What is a lysosome? Let’s unveil the structure and functions of lysosome along with diseases associated with lysosome.
What is a lysosome?
The lysosome is a cytoplasmic organelle present in eukaryotic cells. The lysosome word is made up of two Greek words ‘lyso” means “split or break” and ‘soma’ means ‘body’. It was first discovered by Belgian cytologist Christian de Duve in 1949. It contains digestive enzymes and is involved in the transport and degradation of various cellular metabolites.
Structure of lysosome
The lysosome is membrane-bounded spherical or sac-like organelles present in the cytoplasm. Its size ranges from 50-500 nm in diameter. The lumen of the lysosome has an acidic pH of 4.5 and contains about 60 different types of hydrolytic enzymes to degrade metabolites. These digestive enzymes are produced on rough endoplasmic reticulum and transported to the Golgi apparatus where they are further modified or processed.
The modified or processed enzymes are then pinched off from the Golgi apparatus in the form of vesicles called lysosomes. The different types of digestive enzymes present in lysosomes are amylases, proteases, nucleases, acid phosphatases, and lipases. In a single animal cell, several hundred lysosomes might be present.
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| Diagram of the lysosome. Image created in BioRender.com |
Function of lysosome
The main functions of lysosome are:
1. Phagocytosis
The degradation and recycling of cellular waste is a well-known function of the lysosome. Extracellular products such as food or microorganisms enter the lysosome primarily via endocytosis to food vacuoles. The lysosome fuses with food vacuole to form digestive vacuole and helps in the digestion of food by releasing digestive enzymes. The process of engulfing foreign particles by lysosomes and their digestion is called phagocytosis.
2. Autophagy
The distribution of intracellular materials is mediated by an entirely different mechanism, autophagy (self-eating). A wide variety of cellular stress-inducing conditions cause autophagy and mediate the degradation of protein, lipids, damaged organelles, and intracellular pathogens. The double membrane-bound vesicles called autophagosomes are formed in this process, which sequester cytoplasmic material and then merge with lysosomes.
Lysosomal hydrolases then degrade materials that enter the lysosome, and the resulting breakdown products are used to form new cellular components and energy in response to the cell's nutritional needs. Due to this function of the lysosome, they are also called “suicidal bags”.
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| Worn out cell parts, old mitochondria in mature cells are fused with a lysosome in the autophagy process. Image created in BioRender.com |
3. Lysosomal exocytosis
Lysosomes also play an important role in various physiological processes such as immune response, plasma membrane repair, and bone resorption in an "exceptional" secretory pathway known as lysosomal exocytosis.
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Lysosomal storage diseases (LSDs)
Lysosomal storage diseases (LSDs) are a group of more than 70 diseases that are due to malfunction in the lysosomal enzymes. The mutations in the genes cause the dysfunction of enzymes involved in the degradation of metabolites. Individually, these conditions are rare but affect 1 in 5,000 live births collectively. Some well-known examples of LSDs are glycogenesis type II and Tay-Sachs disease.
Glycogenesis type II also known as glycogen storage disease type 2 or Pompe's disease or acid maltase deficiency is an autosomal recessive hereditary condition caused by a deficiency of lysosomal alpha-glucosidase, which contributes to glycogen accumulation in the lysosome.
Tay-Sachs disease is a rare condition transmitted from parents to children. The defect in the 3-hexosaminidase α-subunit that helps break down fatty compounds triggers this disease between 3-5 months of age. The fatty compounds, called gangliosides, build up in the child's brain to toxic levels and impair nerve cell function.
Some questions and answers
A. Lysosome is membrane-bounded spherical or sac-like organelles ranging in size from 0.1 to 1.2 µm and contains different types of enzymes in its lumen to degrade metabolites.
Q. How many lysosomes are in a cell?
Q. Where are lysosomes created?
A. Lysosomes are pinched off from the Golgi apparatus.
Q. Define Lysosomal storage diseases.
A. Lysosomal storage diseases (LSDs) are a group of more than 70 diseases that are due to malfunction in the lysosomal enzymes. The mutations in the genes cause the dysfunction of enzymes involved in the degradation of metabolites.
Q. Why lysosomes are called suicidal bags?
A. During the process of autophagy, lysosomes are fused with autophagosomes to degrade the cellular products. Due to this function of the lysosome, they are also called “suicidal bags”.
Q. Which enzyme is defective in Tay-Sachs disease?
A. In Tay-Sachs disease, there is a defect in the 3-hexosaminidase α-subunit due to which break down of fatty compounds cannot take place. This disease is a rare condition transmitted from parents to children.
Q. What type of enzymes are present in the lysosome?
A. Lysosome contains about 60 different types of hydrolytic enzymes such as amylases, proteases, nucleases, acid phosphatases, and lipases.
Q. List three main functions of the lysosome.
A. The three main functions of the lysosome are phagocytosis, autophagy, and lysosomal exocytosis.
Q. How lysosomes are formed?
A. The lysosomal enzymes are synthesized in the endoplasmic reticulum and then transported to the Golgi apparatus. In the Golgi apparatus, these enzymes are modified and membrane-bounded vesicles are pinched off from their surface called lysosomes.
References
- Ballabio, A. (2016). The awesome lysosome. EMBO molecular medicine, 8(2), 73-76.
- Bandyopadhyay, D., Cyphersmith, A., Zapata, J. A., Kim, Y. J., & Payne, C. K. (2014). Lysosome transport as a function of lysosome diameter. PloS one, 9(1), e86847.
- Nayak, J. V., Hokey, D. A., Larregina, A., He, Y., Salter, R. D., Watkins, S. C., & Falo, L. D. (2006). Phagocytosis induces lysosome remodeling and regulated presentation of particulate antigens by activated dendritic cells. The Journal of Immunology, 177(12), 8493-8503.
- Platt, F. M., d’Azzo, A., Davidson, B. L., Neufeld, E. F., & Tifft, C. J. (2018). Lysosomal storage diseases. Nature Reviews Disease Primers, 4(1), 1-25.
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