Lysosomes- Definition, Structure, Functions and Diagram

Lysosomes Definition

• Lysosomes are membrane-bound, dense granular structures containing hydrolytic enzymes responsible mainly for intracellular and extracellular digestion.
• The word “lysosome” is made up of two words “lysis” meaning breakdown and “soma” meaning body.
• It is an important cell organelle responsible for the inter and extracellular breakdown of substances.
• They are more commonly found in animal cells while only in some lower plant groups ( slime molds and saprophytic fungi).
• Lysosomes occur freely in the cytoplasm. In animals, found in almost all cells except in the RBCs.
• They are found in most abundant numbers in cells related to enzymatic reactions such as liver cells, pancreatic cells, kidney cells, spleen cells, leucocytes, macrophages, etc.

Structure of Lysosomes

• Lysosomes are without any characteristic shape or structure i.e. they are pleomorphic
• They are mostly globular or granular in appearance.
• It is 0.2-0.5 μm in size and is surrounded by a single lipoprotein membrane unique in composition.

Figure: Diagram of Lysosomes

• The membrane contains highly glycosylated lysosomal associated membrane proteins (LAMP) and Lysosomal integral membrane proteins (LIMP).
• LAMPs and LIMPs form a coat on the inner surface of the membrane
• They protect the membrane from attack by the numerous hydrolytic enzymes retained inside.
• The lysosomal membrane has a hydrogen proton pump which is responsible for maintaining pH conditions of the enzyme The acidic medium maintained by the proton pump that pumps H+ inside the lumen, ensures the functionality of the lysosomal enzymes.
• Inside the membrane, the organelle contains enzymes in the crystalline form.

Lysosomal Enzymes

For degradation of extra and intracellular material, lysosomes filled with enzymes called hydrolases. It contains about 40 varieties of enzymes which are classified into the following main types, namely:

• Proteases, which digest proteins
• Lipases, which digests lipids
• Amylase, which digests carbohydrates
• Nucleases, which digest nucleic acids
• Phosphoric acid monoesters

Collectively the group of enzymes is called hydrolases which cause cleavage of substrates by the addition of water molecules. Most of the lysosomal enzymes function in the acidic medium.

Types of Lysosomes

Primary Lysosomes

• Small sac-like structures enclosing enzymes synthesized by the rough endoplasmic reticulum.
• Simply called as storage granules storing enzymes.

Secondary Lysosomes

• Formed by the fusion of primary lysosome with phagosomes.
• Contain engulfed material plus enzymes.
• Materials are progressively digested.

Functions of Lysosomes

Lysosomes serve two major functions:

1. Intracellular Digestion

• To digest food, the lysosome membrane fuses with the membrane of food vacuole and squirts the enzymes inside.
• The digested food then diffuses through the vacuole membrane and enters the cell to be used for energy and growth.

2. Autolytic Action

• Cell organelles that need to be ridden are covered by vesicles or vacuoles by the process of autophagy to form autophagosome.
• The autophagosome is then destroyed by the action of lysosomal enzymes.

Processes in which lysosomes play crucial roles include:

a. Heterophagy

The taking into the cell of exogenous material by phagocytosis or pinocytosis and the digestion of the ingested material after fusion of the newly formed vacuole with a lysosome.

b. Autophagy

A normal physiological process that deals with the destruction of cells in the body. It is essential for maintaining homeostasis, for normal functioning by protein degradation, turnover of destroyed cell organelles for new cell formation

c. Extracellular Digestion

Primary lysosomes secrete hydrolases outside by exocytosis resulting in degradation of extracellular materials.

Eg. Saprophytic fungi

d. Autolysis

It refers to the killing of an entire set of cells by the breakdown of the lysosomal membrane. It occurs during amphibian and insect metamorphosis.

e. Fertilization

The acrosome of the sperm head is a giant lysosome that ruptures and releases enzymes on the surface of the egg. This provides the way for sperm entry into the egg by digesting the egg membrane.

f. As Janitors of the Cell

Lysosomes remove ‘junk’ that may accumulate in the cell helping to prevent diseases.

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Reference

1. Verma, P.S. and Agrawal, V..K. (1991). Cell biology, Genetics, Molecular Biology, Evolution and Ecology, S.Chand Company.
2. Kar,D.K. and halder,S. (2015). Cell biology genetics and molecular biology.Kolkata, New central book agency.
3. R and suwal,S.N. (2010).Human Anatomy and physiology. Kathmandu, vidyarthi prakashan (p.) ltd.
4. https://study.com/academy/lesson/lysosome-definition-function-quiz.html
5. https://biologydictionary.net/lysosome/
6. https://biologywise.com/lysosome-structure

Lysosomes- Definition, Structure, Functions, and Diagram