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Home
Science
Cell Theory

Cell Theory, Size and Shape of Cell

The “cell theory” was formulated by two biologists, M.J. Schleiden (1838), and T. Schwann (1839).

According to them, all the plants and animals are composed of cells and that the cell is the basic unit of life. The cell theory was further expanded by Virchow (1858) by suggesting that all cells arise from pre-existing cells.

1.0Cell theory, states that 

(i) All living organisms are composed of cells and products of cells.

(ii) All cells arise from pre-existing cells.

2.0Size of Cell

Normal size of human cell - 20 µm to 30 µm in diameter. (1 µm =1×10–6m)

(i) Largest isolated single cell in animals is an Ostrich egg.

(ii) Longest cell in animals is Nerve cell. 

(iii) Smallest cell is PPLO-Pleuropneumonia Like Organism [Mycoplasma], 0.3 μm in length.

Bacteria measure 3-5 µm, while viruses are still smaller and do not have a cellular structure.

The longest living cells in the human's body are nerve cells (or neurons) which can live for an entire lifetime.

Human red blood cells - 7 μm in diameter.

The size of an organism is dependent upon the number of cells not on the size of the cell.

3.0Shape of cell

Shape of cell mainly depends upon the specific function it performs.

Shape of Cell

(i) Elongated & branched - Nerve cell

(ii) Discoidal/Saucer - RBC

(iii) Spindle - Muscle cell

(iv) Spherical - Eggs

(v) Branched - Pigment cell of the skin

(vi) Slipper shaped - Paramecium

(vii) Cuboidal - Germ cells of gonads

(viii)  Polygonal - Liver cells 

Unicellular organisms vs multicellular Organisms 

The organisms which are made up of single cells are called unicellular organisms. e.g. Bacteria, Amoeba, Paramecium, Chlamydomonas.

The single cell is able to perform all life processes, like obtaining food, respiration, excretion, growth and reproduction.

The organisms which are made up of a number of cells are called multicellular organisms. e.g. some fungi, plants and animals.

All the cells of multicellular organisms have similar basic structure and undertake similar basic functions.

Generally, all cells are found to have the same organelles, irrespective of their function and location in an organism. A cell is able to live and perform all its functions because of these organelles.

4.0Also Read

Cytoplasm and Cell Organelles

Vacuoles and Cell Division

Structure of Plant and Animal Cells

Golgi Complex and Lysosomes

Site of Photosynthesis and Its Role in Plant Life

Plasma Membrane

Mitochondria and Plastids

Nutrition in Amoeba

Cell Wall

Table of Contents


  • 1.0Cell theory, states that 
  • 2.0Size of Cell
  • 3.0Shape of cell
  • 3.1Unicellular organisms vs multicellular Organisms 
  • 4.0Also Read

Frequently Asked Questions

All living organisms are composed of cells and products of cells. All cells arise from pre-existing cells.

The shape of a cell is primarily determined by its specific function. For example, nerve cells are elongated for transmitting signals over long distances, while red blood cells are disc-shaped to maximize surface area for oxygen transport.

Unicellular organisms consist of a single cell that performs all life functions independently. Multicellular organisms are made up of many cells, each with specialized functions that contribute to the overall organism.

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