Lysosomes

Origin

Lysosomes may originate directly from the endoplasmic reticulum, or from the Golgi membranes, and then associate with vesicles which have arisen by pinocytosis. Protein granules synthesized by the ribosome are seen in enlargements of the endoplasmic reticulum.

Occurrence

Prokaryotes do not have a well defined membrane system. Organelles comparable to those of eukaryote cells, including lysosomes, therefore appear to be absent. Prokaryotes are, however, known to secrete extra cellular enzymes. Some hydrolyses have been located outside the cell membrane. This has led to the suggestion that such prokaryote cells are surrounded by an extracytoplasmic lysosome. Digestion would thus take place outside the cell, at the site of the location of hyrolases.

Electron microscope and cytochemical studies have revealed the presence of membrane bound structures containing acid phosphates in the Protozoa. These are thought to represent the primary and secondary lysosomes. A number of lysosomal hydrolases have been found in the food vacuoles of many Protozoa. The food vacuoles are equivalent to a type of secondary lysosome. It has been demonstrated that the Protozoa contain heterogeneous populations of lysosomes.
Slime moulds contain particles which appear to be lysosomes. Lysosomal localization of acid phosphates has been demonstrated in the hyphae of many fungi. The fungi secrete a wide variety of extra cellular enzymes, among which are acid hydrolyses. Among the algae, acid phosphates has been located in the lysosomes of Euglena and a few other species.

Lysosomes have been reported in several groups of vertebrates and invertebrates. Almost every animal tissue investigated has yielded evidence of the presence of lysosomes.
It not certain whether structures equivalent to animal lysosomes are present in plant cells. This uncertainty is partly the result of strict adherence to the biochemical definition of lysosomes and partly to the technical difficulties in subcellular fractionation of plant tissues. However, particles containing hydrolases and having the characteristics of lysosomes have been localized in many plant tissues. Particles isolated from tobacco and maize seedlings contain several of the hydrolases found in animal lysosomes. The presence of amylase in these structures is a similarity to some protozoan lysosomes and a difference from animal lysosomes. The balance of the evidence shows that the hydrolase-containing particles found in plant cells have a close affinity with the lysosomes of animals and of algae, fungi, slime moulds and bacteria.


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