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Cell : The Fundamental Unit of Life

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Introduction

If something is living, it will carry out all the seven functions which are [highlight]movement, reproduction, sensitivity, nutrition, excretion, respiration and growth[/highlight]. Some non living things may show one or two of the seven characteristics of living things but not all of them. For ex.-car moves, crystals grow in size during crystallization process.

[pullquote-right]“Thus, the cell (Latin – small rooms) is the fundamental, structural and functional unit of living organisms.”[/pullquote-right]

 

Living organisms are well organized structures i.e. organs made up of tissues; tissues are made up of cells; and cells have smaller components in them.

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Cell – The Fundamental unit of life – Summary

  1.  Credit for the first microscope is usually given to Zacharias Jansen (1590), in Middleburg, Holland.
  2.  Robert Hooke (1665) – He observed a thin piece of cork under his self – designed microscope and noticed many little compartments like the honey comb. He named these compartments ‘cells’. (Latin word; means little room)
  3.  Antony Van Leeuwenhock (1632- 1723) – constructed an improved microscope and discovered: bacteria, sperm cells, blood cells.….
    • The kind of microscope that consists of a single biconvex lens is called simple microscope.
    • The kind of microscope that consists of a two lenses is called compound microscope.
    • The concave mirror used for throwing light on the object in Hooke’s microscope.
    • The maximum magnification that can usually be achieved by an electron microscope is 200,000 times as against the ordinary compound microscope which magnifies an object upto a maximum of about 2,000 times.

     

  4. Robert Brown (1831) – discovered the nucleus in the cell.
  5.  Purkinje (1839) – coined the termed ‘protoplasm’ for the fluid substance of the cell.
  6.  Cell Theory (1839) – proposed by Schwann and Schleiden. “ All plants and animals are made up of cells.”
  7.  Rudolf Virchow (1858) – He expanded the Cell Theory by adding “Omnis cellula e cellula” i.e. all cells arise from pre-existing cells.
  8.  Electron microscope (1940) – It made possible to observe and understand the complex structures of the cell and its various organelles.

 

Cell Theory

  1.  All organisms (plants and animals) are composed of one or more cells. (Schwann and Schleiden)
  2.  The cell is the fundamental, structural and functional unit of living organisms. (Schwann and Schleiden)
  3.  All cells arise from pre-existing cells. (Rudolph Virchow)
  4.  Every organism starts its life as a single cell.
    [pullquote-right]“Thus, a cell is defined as a mass of protoplasm having DNA and limited by a outer membrane. It is the fundamental, structural and functional unit of living organisms which is capable of independent existence.”[/pullquote-right]All cells have three basic parts –
    (a) Outer / Cell membrane – It limits the cell and gives it shape.
    (b) DNA – It may be contained in a nucleus.
    (c) Protoplasm – It is a living substance filled in the space within the cell.

 

Organism

It is any living thing (plant or animal) that exist in this world. An organism can be unicellular or multicellular.

(A) Unicellular organism (Uni = single ) –
It is an organism which only has one cell. Unicellular organisms are usually quite active and always moving around. They can be easily found in water such as ponds, lakes, rivers and any moist area.
Examples – Amoeba, Plasmodium, Paramecium, Chlamydomonas, bacteria etc.

A living thing which can only be seen under a microscope is called a microorganism.

(b) Multicellular organism (Multi = many) –
It is an organism that is made up of more than one cell. Most animals and plants are multicellular organisms. Multicellular organism have different types of cells. Each type of the cells contribute different functions. In humans, there are trillions of cells group into specialized tissues and organs.
Examples- Hydra (animal), Spirogyra (plant), Ostrich, Pine tree etc.

 

 

When we observe temporary slides of tip of roots of onion, peels of onions of different sizes, upper and lower surface peel of tradescantia or rheo leaf, leaf of maize and mustard etc., we can find the answer of the following questions –

  • Do all cells look alike in terms of shape and size?
  • Do all cells look alike in structure?
  • Could we find differences among cells from different parts of a plant body?
  • What similarities could we find?

Cells of a multicellular organism vary greatly in shape, size and structure. Cells of different parts of a plant and an animal body are different in shape, size and structure. Basic similarities among all cells of higher organisms are that they all have plasma membrane, cytoplasm with organelles and a nucleus.

 

Division of Labour

For multi-cellular organisms, it refers the separation of a work process into a number of tasks, with each task performed by a separate group.
For e.g. –

  1. In the human body, many processes take place such as digestion, circulation of blood, movement etc. Digestion cannot take place unless all organs work together. The mouth must chew, the gullet must push the food into the stomach and so on. All systems have a part to play as each system have a specific role in digestion. It enables the smooth and efficient functioning in the human body so it can survive better.
  2. Within a single cell, there are certain specific components called cell organelles. Each kind of cell organelle performs a special function, for e.g., making of new materials in the cell like protein synthesis by ribosomes, starch / glucose synthesis by chloroplasts, clearing up the waste material from the cell by the lysosomes and so on. Due to these organelles, a cell is able to perform all its functions properly. Thus, these organelles combinedly form the basic unit called the cell. It must be noted that all cells are found to have the same organelles, no matter what their function is or what organism they are found in.

 

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Morphology of Cells – Size, Shape and Structure