What is Lysosomes?

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Ned
What is Lysosomes, Origin and Occurrence,
Morphology, Mechanism and its Functions.
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Lysosomes are small unit membrane bound sacs that store acid hydrolases . It is also
known as "Suicidal bags". Lysosomes helps to digest the food, outside bodies and
intracellular debris . It also help to recycle cell components. We can only see lysosomes
under an electron microscope .
In 1955, Belgian cytologist and biochemist Christain de Duve firstly reported the
lysosomes on the basis of biochemical study. But the term lysosome was coined by
Novikoff. In 1956 Novikoff observed lysosomes in the cell with the help of an electron
microscope.
Origin and Occurrence - The lysosomes arise from the Golgi complex. Their membrane
and hydrolytic enzymes are synthesized on the Rough Endoplasmic Reticulum, then
transported in transport vesicles to the Golgi complex for modification and packaging.
Secretory vesicles filled with lysosomal enzymes bud off from the trans face of the Golgi
complex as primary lysosomes.
The lysosomes occur in animal cells, Plant cells, fungi and protozoans. But they are less
prevalent in plant cells than in animal cells.
Animal lysosomes
Morphology - The lysosomes are generally in spherical form and vary in size from 0.2 to
0.8 micrometer. It is a tiny sac bounded by a single unit membrane of lipoprotein.
Lysosomes contain a dense, finely granular fluid consisting of digestive enzymes for
hydrolysis called acid hydrolases. It includes lipases, nucleases, proteases, acid
phosphatases. More than 40 different digestive enzymes present in human lysosomes.
These enzymes break all the major biological macromolecules that present in the cells.
Lysosomes pH is 5 and these are active in acidic medium.

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Structure of lysosome
MECHANISM -
As we know that lysosomes store the hydrolyzing enzymes in the cell. Their membrane
prevents the enzymes from escaping into the cytoplasm and destroying it. The material
needing hydrolysis must enter the lysosomes because enzymes remain confined within
them. When the cells are injured and dead, the lysosome membrane ruptures and
releases the enzymes that lyse the weakened cells. This process is known as post-
mortem degeneration. Also, when an outside particle enters inside the cells, the
lysosomes engulf them and are disposed of by exocytosis.
General Mechanism of Lysosomes
Kinds - according to the time, the lysosomes change their nature of contents in the same
cell. This change or this variation is referred to as polymorphism.

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Four types of lysosomes are recognized on the basis of their contents.
1) Primary lysosomes - Those lysosomes which are newly formed and only contain
enzymes are known as primary lysosomes. In primary lysosomes the enzymes are in an
inactive state.
2) Secondary Lysosomes - The fusion of a primary lysosomes with a vacuole or secretory
granule is known as secondary lysosomes. In this, some material to be digested enters
primary lysosomes.
3) Residual bodies - After the digestion of incoming material the indigestible matter
remains in the secondary lysosomes are known as residual bodies. This residual body
after that meets with the cell membrane and by exocytosis released from the cell.
4) Autophagic Vacuoles - when a cell digests its own organelles known as autophagic
vacuoles. It is also known as Autolysis means breakdown.
Plant Lysosomes -
In plants, lysosomes contain certain reserve food material beside the digestive enzymes.
Plant lysosomes are 3 Kinds -
1) Spherosomes - The Spherosomes are spherical bodies about 0.5-1 micrometer wide
and enclosed by a single membrane. These spherosomes contain granules that are rich
in lipids but also have some protein. Spherosomes arise from endoplasmic reticulum.
They occur in most plants. Also, in Maize root tip the spherosomes have hydrolytic
enzymes.
2) Aleurone Grains - are membrane -bound, spherical bodies containing proteins and
phosphate. They occur in the cells of endosperm and cotyledons of seeds.
3) Vacuoles - are rounded sacs bounded by single unit membranes containing a variety of
hydrolytic enzymes.

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Functions -
The main function of lysosomes is to take part in several decomposition processes in a
cell. Like -
1) Digestion of Harmful material - The lysosomes dispose of harmful particles like viruses,
bacteria and toxic molecules by hydrolyzing them in certain leukocytes and macrophages.
This process is known as natural defence.
2) Digestion of Useful materials - Cells take the organic substance in vacuoles from the
environment and then digest it in lysosomes. This is called intracellular digestion.
3) Cell's Protection - The lysosomal membrane protects the cell contents from autolysis
by lysosomal enzymes.
4) Renewal of Cells and Organelles - The lysosomes have a facility to renew the cells and
organelles.
5) Digestion of unwanted material - The lysosomes digest the dead cells and debris that
accumulate at the site of injury.
Some Important Questions: -
Q1) What is Autolysis?
Ans. Auto means itself and lysis means breakdown. The breakdown of a cell by its own
enzymes called autolysis.
Q2) What is Exocytosis?
Ans. Some unwanted material is disposed of from the cell membrane called exocytosis.
Reference book- Pradeep's Biology Text Book .

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