*INTRODUCTION :
-> In 1949, De Duve, using the cell fractionation technique, isolated a class of particles which were found to have a high content of ACID PHOSPHATASE and other HYDROLYTIC ENZYMES. Because of these hydrolytic properties, these particles were called LYSOSOMES.
-> They are cell organelles found both in plant and animal cells as well as in protozoa, but they are absent in bacteria (which have periplasmic space instead of lysosomes ).
-> They are an animal cell’s digestive organelles, and their main function is INTRACELLULAR and EXTRACELLULAR DIGESTION.
-> These organelles function in one of the following ways:
•Digestion of food taken in by phagocytosis.
•Digestion of parts of the cell by Autophagy
(Auto= self ; Phagy =eating )
•Breakdown of extracellular material by the release of enzymes into the surrounding medium.
* STRUCTURE:
-> Lysosomes are membrane-bound spherical vesicles, approximately 0.2 - 0.5 micrometres in size.
-> They are also referred to as suicidal bags" because their rupture can destroy the cell.
# KEY COMPONENTS :
A) SINGLE MEMBRANE: A protective lipid bilayer which contains a glycosylated lysosome-associated membrane and integral proteins.
B) ACIDIC LUMEN: The interior maintains an acidic pH essential for the activity of hydrolytic enzymes.
C) HYDROLYTIC ENZYMES: It contains 50 types of enzymes to break down fats, etc.
* PROPERTIES:-
-> A typical lysosome contains at least 50 different hydrolytic enzymes (produced in the RER ). When taken together, lysosomes can hydrolyse every type of biological macromolecule.
-> All these enzymes share an important property, i.e., all of them have their optimal activity at an acidic pH and therefore are ACID HYDROLASES.
-> An important property of lysosomes is their stability in the cell. The enzymes are enclosed in a membrane and are not readily available to the substrate.
-> This latency of lysosomal enzymes is due to the presence of membranes. Lysosomal membranes contain a variety of highly glycosylated integral proteins whose carbohydrate chains form a protective lining which shields the membrane from attack by the enclosed
enzymes and also protects the rest of the cell from the destructive effects of the enzymes.
* POLYMORPHISM:-
-> The most remarkable morphological characteristic of lysosomes is their polymorphism regarding their size and internal structure. It is the result of a primary lysosome.
-> At present, four types of lysosomes are recognised, of which only the first one is PRIMARY LYSOSOME, and the other three may be grouped as SECONDARY LYSOSOMES.
1)Primary lysosomes :
• This is a small body whose enzymatic content is synthesised by the ribosome. From there, the enzymes are transported via the E.R to the Golgi complex, in which the first acid phosphatase reaction takes place.
2)Secondary lysosome:
• Full complement of acid hydrolases is present in them.
-> The secondary lysosomes are of the following types:-
a) Heterophagosome /Digestive vacuole :
. This results from the phagocytosis or pinocytosis of foreign material by the cell. The digestive vacuole contains a positive phosphatase reaction. The engulfed material is digested by the hydrolytic enzymes which have been incorporated into the lysosome.
. Under ideal conditions, digestion leads to products of low molecular weight, which pass through the lysosomal membrane and are incorporated into the cell to be used again in many metabolic pathways.
• This is formed if the digestion is incomplete. In amoeba and other protozoans, the residual body is eliminated by defecation.
c) Autophagic vacuole/ cytolysosome/ autophagosome :
During starvation, the liver cells show numerous autophagic vacuoles. This is a mechanism by which the cell degrades its own constituents, such as mitochondria and endoplasmic reticulum.
* FUNCTIONS:-
1). Intracellular Digestion:
. Within the secondary lysosomes, carbohydrates are hydrolysed to monosaccharides, proteins- dipeptides.2). Autophagy:
. Autophagy is a mechanism by which less important cell components are broken down to facilitate survival.
3). Removal of cell and extracellular material during developmental process:
• Lysosomes bring about shedding or remodelling of tissues with the removal of whole cells and extracellular material during the developmental process.
4). Release of lysosomal enzymes into the extracellular medium:
• The contents of the primary lysosomes may be released into the extracellular medium by the process of exocytosis. This process is activated by parathyroid hormone and inhibited by calcitonin.
5). Release of thyroid hormone:
• Thyroid hormones (T3 &T4) are released from a large protein molecule called thyroglobulin stored within the thyroid follicles.
6). Lysosomes are important in germ cells and fertilisation:
. The acrosome of the spermatozoan can be considered as a specialised lysosome which contains PROTEASE and HYALURONIDASE and abundant ACID PHOSPHATE. During fertilisation, hyaluronidase disperses the cells of cumulus orphorus and protease digests zone pellucide, making a channel through which the sperm nucleus penetrates the egg.
. In eggs, lysosomes play a role in the digestion of stored reserves.
7). Waste Removal :
. Lysosomes break down cellular waste materials into simpler substances, which can be reused by the cell or expelled outside.
8). Defence Mechanism :
. In white blood cells, lysosomes destroy harmful microorganisms, helping in immunity.
9). Cellular Balance (Homeostasis ):
. By continuously degrading and recycling materials, lysosomes maintain internal balance and prevent accumulation of toxic substances.
Thus, lysosomes are vital cell organelles and act as the "suicidal bags " of the cell, helping in intracellular digestion, waste removal and maintaining cellular health by removal or breakdown of unwanted or harmful substances.
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