The organs of a system function in a coordinated manner to carry out major life processes. For example, the digestive system includes the alimentary canal and associated glands, which are composed of epithelial, connective and muscular tissues, which ultimately are formed of various types of cells. But how did we come to know about these concepts? Who gave the idea of a cell? What are the basic concepts that form a cell theory? What is its modern interpretation? To know about all these, scroll down to the article.

Concept of Cell

Cells are a basic structural and functional unit of life. Every organism is made up of cells. Organisms may be unicellular or multicellular, depending on the number of cells present in the organisation of the body.

Interestingly, the beginning of every organism has a single cell (called a zygote, for sexually reproducing organisms).

Each cell is capable of performing all life functions. Depending on the nature of cells, a prokaryotic cell is considered primitive, as it does not have a membrane-bound nucleus and organelles. Conversely, eukaryotes are advanced cells with various organelles and membrane-bound nuclei. Each cell functions as a self-contained unit. It can independently carry out nutrition, growth, respiration, reproduction and self-regulation. In multicellular organisms, cells are not totally independent of one another but interact and cooperate with one another.

Discovery of Cell

The history of cell studies began when a book called ‘Micrographia’ got published by an Englishman, Robert Hook, in \(1665.\) The cell was discovered by Robert Hook. He was observing the slide of a cork. In that, he saw honeycombs like patterns or some compactly packed structures or compartments under his simple primitive microscope. He called it a “cell”. This cork was of plant Quercus. The wall which he saw was actually the cell wall in which suberin was deposited, and there was nothing inside. He saw dead cells. Robert Hook discovered the cell, but it was a dead cell. Anton Van Leeuwenhoek observed living cells for the first time.

Fig: Robert Hook’s Drawing of Structure of Cork Observed under a Primitive Microscope

Major Discoveries of Structural Organisation of the Cell

Year	Discovery
\(1665\)	Robert Hook discovered Cell.
\(1781\)	F. Fontana discovered a nucleolus.
\(1831\)	Robert Brown discovered the nucleus.
\(1838\)	Schleiden discovered plant cells.
\(1838\)	Schwann discovered animal cells.
\(1839\) & \(1846\)	Prukinje and Von Mohl renamed sarcode or jelly like substance of cells a protoplasm.
\(1839\)	Schleiden & Schwann jointly put forward the cell theory.
\(1855\)	Rudolf Virchow added to cell theory “omnis e cellula e cellula”
\(1866\)	E. Haeckel introduced Plastid.
\(1873\)	Anton Schneider described chromosomes for the first time.
\(1875\)	Van Beneden observed Centriole.
\(1879\)	W. Flemming described chromatin.
\(1898\)	Camillo Golgi described the Golgi apparatus.
\(1931\)	J.Q. Plowe coined the term Plasmalemma.
\(1945\)	K. R. Porter and Thompson discovered Endoplasmic Reticulum independently.

What is a Cell Theory in Science?

Cell theory was given by Schleiden and Schwann in \(1839.\) It states that all organisms are made of cells, i.e. a cell is a structural and functional unit of an organism. Original cell theory had two points. Later, one point was added by Rudolf Virchow, a German Physician, in \(1858.\) He gave a phrase in his own language, “omnis cellula e cellula”, which means all the cells arise from the pre-existing cells. Cell theory states that-

1. Cells are the building blocks of every living organism.
2. All living organisms are made up of one cell or more than one cell.
3. All the functions of the body are the product of the functions of independent cells.
4. All the cells arise from pre-existing cells.

The idea of Rudolf Virchow was already given by Robert Remak, so Virchow was not given credit. Remak worked on cell division and, in \(1852,\) claimed that observations of Schleiden and Schwann were incorrect and proposed that reproduction in animal cells occurs through binary fission, which was already introduced by Dumortier.

Modern Interpretation of Cell Theory

1. All living organisms are made up of one cell or more cells.
2. All the metabolic functions of the organism are the product of the functions of independent and individual cells.
3. All the cells arise from pre-existing cells.
4. Energy flow occurs within cells. This is due to various metabolic activities occurring inside the cell.
5. Each cell contains the whole complement of genetic information, not only for itself but for the whole organism.
6. All cells are basically alike in chemical composition and metabolic activities.
7. The cells are totipotent and can form the whole organism under a specific set of conditions.

In recent years a large number of subcellular structures have been discovered, which demonstrate that a cell is no longer a basic unit of life. Life can exist without a cell.

Drawbacks of Cell Theory

Cell theory also has certain exceptions and objections–

1. Viruses do not have a cell (acellular organisms) or cell machinery for vital functions. It does not obey the theory of “omnis cellula e cellula”.
2. In some organisms, a number of nuclei are present in the body where differentiation of cells is absent, like Rhizopus (fungus) and Vaucheria (alga), cells are coenocytic.
3. In sieve tube elements and mature RBCs of mammals, the nucleus is lacking.
4. The well-defined nucleus is absent in prokaryotes. Instead, a nucleoid is present.
5. Prokaryotes also lack many cell organelles which are present in the eukaryotes.
6. Schleiden believed new cells to arise from cytoblast or nucleus, but Schwann thought through crystallisation. However, the chemical origin of life and cells are not

Summary

Every organism is made up of cells. Each cell functions as a self-contained unit. It can independently carry out nutrition, growth, respiration, reproduction and self-regulation. The cell was discovered by Robert Hook in \(1665.\) Schleiden and Schwann, in \(1839,\) gave cell theory. Cell theory says all living organisms are made up of one or more cells, and all the functions of the body are the product of functions of independent cells. Rudolf Virchow, in \(1858\) added his findings to cell theory that all the cells arise from pre-existing cells.

Now, a modern version of cell theory is followed, which includes some more ideas as energy flow occurs within cells. Genetic information is passed on from cell to cell, and all cells are basically alike in chemical composition and metabolic activities.

FAQs

Q.1. What are the \(3\) principles of cell theory?
Ans: 1. All living organisms are made up of one cell or more than one cell.
2. All the functions of the body are the product of the functions of independent cells.
3. All the cells arise from pre-existing cells.

Q.2. Who found the first cell?
Ans: Robert Hook found the first cell.

Q.3. What is smaller than a cell?
Ans: Cell organelles are found inside the cell. So, these are smaller than a cell.

Q.4. What are the two drawbacks of cell theory?
Ans: a. Viruses do not have a cell (acellular organisms) or cell machinery for vital functions. It does not obey the theory of “omnis cellula e cellula”.
b. In some organisms like Rhizopus (fungus) and Vaucheria (alga), cells are coenocytic.

Q.5. What is the Coenocytic condition?
Ans: Coenocytic condition refers to the cell which has multiple nuclei.