Lysosomes

The lysosomes are organelles globular, formed by the Golgi apparatus, present in both animal cells and plant.

Lysosomes are membranous vesicles, inside which intracellular digestion takes place. Its membrane is of the 75 Å unitary type, covered on the inside by a thick layer of glycoproteins that protect it from destruction by the action of hydrolase enzymes. The lysosome membrane contains a proton pump that introduces H⁺ and maintains the acidic pH, below 5, into the lysosome, and transport proteins that allow the products resulting from digestion to pass into the cytosol.

By Alejandro Porto (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Chemical composition and types of lysosomes

The lysosomes containing about 50 different enzymes type acid hydrolases, synthesized in the rough endoplasmic reticulum. They are enzymes capable of digesting macromolecules and function optimally in a medium with a pH=5. The pH remains acidic thanks to the proton pumps of the lysosome membrane. They also have transport proteins that allow digested products to pass into the cytosol. The lysosome membrane contains highly glycosylated integral acid proteins, which protect it from the enzymes it contains.

Lysosomes contain a large number of hydrolytic enzymes, but their content can be very different. Two types of lysosomes are distinguished:

• Primary lysosomes (also called "hydrolase vesicles"). They only contain hydrolytic enzymes. They are vesicles of secretion newly formed by budding from the cisternae of the Golgi apparatus. They can form secondary lysosomes or expel their contents to the outside.
• Secondary lysosomes. They also contain hydrolases, but also substrates in the process of digestion . They are primary lysosomes fused with other substances, of internal or external origin. According to the substance they hydrolyze, two types are distinguished:
  • Heterophagic vacuoles: fusion of a primary lysosome with particles that come from outside the cell (phagolysosome).
  • Autophagic vacuoles: fusion of a primary lysosome with different parts of the cell that are no longer needed or damaged areas (autophagolysosome).

Functions of lysosomes

The lysosomes are responsible for the digestion of macromolecules, although in some cases, serve as temporary storage for reservations:

1.- Cell digestion. Lysosomes can digest macromolecules, both internal and external. This function is performed in two ways:

• Autophagy. It matters that digests is a part of the cell, as fragments of the endoplasmic reticulum or Golgi apparatus cisterns. Autophagy is important for cells, as it destroys damaged areas, intervenes in cell development, and allows cell nutrition when conditions are unfavorable.
• Heterophagy. What is digested has an external origin. It has the dual purpose of nurturing and defending the cell. The substrates enter the cell by endocytosis, forming a vesicle that fuses to a primary lysosome, originating a digestive vacuole, where digestion occurs. The products of digestion pass into the hyaloplasm, where they are reused. What cannot be digested, is expelled to the outside by exocytosis, being able to fuse with the plasma membrane.

2.- Storage of reserve substances. In seeds there is a special type of secondary lysosomes, aleurone grains, which store reserve substances. Cellular digestion does not occur until germination time. Hydrolytic enzymes are then activated, initiating intracellular digestion, the products of which will be used in the development of the embryo.