Cell theory : Cell is Unit of Life – An overview

What is a Cell ?

Cell is the fundamental structural and functional unit of all living organisms. Cell in latin word means “small room” . Just like bricks are the building blocks of a house or school , cells are the building blocks of life . Because you are alive, you are made up of cells. Let us observe a unicellular organism (e.g., Amoeba) under a microscope. We will find that a single cell is capable of independent existence and also performing the essential functions of life. In fact, all cells have certain structures in common to carry out basic life processes, but differential distribution of organelles gives cells distinct characteristics. In below lets talk about cell theory.


In other words, cells make up living things and carry out activities that keep a living thing alive.


Generally cell size ranges between 0.2ųm – 20ųm.

Smallest cell : PPLO : Mycoplasma gallisepticum : 0.1ųm

Bacteria : Unicellular : About 10 times smaller than animal cell.

Escheritia cell : Rod shaped Bacteria : 1 ‐2 micron long.

Protozoa : Amoeba proteus : 220 – 760 ųm .

Generally plant cells are larger than animal cell.

Generally Animal cell range from 10ųm to 30 ųm & plant cell 10ųm to 100ųm except ostrich egg cell :75 mm long and acetabularia 10cms in length .

Cell Size


There is great variability in cell shape i.e. spherical, polygonal, disc like, cuboidal, columnar , spindle like or fibre like .

Generally cells are spherical but in multicellular forms due to pressure become polyhedral . In plants as cells have cell wall appear hexagonal or polygonal.

Cell Shapes

Type of cells

Dougherty (1957) classified cells into prokaryotic (Pro meaning primitive , karyon meaning nucleus) and (Eu.meaning true , karyon meaning nucleus) types on the basis of structural organization of their nucleus.

Dodge et al (1966) proposed a third type Mesokaryotic (nucleus is larger in size; these types of cells which are intermediate between prokaryotic and eukaryotic) , which can be placed in between prokaryotic and eukaryotic cells.

Prokaryotic cells are generally smaller than eukaryotic cells.

Cell Types

Why Do the Organisms have Cells ?

In the living organisms, the metabolic reactions can occur only in a delicately balanced environment that is different from any found in the non-living objects. Cells are the life- supporting chambers which have such a special environment. A living cell keeps its chemical composition steady within its boundary. This condition is called homeostasis (G. homoios = same; stasis = standing). In the controlled environment of a cell, all life activities can occur in a desired manner.


In biology, cell theory is the explanation of all types of cells that were discovered by different scientists in different things.

History of Cell Theory

  • Around the year 1590, two Dutch lens makers by the name of Hans and Zacharias Janssen invented the first compound microscope when they put two of their lenses together in a tube.
  • The term cell was introduced by an English scientist Robert Hooke in his book Micrographia published in London in 1665. With this, he launched the study of microscopic anatomy. He examined thin slices of cork under a primitive microscope he had assembled . He saw tiny, empty compartments in the slices . He called them cellulae, now termed cells; and thought them to act as passages for conducting fluids. Since cork is the dead bark of a spanish oak, Quercus, Hooke saw only the dead walls of plant cells. These cells had lost their living contents and looked like small rooms. Hence, he gave each compartment an ap propriate name, the cell (L. cella = chamber,cellula = dim. of cella). Hooke also observed many other plants under the microscope and found the same structure.
  • After Hooke, the early microscopists started studying the fine structure of the organisms. The greatest among these was Anton van Leeuwenhoek . He was a Dutch merchant and pursued microscopy as a hobby. He designed remarkable simple microscopes, and with these he first describe free cells such as human sperm cells, bacteria, protozoans and red blood corpuscles from 1675 to 1680. He called the swimming creatures “animalcules” .
  • Al- fonso Corti, in 1772, noted living material in the cells.
  • Nehemia Grew in his two volumes on microscopic plant anatomy published in 1682, laid the foundation for the cell concept,i.e. a cell is the unit of structure in organisms.
  • In 1809, Lamarck stated that “no body can have life if its constituent parts are not cellular tissue or are not formed by cellular tissue”.
  • In 1831, a Scotch botanist Robert Brown discovered the nucleus in an orchid root cell.
  • In 1838 , a German botanist , M.J. Schleiden (1804-1881) , announced that all plants were composed of cells.
  • In 1839, German zoologist , Theodore Schwann (1810-1882), stated that all animals were also formed of cells.

