Cell Theory

Cell Theory

In the year 1838, a German botanist called Matthias Schleiden discovered that all plant tissues are made up of different types of cells. A British Zoologist, Theodore Schwann, who was a contemporary of Schleiden, reported the presence of athin outer layer in the animal cells which is now known as the plasma membrane.  Schwann also discovered that along with plasma membrane,an additional unique outer layer is also found exclusively in plant cells i.e. the cell wall. Both Schleiden and Schwann together postulated the Cell Theory, based on their findings. However, this theory did not give any indication of the genesis of cells.

In 1855, a German biologist named Rudolf Virchow first explained the origin of cells from pre-existing cells. This led to the modification of the original cell theory proposed by Schleiden and Schwann which is today understood as:

1. All living organisms are made up of cells and cell products.
2. Every cell is made up of pre-existing cells.

CELL THEORY

The theory was formulated by Schleiden and Schwann.

The various points of cell theory are:

1. Each cell is made of a small mass of protoplasm having a nucleus and bounded by a cell membrane with or without cell wall.

2. All cells are basically alike in structure and metabolism.

3. Organisms are composed of cells and their products.

4. The functions of an organism is an outcome of activities and interactions of its constituent cells.

But, it did not explain as to how new cells are formed.

Cell theory was first modified in the light of Virchow's findings that cells develop from pre-existing cells i.e.,"Omnis cellula e cellula". It is known as law of cell lineage. Number of other modifications were carried out in cell theory. The modern cell theory is known as cell principle or cell doctrine.

Drawbacks of Cell Theory

Important drawbacks of cell theory are given below:

1. Viruses cannot be explained using this theory.

2. Bacteria and blue-green algae do not have an organized nucleus.

3. Certain fungi, such as Rhizopus, have hyphae composed of a multi-nucleated mass of cytoplasm (coenocyte).

4. Acetabularia (unicelled, marine green algae) has a uninucleated differentiated body (acellular).

5. Sieve tube and mature RBC lack nuclei.

6. Volume of the cell is occupied by a semi-fluid matrix called cytoplasm, and is main arena of cellular activities in both pro and eukaryota.

7. Cells differ greatly in size, shape and activities e.g., Mycoplasma -the smallest cells (0.3 µm length), Bacteria (3 -5 µm), Human RBC (~7.0 µm)