Cell The basic unit of life

PART -1



INTRODUCTION

  •    About 4 million of species are there including monerans , protists ,fungi, plants and animals. These organisms show great variety not in shape, colour , structure but also in functioning. But the similar thing among all these organism is that they are made up of cells.                

  •    Some organisms like Amoeba ,Paramecium,Bacteria, Chlamydomonas,etc. are made up of single cell (unicellular) only

 

  •        There are large number of other organisms like plants and animals are made up of million of cells (multicellular).

 

  •                  Cell is the structural and functional unit of life for all living beings.

 

 HISTORY 

  •              Robert hook (1665) coined the term cell. He explain his observations in a book namely, Micrographia.      
  • Anton van Leeuwenhoek (1674) observed first living cells like bacteria from pound and tartar of teeth, erythrocytes pf a fish,sperm and protozoans( eg.Vorticella_)  .
  • N. Grew proposed cell concept which states that cell is unit of structure

    of organisms.
  • Robert Brown (1831) reported nucleus in the root cell of orchids .
  • Dujardin (1836) reported a semifluid material around the nucleus of muscle cells named sarcode.
  • Purkinje (1839) named this jelly like material "protoplasm".
  • Huxely term this protoplasm as "physical basis of life".




CELL THEORY

  • The actual credit for cell theory goes to two scientists, a German Botanist M.J. Schleiden (1838) and a British Zoologist T. Schwann (1839). They gave the concept "all living organisms are composed of cells" and products of cells. Schleiden and Schwann together formulated the cell theory but this theory did not explain how new cells were formed.
  • Viruses, viroids and prions are exceptions to the cell theory as they are obligate parasites (subcellular in nature).
  • Rudolf Virchow (1855) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Cell theory states that–
    • All living organisms are composed of cells and products of cells.
    • Membrane bound cell organelles of the protoplasm do not survive along or outside the protoplasm.
    • All cells have similar fundamental structure and metabolic reactions.

    • Genetic information is stored as DNA in the chromosomes present in the nucleus.

    • All cells are arise from pre-exsisting cells.
    • This is further supported by Louise Pasteur(1862) that the life orignates from pre-exsisting life.(Biogenensis).

        Exception to cell theory

  1.             Viruses lack protoplasm. They are made up of protein and one of nucleic acid i.e.                  DNA or RNA.
  2.           Prokaryotic cells lack organised nucleus.
  3.         COENOCYTIC HYPHAE of Rhizopus or fungus and algae like Vaucheria ane multinucleated and aseptate.

               Cell diversity

  1. Cell shape                                                                                                                            The shape of cell varies from organism to organism and and in the part of same organ depending upon their function.      

  1. Cell size                                                                                                                       Mycoplasmas, the smallest cells, are only 0.3 µm in length while bacteria could be 3 to 5 µm. Among multicellular organisms, human red blood cells are about 7.0 µm in diameter. Nerve cells are some of the longest cells.      

               TYPES OF CELLS 

On the basis of nature of nucleus cells are divided into two main categories-

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