Living beings or organisms are made of one or more cells. Cell is a basic unit of structure and function of organisms. It was discovered by Robert Hooke (1665). He wrote a book ‘Micrographia’ and coined the term cell. Leeuwenhoek (1672) was the first to see free cells. He observed bacteria, protozoa, red blood cells, sperms etc. under his microscope.

In 1809, Lamarck came to the conclusion that all living beings are formed of cells. Dutrochet (1824) believed in individuality of cells. Living substances was first observed by Corti (1772) and was named sarcode by Dujardin (1836). Robert Brown (1831) discovered nucleus while Von Mohl (1838, 1846) & Purkinje (1839) named the jelly-like living substance or sarcode of the cell as protoplasm.

Cell Theory:

In 1838, Schleiden found that all plant cells have essentially similar structure. Schwann (1838) working independently observed that animal cells do not have cell wall but are otherwise similar amongst themselves. He put forward cell hypothesis bodies of animals and plants are made up of cells and their products. Schleiden and Schwann (1839) compared their findings and formulated cell theory which states that:


(i) Living beings are made of one or more cells

(ii) A cell is a mass of protoplasm having a nucleus.

(iii) Cells are similar in basic structure and metabolism.

(iv) The functions of an organism are due to activities and interactions of its cells.


However, there are several short comings in Cell Theory:

(a) Viruses do not have cellular structure

(b) Monerans and protistans are not divisible into cells. They are acellular.

(c) Certain organisms are multinucleate.


(d) A typical nucleus is absent in procaryotes

(e) Connective tissues have a lot of non-living material as compared to living matter.

(f) Certain cells lose their nuclei in the mature state, e.g. RBC, sieve tube elements.

Cell theory was first modified in the light of Virchow’s findings (1855, 1858) that cells develop from pre-existing cell-Omnis cellula-e-cellula. It is known as law of other modifications was carried out in the cell theory. The modern cell theory is called ‘cell principle’.


Protoplasm Theory:

It was proposed by Max Schultze (1861) which states that living matter of an organism in protoplasm and the cell is simply an accumulation of protoplasm limited by an outer membrane and containing a nucleus.

Organismal Theory:

Sachs (1874) proposed that the whole organism functions as a single entity which is made of a continuous mass of protoplasm incompletely divided into cells.


Cellular Autonomy and Multicellularity:

Cells are autonomous or self contained units because

(ii) They obtain or manufacture food

(ii) All cells require energy for overcoming entropy, performing body activities and biosynthesis. For this they oxidise food materials in respiration.


(iii) Cells convert nonliving materials into components of living protoplasm.

(iv) Worn out parts are replaced by news ones.

(v) There is exchange of gases.

(vi) Cells discard waste materials.

(vii) They are able to regulate their activities through flow of energy and information.

(viii) Cells maintain their own internal physicochemical environment

(ix) They may divide and form daughter cells.

(x) There is a definite life span. Cells of unicellular organisms lead independent existence with no dependent on others for any function, material or information. They depend upon their own intrinsic information. Irritability is, of course present. Cells of multicellular organisms possess autonomy but show various interactions and specialties.

Multicellularity is more advantageous then unicellularity because it

(i) Increases survival

(ii) Induces specialisation

(iii) Ensure uninterrupted activity

(iv) Provides for division of labour

(v) Outer cells became specialised to protect internal cells.

(vi) Internal cells develop their own environment

(vii) Keeps the cells small except where they take part in conduction and support.

(viii) Death of a few cells or reproduction does not kill the organisms. Rather some dead cells are functionally important, e.g. tracheary elements.

(ix) Differentiated cells may take over the function of division through the process of dedifferentiation e.g. plant cells.

(x) In some cases differentiation leads to loss of some basic activities like:

(a) RBCs do not have aerobic respiration, DNA replication and RNA synthesis since nucleus is lost towards maturity.

(b) Sieve tube cells become enucleate

(c) Nerve cells posses nuclei but are unable to differentiate.

(d) Liver and muscle cells do not divide normally but retain the ability to do so.