Different types of lysosomes

Lysosomes are structurally heterogeneous group of organelles that are formed from the Golgi complex and are membrane enclosed spherical vacuoles. The existence of lysosomes was discovered and named by Nobel Prize Laureate Christian de Duve and his co-workers. Lysosomes are bound by only a single membrane and lack internal structure, but they contain as many as 50 different kinds of powerful hydrolytic enzymes ,capable of hydrolyzing proteins, polysaccharides, nucleic acids, cell-invading bacteria. Human white blood cells, which ingest bacteria through phagocytosis, contain numerous lysosomes. They also play a role in processes such as cell death (apoptosis) and immune defence.

Different types of lysosomes

Several different lysosomal forms have been identified within individual cells, including: primary, secondary, and tertiary (known as residual bodies). Each represent distinct stages of their life cycle and function within the cell.

01. Primary Lysosomes

Primary lysosomes are the initial stage in the lysosomal life cycle. They are produced by the Golgi apparatus, an organelle that packages proteins and other molecules for transport within the cell. The Golgi apparatus forms small vesicles that carry digestive enzymes, which develop into primary lysosomes. These enzymes have the potential to break down various biomolecules, such as proteins, lipids, nucleic acids, and carbohydrates. However, in primary lysosomes, these enzymes remain inactive until they are needed.

The primary lysosome acts as a storage unit, holding these enzymes in a safe state until they are needed. When the cell encounters material that requires digestion, such as when it engulfs external particles through endocytosis, ingests larger objects through phagocytosis, or starts recycling its own damaged parts through autophagy, the primary lysosome fuses with the vesicle containing this material. This fusion activates the enzymes, turning the primary lysosome into a secondary lysosome. In the secondary lysosome, the enzymes begin to break down the contents, facilitating the digestion and recycling of cellular materials.

02.  Secondary Lysosomes

A secondary lysosome is an important stage in the lysosomal lifecycle, formed when a primary lysosome fuses with another vesicle that contains material intended for digestion. This fusion process activates the digestive enzymes within the primary lysosome, transforming it into a secondary lysosome where the breakdown of materials takes place.

The material to be digested can come from different sources:
  • Endosomes:
    • Endosomes are membrane-bound vesicles within cells that transport materials taken up from the extracellular environment via endocytosis. During endocytosis, the cell engulfs external substances such as nutrients, hormones, or other molecules, forming a vesicle. This vesicle then fuses with early endosomes, which mature into late endosomes. Endosomes are involved in sorting these materials, directing them either to lysosomes for degradation or back to the cell membrane for recycling.
  • Phagosomes:
    • Phagosomes are specialized vesicles formed when a cell engulfs large particles, such as bacteria, dead cells, or other debris, through a process called phagocytosis. During phagocytosis, the cell membrane extends around the particle to form a phagosome. This phagosome then fuses with a lysosome (often becoming a secondary lysosome), where the engulfed material is broken down by digestive enzymes. 
  • Autophagosomes:
    • Autophagosomes are vesicles that form during a process called autophagy, where the cell degrades its own damaged or surplus organelles and proteins. The formation of an autophagosome begins with the encapsulation of damaged or excess cellular components within a double-membraned vesicle. This vesicle then fuses with a lysosome to form an autolysosome, where the enclosed material is degraded by lysosomal enzymes.


Once the primary lysosome merges with these vesicles, the enzymes inside become active and start digesting the contents. The digestion process breaks down complex molecules into simpler ones, such as amino acids, fatty acids, and sugars. These simpler molecules are then either recycled within the cell to be used in various cellular processes or prepared for removal from the cell.

Secondary lysosomes play a crucial role in maintaining cellular health by ensuring that waste and damaged components are effectively broken down and processed. This recycling process helps in managing cellular resources efficiently and clearing out potentially harmful substances. By handling and processing these materials, secondary lysosomes contribute significantly to the cell’s ability to function smoothly and stay healthy.

03. Tertiary Lysosomes (Residual Bodies)

Tertiary lysosomes, also known as residual bodies, are the last stage in the life of a lysosome. They form after a secondary lysosome has finished. Once this process is complete, any material that the cell cannot break down further or use is left behind. This leftover material collects in the tertiary lysosome. These lysosomes act as storage for waste that the cell cannot get rid of or use. They act as a place for the cell to keep waste that remains after the digestion process is complete.

These residual bodies often contain indigestible substances, such as certain pigments or complex lipids, that the cell cannot process further. Over time, especially in long-lived cells like neurons (nerve cells) or muscle cells, these residual bodies can build up. This accumulation can lead to the formation of lipofuscin, a brownish-yellow pigment often referred to as "age pigment." Lipofuscin is commonly seen in older cells and is considered a hallmark of cellular aging.

While tertiary lysosomes and the residual materials they contain are generally not harmful to the cell, their accumulation can reflect the cell's aging process and reduced efficiency in waste disposal. In some cases, if too many residual bodies accumulate, they could potentially interfere with cell function, though this is more of a concern in the context of aging or certain diseases.

Tertiary lysosomes are essentially the cell's final storage for waste that cannot be removed. They serve as a record of the cell's history, showing what materials the cell has encountered and processed over its lifetime. In some cells, these residual bodies can persist for years, slowly accumulating more indigestible material as the cell continues to operate.



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SAQ 1

  1. What is a cell? What are the essential characteristics of cells?
  2. Explain the fluid mosaic model of the plasma membrane
  3. Which organelles are involved in photosynthesis?
  4. Why the mitochondria is called the powerhouse of the cell?
  5. Which organelle contains enzymes for cellular respiration?
  6. Why mitochondria and chloroplast are called semi-autonomous?
  7. Mention any two advantages of the extensive network of the endoplasmic reticulum
  8. What is the function of peroxisomes in plant cells?
  9. Explain the following terms: (a) chromatin network (b) chromosomes (c) Nucleosome (d) Solenoid Model
  10. What is the function of the nucleolus in the cell?

SAQ 2



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