Cell theory definition

Cell theory is a collection of ideas and conclusions from many different scientists over time that describes cells and how cells operate.

Statement of Cell Theory

The cell theory states that-

(i) all living things are composed of minute units, the cells, which are the smallest entities that can be called “living”.

(ii) a cell is a mass of protoplasm containing a nucleus and bounded by a cell membrane , and in many cases by a cell wall also.

(iii) all cells are basically alike in structure and metabolic activities .

(iv) the function of an organism as a whole is the result of the activities and interactions of the constituent cells.

Principle of cell theory

The cell principle states that-

1. Life exists only in cells.

2. Living objects are composed of cells and cell products, or protoplasm, are multinucleate mass of or are like a single free cell.

3. A cell is a small mass of protoplasm usually containing a nucleus or nuclear material and some other organelles, and is bounded by a cell membrane. A cell organelle does not survive alone.

4. Cell is also a unit of function, reproduction, beredity and disease, besides being a unit of structure.

5. Cells always arise from the preexisting living cells by division. They never arise de novo. The new cells are like the parent cell in all respects.

6. All cells existing to-day can trace their ancestry back to the original cells of ancient time. This also holds good for the cell organelles called chromosomes.

7. All cells have fundamental similarity in physical structure (organelles), chemical composition and basic metabolic reactions.

8. Cells maintain homeostasis to remain alive.

9. Structure and working of a cell is controlled by DNA .

10. A cell, though an integral part of an organism, can act (grow, divide and die) independently of the other cells around it.

11. Each cell is capable of regulating its vital processes.

12. Genetic information is stored and expressed within the cells.

Cell structure

Cell Structure

Cell membrane

  • selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells.


  • The living material surrounding the nucleus.


  • It is considered to be the brain of the cell ; contains genetic material of cell (DNA) and nucleoli; site of RNA synthesis and ribosomal subunit assembly.


Contains enzymes that digest material taken into the cell.

Phagocytic vesicle

  • Contains materials produced outside the cell; secreted by endocytosis.


  • Faciliate the movement of the chromosomes during cell division.


  • Supports cytoplasm ; assists in cell division and forms components of cilia and flagella.


  • Increase surface area of certain cells.


  • Move substances over surface of the certain cells.

Secretory vesicles

  • Contains materials produced in the cell; formed by the golgi apparatus ; secreted by exocytosis.

Golgi apparatus

  • Modifies protein structure and package protein in secretory vesicles ; Protein ‘packaging plant’ ; Move materials wthin or out of the cell.

Smooth endoplasmic reticulum ( SER)

  • Site of lipid synthesis; particiapates in detoxificatiin.

Rough endoplasmic reticulum ( ROR)

  • Has many ribosomes attached; site of the protein synthesis.


  • Site of protein synthesis and made of RNA


  • This is also called as ” Powerhouse of the Cell “ .

Produces energy through chemical reactions–

Breaking down fats and carbohydrates .

Controls level of water and other materials in cell.

Recycles and decomposes proteins, fats and catbohydrates.

Important of points of exams

Father of Biology and Zoology – Aristotle, Greek philosopher and scientist , 384-322B.C.

Father of Botany– Theophrastus, a Greek philosopher and scientist, 372-287 B.C.

Father of Microscopy– Anton van Leeuwenhoek, a Dutch microscopist, 1632-1723. He was the first to examine free cells (bacteria, protozoans, red blood cells,human spermatozoa).

Father of Cytology – Robert Hook, a British experiment physicist , 1635-1703.

Tissue Culture – Developed by white in 1932.

First Microscope– Built by Zacharias Janssen in 1590.

Protoplasm theory – Max Shultze,1861 .

Spherosomes – Discovered by Perner in 1953.

Microfilaments – Discovered by Paleviz et. al. in 1974.

Plasma Membrane – It was named cell membrane by Nageli and Cramer in 1855.

Plasmalemma – Discovered by Plowe in 1931.

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