Oct 042012

The word ‘lysosome’ is derived from two Greek words ‘lysis’ and ‘soma’ where the word ‘lysis’ means digestive and ‘soma’ means body. Hence, in simple words, we can say that lysosome is a structure which helps in digestion. Lysosomes were first discovered by de Duve in 1955 A.D. He found them in the liver cells of rats. These are present in all animal cells and in protists, fungi etc. They are more common in macrophages. They are mostly found in liver cells, White Blood Cells (W.B.C.), pancreas etc. which perform digestive functions.

Lysosomes are bound by single membrane. They contain many digestive enzymes which are known as acid hydrolases. These enzymes are capable of hydrolyzing or digesting all classes of macromolecules. Lysosomes are formed directly from Endoplasmic Reticulum (E.R.) and Golgi bodies (G.B.). The enzymes are synthesized by Endoplasmic Reticulum and transported to Golgi bodies. Then the enzymes are concentrated and then packed into vesicles which form primary lysosomes.

On the basis of morphological contents and functions, lysosomes can be categorized into four types which are listed and described below:

  1. Primary Lysosomes: Primary lysosomes are small bodies which are just pinched off or separated from Golgi bodies. They enclose enzymetic contents which are usually secreted by Endoplasmic Reticulum.
  2. Digestive Vacuoles or Phagosomes (Secondary Lysosomes): When the primary lysosome fuses with other vacuoles containing extracellular or intracellular materials, it is called secondary lysosome. Within secondary lysosome, the material is hydrolyzed by enzymes and then it is digested.
  3. Residual Lysosomes (Tertiary Lysosomes): In secondary lysosomes, food materials are digested. The digested food materials are absorbed into cytoplasm and undigested materials i.e., residue are left. The lysosomes having residue are called residual bodies or residual lysosomes. They move near to the cell surface membrane and they throw the contents by exocytosis process.
  4. Autophagic Vacuoles (Autolysosomes or Autophagosomes): When a lysosome contains a part of its one cell and digests it, it is called autophagic vacuole. It is a part of normal activity by which old, damaged and functionless organelles are digested.

Functions of Lysosomes
The major functions of Lysosomes are listed below:

  • Digestion of large extracellular particles like food materials, that enter the cell is done by lysosomes.
  • Digestion of intracellular substances like cell organelles during starvation to provide energy is carried out by lysosomes. This process is also known as autolysis. There can be self-destruction by the release of the contents of lysosomes within the cell. So, lysosomes are called the suicidal bags of the cell.
  • Digestion of harmful substances is done by lysosomes.
  • Lysosomes actively participate in the digestion of a cell as a whole during pathological conditions when the cells die due to attack of foreign bodies or lack of oxygen or poisoning etc. They digest damaged, old and functionless cells.
  • Lysosomes even help in the digestion of substances outside the cell. Lysosomes move near to the cell surface membrane and discharge their contents to digest the materials.
Sep 262012

Endoplasmic Reticulum is one of the most important parts of a cell (the functional and basic unit of life). It conducts various functions in a cell which helps the cell to work in an efficient and smooth manner. It is known as E.R. in abbreviated form. It is a complex system of membranes which runs throughout the cytoplasm of all eukaryotic cells except mature R.B.C. of mammals, eggs and other embryonic cells.

It was first discovered by Porter in the year 1945 A.D. Endoplasmic Reticulum is extensive or well-developed in metabolically active cells such as liver cells, pancreatic cells etc. But Endoplasmic Reticulum is simple in storage cells like adipose cells. It is generally connected with nuclear membranes and sometimes extended upto the cell membranes.

Endoplasmic Reticulum occurs in three different morphological forms which can be enlisted as:

  1. Cisternae
  2. Vesicles
  3. Tubules

1. Cisternae
Cisternae are elongated, flat, sac-like unbranched structures which are bounded by single membrane. These structures are generally found to be parallel to each other. They are about 40 to 50mμ in diameter. Cisternae are generally covered with ribosomes.

2. Vesicles
Vesicles are round, oval or spherical, membrane-bound structures which are found in the cytoplasm of cell. They are about 25-50mμ in diameter. They do not contain ribosomes in them.

3. Tubules
Tubules are branched, irregular structures which form a reticular or net-like structure along with the cisternae. They are about 50-200mμ in diameter.

Types of Endoplasmic Reticulum (E.R.)
There are two types of Endoplasmic Reticulum. They are:

  • Smooth Endoplasmic Reticulum (S.E.R.): If endoplasmic reticulum is not associated with ribosomes, it is called smooth endoplasmic reticulum. It has a smooth membrane. It doesn’t participate actively in the process of protein-synthesis. It is associated with the synthesis of steroids and lipids. It is connected with Golgi bodies and plasma membrane.
  • Rough Endoplasmic Reticulum (R.E.R.): If endoplasmic reticulum is associated with ribosomes, it is called rough endoplasmic reticulum. It has a rough and granular surface due to the presence of ribosomes. It mainly contains cisternae and few tubules. It participates actively in protein-synthesis process. It is connected with nuclear membrane.

Functions of Endoplasmic Reticulum
The functions of Endoplasmic Reticulum are given below:

  • Endoplasmic Reticulum (E.R.) gives mechanical support by forming a network in the cytoplasm.
  • It helps in the formation of an intracellular transport system which leads to the exchange of materials from one part of the cell to another.
  • It consists of many enzymes which helps in performing many metabolic activities. It also produces enzymes of primary lysosomes.
  • It helps in protein-synthesis by Rough Endoplasmic Reticulum (R.E.R.). It also helps in the synthesis of lipids and steroids by Smooth Endoplasmic Reticulum (S.E.R.).
  • It helps in the exchange of materials between the cells and the immediate environment.
  • It helps in the formation of nuclear membrane, cell membrane and cell wall also.