Cell: Meaning and Types of Cell

In this article we will discuss about the meaning and types of cell.

Meaning of Cell:

Cell is the structural and functional unit of all living organisms except viruses which do not have a cellular organisation. Viruses are not self-reproductive units.

They are able to show self-reproduction when they are in contact of living bodies, otherwise they remain dead or totally inactive. The suitable term which is used for viruses is virus particles. It can be seen by electron microscope only. These are not found in isolated form. These are parasites.

The following word is most appropriate for viruses:

“Primitive organisms that have not reached a cellular state.”

Other definitions of cell may be given as follows:

“The smallest living unit capable of independent existence.”

or

“A small mass of living matter containing a nucleus or nuclear material.”

or

The cell is a piece of nucleated cytoplasm surrounded by a wall or membrane existing singly or in groups and containing structures of various sorts

—R.C.Mc Clean and W.R.Ivimey Cook

or

Cell as “a unit of biological activity delimited by a selective permeable membrane and capable of self-reproduction in a medium free of other living systems” — A.G. Loewy and Siekevitz (1963).

“On the basis of recent investigations, cell is now supposed as well equipped molecular factory” —J. Brachet, 1961

Cell may be different in shape and size.

The shape may be as follows:

(i) Spherical or polyhedral e.g., undifferentiated parenchyma in mesophylls and cell of roots, stems etc.

(ii) Cuboidal or flattened or brick-shaped, e.g., epidermal cells including cambial cells.

(iii) Elongated, e.g., root hairs, filamentous fungi, phloem, xylem and bast fibres, collenchyma etc.

(iv) Kidney shaped, e.g., guard cells. Despite it, others shapes which are found within the cell are cylindrical (palisade parenchyma), branched etc.

The cell size also varies in plants and animals. However, the majority of cells have a diameter in the range of 1 µm to 100 µm. The largest cells are seen in green alga Nitella, which may be several centimeters in length. In animals, the largest known cell is the egg of an ostrich which can be as long as 170 mm (170 x 135 mm). The smallest cell is perhaps yet to be discovered.

However, the bacterial cells and the cells of some blue-green algae are the smallest, usually in the range of 0.2 to 1.5 pm.

The factors governing the size of the cells are:

(i) The nucleocytoplasmic ratio, that is, the ratio between the volume of the nucleus and the cytoplasm

(ii) The ratio of the cell volume and

(iii) The rate of metabolism.

It has also been found in some higher plants that the length of fibres ranges from 20 mm to 550 mm. Cannabis sativa, Corchorus, Crotolaria etc. come under this category.

Types of Cells:

1. Prokaryotic cells

2. Eukaryotic cells

1. Prokaryotic Cells (Gk; pro = primitive; karyon = nucleus):

Organisms in which the nuclear material is not bounded by a definite nuclear membrane are called Prokaryotes. Nucleolus, mitochondria, ER, Chloroplast, Golgi bodies are found absent. Division of cell is amitosis or direct. Examples are bacteria, blue-green algae etc.

2. Eukaryotic Cells (Gk; eu = well or good; karyon = nucleus, i.e.; true nucleus):

Organisms in which the nucleus has a definite nuclear membrane are known as Eukaryotes, e.g., all other plants and animals. The nuclear materials are found in nucleoplasm and never unevenly distributed in cytoplasm.

Fundamentally prokaryotes and eukaryotes differ in many ways as given below: