Apr 16, 2013

Lysosome vs Peroxisome

Lysosomes and peroxisomes both break down substances within the cell. Learning which substances they break down and in what contexts they function will make it easier to tell them apart.

Characteristics of lysosomes:
  • fuses with food vacuoles and membranous vesicles
  • contains hydrolytic enzymes for digestion
  • digests macromolecules
  • breaks down damaged organelles
  • built with proteins from the rough ER
Lysosomes function within the context of the endomembrane system, which moves membrane lipids and proteins through the cell and cell membrane. For example, the rough endoplasmic reticulum (rough ER) sends membranes and proteins to the golgi apparatus for packaging and modification. The golgi apparatus sends its modified lipids and proteins out to fuse with the cell membrane. Lysosomes participate in the endomembrane system by fusing with membrane bound vesicles and organelles.

Lysosomes digest substances such as macromolecules, once they fuse with membrane bound vesicles. In this way lysosomes help break down substances which can be recycled for other cellular needs. For example, as amoebas engulf food via phagocytosis, part of the cell membrane forms around it as it moves into the cell. The membrane enclosed food is called a food vacuole. A lysosome will then fuse with the food vacuole to digest its contents.

Characteristics of peroxisomes
  • break down substances via oxidation
  • detoxify
  • break down fatty acids
  • built with proteins from ribosomes in the cytosol
  • do not bud off of the endomembrane system
Peroxisomes are not technically part of the endomembrane system. Each peroxisome is enclosed by one membrane and considered to be an organelle. However, peroxisomes do not obtain their necessary building blocks like other membrane enclosed organelles. For example, lysosomes receive their proteins from ribosomes bound to rough ER. Ribosomes in the cytosol, not the rough ER, provide peroxisomes with their proteins. Peroxisomes can synthesize some of their own membrane lipids, but also obtain some from the ER. 

Peroxisomes can increase in number by dividing once they grow large enough.
Peroxisomes break down substances by oxidation. Oxidation produces hydrogen peroxide by transferring a hydrogen from the target substance to oxygen. You can remember the oxidative function of peroxisomes by the "oxi" part of this organelles name, which sounds much like hydrogen "peroxide." The oxidative function of peroxisomes is useful for breaking down toxins. For example, the peroxisomes in liver cells help break down alcohol. 

Peroxisomes safely sequester and break down toxic substances, including hydrogen peroxide and products of oxidation, from the cell and other endomembrane organelles.
In addition to toxins, peroxisomes also break down fatty acids which are then sent to the mitochondria for cellular respiration. Glyoxysomes, a specialized type of peroxisome, convert fatty acids into sugar in germinating plant seeds.