Cell: The Unit Of Life

All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while others, like us, composed of many cells, are called multicellular organisms.

What is a Cell?

Unicellular organisms are capable of (i) independent existence and (ii) performing the essential functions of life e.g. reproduction, growth, etc. anything less than a complete structure of cell does not ensure independent living. Hence, cell is the fundamental structural and functional unit of all living organisms.

Anton Von Leeuwenhoek first saw and described a live cell. Robert brown later discovered the nucleus.

Invention of microscope and electron microscope revealed all the structural details of the cell.

Cell Theory

In 1838, Matthias Schleiden, a german botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. About the same time Schwann (1839), a British Zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’. He also concluded, the presence of cell wall is a unique character of plant cell. On the bases of this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.

Schleiden and Schwann together formulated the cell theory. This theory however, did not explain as to how new cells were formed. 

It was Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells. He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Cell theory as understand today is:

• All living organisms are composed of cells and products of cell.
• In organisms, the cell is the fundamental unit of structure and organization.
• All cells arise from pre-existing cells. (omnis cellula e cellula).

The Modern View of Cell Theory

The following are some of the most widely acknowledged aspects of current cell theory:

• All known living organisms are made up of one or more cells.
• By the process of division, all live cells can developed from pre-existing cells.
• In all of the living species, the cell is the basis structural and functional unit.
• The cumulative activity of separate cells determines an organism’s activity.
• Within cells, energy flow (metabolism and biochemistry) happens.
• DNA is present particularly in the chromosome, whereas RNA is found in the cycles and cytoplasm of cells.
• In creatures of comparable species, all cells have a chemical makeup that is almost identical.

Prokaryotic cells have 70s ribosomes while eukaryotes have 80s ribosomes.

An Overview of Cell

The onion cell which is a typical plant cell, has a distinct cell wall as its outer boundary and just within it is the cell membrane. The cells of the human cheek have an outer membrane as the delimiting structure of the cell.

Inside each cell is a dense membrane bound structure called nucleus. This nucleus contains the chromosomes which in turn contain the genetic material, DNA. Cells those have membrane bound nucleus are called eukaryotic cells whereas cells those do not have membrane bound nucleus are called prokaryotic cells.

In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cells. Various chemical reactions occur in it to keep the cell in the ‘living state’.