Lysosomes | Definition, Types, Function, Discover, Structure

Hello friends, in this article we will know what is lysosomes, and will also learn about them – Its Definition, Types, Structure, Discovered, functions, etc. So let’s start without wasting time.

What is Lysosomes’ definition?

Lysosomes are small membrane-bound organelles found in the cytoplasm of Eukaryotic cells.  These enclose tissue dissolving or hydrolytic enzymes. These are after described as suicide bags because if by chance the lysosome gets ruptured the dissolved the cell in which these have got ruptured.

Who discovered Lysosome?

Lysosomes were first seen in 1949 by Christian de-Duve and were named lysosomes in 1955. 

Structure of Lysosome –

Structurally lysosomes are rounded or spherical spaces averaging From 0.4 to 0.8. In certain cases, these are especially enlarged and attain a size of about 5 as the cells of the Uriniferous tubules of the kidney. Each is bounded by a single-unit membrane and encloses a dense matrix.

Lysosome Diagram –

lysosomes
Lysosome, Image credit – dreamstime.com

The chemical composition of lysosome –

The matrix contains tissue-dissolving enzymes or hydrolases and acid phosphatase.

Polymorphism in lysosomes –

The internal contents of lysosomes vary with the stage of digestion. Based on these four types of lysosomes have been distinguished.

  1. Primary lysosomes or storage granules –
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These are newly formed lysosomes. Did originate from the Golgi complex. These contain only hydrolytic enzymes. These have a diameter of about 100 nm.

2. Secondary lysosome or heterophasic vacuoles or Hetero Lysosome – 

The Heterolysosome are formed by the fusion of primary lysosomes with the phagosomes. Therefore, these contain lysosomal enzymes and the substances to be digested. Dissolution of substances occurs inside the phagosomes. These are also known as phagolysosomes.

Secondary lysosomes may be of the following three types –

(i) Large digestive vacuoles or Heterolysosomes are formed by the fusion of primary lysosomes with phagosomes containing large particles.

(ii) Multivesicular bodies contain many endocytic vesicles fused with one lysosome.

(iii) Autophagic vacuoles contain and digest intracellular cell membranes or organelles. These are present in the cell only under special physiological conditions When cells or some parts of the cells are to be removed. These contain the hydrolyzing enzymes and a part or a component of the cell that is to be dissolved. Thus, these bring about auto dissolution or auto digestion of  Cellular organelles.

3. Cyto Lysosomes are enlarged lysosomes observed in Dying organisms under a variety of physiological and pathological conditions. These cyto lysosomes are rich in acid hydrolases.

4. Residual bodies — Exhausted lysosomes with undigested wastes are called residual bodies or telo lysosomes. These are large and irregular.

Biogenesis of Lysosome –

Lysosomes originate from the Golgi complex. The proteins synthesized by the ribosome are discharged in the cavity of the Endoplasmic reticulum.

These move into the smooth Endoplasmic reticulum and from there into the Golgi membrane at the cis face.

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Here these enzymes are concentrated and stored in the storage granules or for the primary lysosome. These separated out from the trans face of the Golgi complex.

What is the function of lysosomes?

  1. Heterophagy or digestion of external particles – 

The enzymes can catalyze The breakdown of polysaccharide-protein and nucleic acid molecules. So the lysosome (lyso, lytic or digestive; soma body) helps in the intracellular digestion of foodstuff.

The pinocytic vacuoles or phagocytic vacuoles are formed by endocytosis. Fuse with lysosomes which liberate the tissue digestive or tissue hydrolyzing Enzymes and bring out the digestion of Phagocytosed materials or bacteria.

This process of digestion is known as heterophagy. This method of digestion is also called intracellular digestion and is found in protozoans, sponges, and coelenterates.

2. Autophagy –

The lysosome also helps in the removal of dead and decaying cells or tissues. The lysosome in these cells becomes enlarged and later On Ruptures releasing the enzymes into the cell. As a result, the complete cell is digested.

This is known as autophagy or autolysis. It is found in a starving cell or tissue or organ undergoing differentiation repair are metamorphosis. The process of tissue degeneration is known as nacrosis and the lysosome is termed suicide bags (de-Duve,1959).

Significance of lysosomes –

  1. Nutrition by digestion of foodstuff in protozoans and many metazoans having intracellular digestion.
  2. Nutrition by autophagy during cellular degeneration under special conditions.
  3. Lysis of organelles during cellular differentiation and metamorphosis.
  4. Scavenging worn-out cell parts and denatured proteins.
  5. Removal of exhausted cells.
  6. Defense against invading bacteria and viruses by macrophage.
  7. Digesting Yolk during embryonic development.
  8. Bone resorption (osteolysis during osteogenesis).
  9. Penetration of sperm in the ovum during fertilization.
  10. Chromosomes are caused by ruptured lysosomes. This may lead to cancer. 
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Lysosome Disease –

Abnormal functioning or defects in the normal functioning of lysosomes produce the following disease.

  1. Silicosis – 

In industrial workers Inhalation of silica of asbestos fibers causes this disease. The macrophages in the lung ingest silica fibers to remove them from the air we respire. The fiber becomes enclosed in the secondary lysosome and is not dissolved or digested.

The presence of silica fiber in the lysosome makes their plasma membrane leaky. The hydrolytic enzyme causes damage. Thus, the condition is characterized by the destruction of the lungs and air passages.

2. Rheumatoid arthritis –

This is an inflammatory disease of the body’s joints. This is caused by the release of lysosomal enzymes from the cell into The extracellular space causing damage to the material.

Differences between Primary lysosomes and Secondary lysosomes –

Primary LysosomesSecondary Lysosomes
Primary lysosomes or lysosomes are   Newly formed Lysosomes.Secondary lysosomes are formed by the fusion of phagosomes and primary lysosomes.
These contain hydrolytic enzymes.These contain lysosomal enzymes and substances or organisms to be digested or dissolved.
These are also called storage granules.These are also called  Heterolysosomes, phagolysosomes, or heterophagic vacuoles.

What is Autophagic vacuoles and Digestive Vacuoles?

Autophagic vacuoles digest intracellular cell organelle. These are created only in special physiological conditions. These contain hydrolytic enzymes of primary lysosome and some cell organelle or some cell membrane of their own cell. These result in auto-dissolution or auto-digestion.

Digestive vacuole digest some foreign substances or organisms. These are formed by the fusion of primary lysosomes and phagosomes.

Where are lysosomes found?

Lysosomes are found in animal cells and a few plant cells.

Are lysosomes in plant and animal cells? Or do plant cells have lysosomes?

Yes

What do lysosomes do?

Hint — Read the functions of lysosomes.

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