]. 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 <a href="Vacular ATPase proton pump">vacular ATPase proton pump</a> 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 <a href="Endocytosis">endocytosis</a>, <a href="Autophagy">autophagy</a> and <a href="Phagocytosis">phagocytosis</a>. 

Endocytosis

In endocytosis, <a href="Molecule">molecules</a> are taken in from the outside the cell and packaged into <a href="Vesicles">vesicles</a>. These vesicles containing macromolecules and other small substances fuse with <a href="Organelles">organelles</a> inside the cell called <a href="Early endosomes">early endosomes</a>, which mature into <a href="Late endosomes">late endosomes</a> by becoming more acidic (roughly pH 6) due to the <a href="V-ATPase">V-ATPase</a>. It is here, in the late endosomes, where the digestion of the macromolecule starts. <a href="Late endosomes">Late endosomes</a> are sometimes known as <a href="Multivesicular bodies">multivesicular bodies</a> (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 <a href="RAB5">RAB5</a> <a href="GTPase">GTPase</a> with <a href="RAB7">RAB7</a> [5].

Autophagy

Autophagy or autophagocytosis, is a process involving the use of lysosomes to digest intracellular components of the cell. There are two different autophagic pathways: selective and non-selective autophagy. Selective autophagy is the specific degradation of certain damaged organelles or protein aggregates in order to maintain homeostasis. Non-selective autophagy involves the random sequesteration of cytoplasm to then by degraded in a lysosome so that the cytoplasm can be recycled - the resulting metabolites are needed by the cell for survival since this type of autophagy occurs when the cell is put under stress (e.g. starvation).

The autophagic pathway involves the formation of an autophagosome, a double membrane structure, around the unwanted molecule. The autophagosome then fuses with a lysosome, the contents of which are digested by the hydrolytic enzymes. Permeases in the lysosomal membrane allow the resulting amino acids to be translocated into the cytoplasm where they can then be used to synthesise newn proteins[6].

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

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