Digestive system of the cell – Lysosomes

Discovery and nomenclature of lysosome:

They were discovered by De Duve in 1955. Initially these organelles were named as peri-canalicular dense bodies based on their location inside the cell. Later on digestive enzymes were discovered in them, which were capable of cell lysis. So, their name became Lytic bodies; which was further modified and the name “lysosome” came.

Why lysosomes were called suicidal bags

Initially De Duve observed that hydrolytic enzymes of lysosomes are capable of digesting the entire cell. So, he named them suicidal bags. Later on, it was found that leaked enzymes of lysosomes become inactive in the neutral pH of cytosol. Lysosomal enzymes can degrade only a aged cell (i.e. a cell which has undergone ageing) or which is useless for the human body, like 110-120 day old RBC. So, the action is actually not “suicide” but is apoptotic. It is also important to note that pH of cytosol of an aged cell starts reducing from 7.2 gradually.


They have a range of morphogenetic shapes, even in a single cell, due to which their identification is very difficult in the microscope. I mean to say, in a single cell, many lysosomes are present with different shapes. Sometimes, these organelles are confused with micro bodies and vacuoles present in the cell. Their actual identification is possible only with the help of lytic enzymes.

Internal organization:

The lysosomes contain two components:

1) Granulated stroma- contains about 50 types of hydrolytic enzymes. These include nucleases, phosphatases, sulphatases, glycosidases, lipases, phospholipases. All these enzymes are active only in acidic pH range. If lysosome splits open, then all the enzymes will be released in the cytosol, and become inactive. This is so because, hydrolytic enzymes of lysosomes are active only in acidic pH range, and the pH of cytoplasm is 7.2.

2) Vacuole- discussed later.

Types of lysosomes

  1. Primary lysosomes – These are also known as storage granules. The primary lysosome is a tiny spherical body present inside the cell. It contains a lot of enzymes; because of which they are known as storage granules.

Sometimes, they are also called as virgin lysosome, or proto-lysosome.

  1. Secondary lysosomes- When primary lysosome, fuses with any extracellular or intracellular body, it is known as secondary lysosome. It is also known as hetero-phagosome or digestive vacuole. All acid hydrolase enzymes are found in secondary lysosomes. All the enzymes of secondary lysosome carry out the process of cellular digestion; so, it is actually the secondary lysosome where the actual processes of cellular digestion occur. After digestion, the digested components are released into the cytoplasm. This can be further of two types:
  • Heterolysosome – When primary lysosome fuses with a phagocytic particle which has come from outside the cell, then it is known as hetero-lysosome. Substances are also brought into the cell via endocytic pathway and form a vesicle known as endosomes. These endosomes combine with primary lysosome to form hetero-lysosome; also known as heterophagic vacuoles or phagolysosome (because the substance entered the cell via phagocytosis).
  • Cyto-lysosome – The other names given to this organelle are auto-phagosome, auto-lysosome or autophagic vacuole. Sometimes, cell components like ER, Golgi complex, mitochondria etc. break open inside the cell. These debris combine with primary lysosome to form cyto-lysosome.
  1. Residual bodies- Even after digestion of foreign/waste material, sometimes, the engulfed material remains undigested and collects in the cell; it is known as residual body in eukaryotes. Greater the number of residual bodies inside the cell, faster is the ageing process.

Pathways which lead to secondary lysosome formation:

There are three pathways which lead to formation of secondary lysosome inside the cell:

  1. Endocytic pathway – In this external substance which has entered the cell by the process of endocytosis combine with primary lysosome to form hetero-lysosome.
  2. Phagocytic pathway – In this external substance which has entered the cell by phagocytosis combines with primary lysosome to form phagolysosome.
  3. Autophagic pathway- In these intracellular broken organelles enters the primary lysosome forming cyto-lysosome.

Important points:

  • These are dense bodies present inside the cell, and are surrounded by a membrane.
  • Their size ranges from 0.23 – 0.50 micrometers.
  • pH of lysosomes ranges from 5-6 unlike cytosol whose pH is 7.2.
  • Contains hydrogen pump, which maintains the intra-cellular pH.
  • Important point: If lysosomal enzymes leak into the cytosol, they will become inactive; because all acid hydrolases are inactive at the cytosolic pH.

Functions of lysosomes:

  1. Digestion of macro-molecules- Large macro-molecules enters the cell via endocytosis. After digestion, macro-molecules become micro-molecules; these can be further used by the cell.
  2. Digestion of un-useful particles- The particles which are useless for the cell need to be digested. Therefore, the lysosome is also known as digestive system of the cell.
  3. Sometimes, the cell needs to digest intracellular substances for maturation, and growth. For e.g. tadpole becomes a frog only after its tail is digested. This digestion is also performed by lysosome of the cell.
  4. Auto-digestion- After cell death, lysosome membrane ruptures, and digests the entire cell. This phenomenon is known auto-digestion.
  5. Some enzymes of lysosome are liberated out of the cell in Extra-cellular matrix. For e.g. melanocytes (melanosomes) of skin cell release the melanin pigment in Extra Cellular Matrix (ECM). Human sperm penetrates ova when lysosome of sperm releases enzymes into the ECM. Both these mechanisms accomplish when lysosome fuses with plasma membrane.