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Introduction to Cell

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Introduction to Cell - Lesson Summary


Cell
All living organisms are made up of cells. Cell is the basic structural and functional unit of complex organisms. Cells perform all the metabolic activities taking place at cellular level. The discovery of cells was possible with the invention of microscope.

History of cell
    •  Marcello Malpighi, in 1661, proposed that plants are made of tiny structural units called 'Utricles'.
    •  Robert Hooke, in 1665, observed many tiny, hollow, room like structures in a thin slice of cork through a compound microscope and called them ‘cells’.
    •  Leeuwenhoek, in 1674, with the improved microscope, discovered free-living cells in pond water for the first time.
    •  Robert Brown in 1831 discovered the nucleus in the cell.
    •  Purkinje in 1839 coined the term 'protoplasm' for the fluid part of the cell.
    •  Schleiden in 1838 and Schwann in 1839 proposed the cell theory which stated that all plants and animals are composed of cells.
    •  Rudolf Virchow in 1855 further expanded the cell theory by saying ‘omniscellula-e-cellula’, which means all cells arise from pre-existing cells.
    •  Cell is derived from the Latin word "Cellula" which means "a little room".

Microscopes
The two types of microscopes used to visualise the cell are compound microscopes and electron microscopes.
                                                                  
a) Compound microscopes are also called as light microscopes.
    •  Compound microscope is an instrument used to magnify the specimen up to 4,000 times.
    •  Compound microscope consists of a stage on which the specimen is placed under an objective piece.
    •  The light reflected from the mirror is used to illuminate the specimen.
    •  From the eye piece, a magnified image of the specimen is seen.
    •  The organisms which cannot be seen by naked eye are selected as specimen.

     

                     
b) Electron microscope are used for very large magnification of the specimen.
    •  It uses a beam of electrons to magnify the specimen. Electrons are spread by electromagnets on to a photographic plate to obtain enlarged image of the specimen.
    •  It magnifies the specimen upto 1,00,000 times.
    •  It is used to observe very small structures of the size of 10 Ao.

Cell theory
M.J.Scleiden and Theodre Schwann proposed cell theory for the first time.The cell theory can be stated as follows.
    •  Cell is the basic structural unit of all the organisms.
    •  Preexisting cells only gave rise to new cells.
    •  An organism has its body composed of cells.
    •  A single cell transfers life from one generation to another generation.

Microscopic examination of cells
Plant and animal cells are only visible only under a microscope.
     

                                                                               

    •  The microscopic examination of a plant cell enables us view a prominent vacuole, nucleus, and cytoplasm.
    •  For the microscopic examination of an animal cell, spread the specimen on a glass slide, and add a drop of water and methylene blue. Cells with darkly stained spherical nuclei at their centre can be observed.

Shape of cells
    •  Cells differ in their shape. The shape of a cell is related to the specific function that it performs. Cells can be round, spherical, elongated, pointed and even branched.
    •  Shape of the cell also depends on age, pressure, internal skeleton, external skeleton etc.
    •  Amoeba is irregular in shape.
    •  Neuron, the nerve cell is a branched structure.
    •  White Blood Corpuscle is the only animal cell that changes its shape.
    •  Red Blood Corpuscles are round and flattened.
    •  Muscle cells are spindle-shaped.
    •  Plant cell has definite shape.

Motile or Non-motile
    •  Cells which change their shape from time to time in order to perform locomotion are termed to motile cells. Cells like amoeba (motile) change their shape for motility.
    •  Cells whcih have fixed shape and do not exhibit any change during locomotion are termed to be non-motile cells. Cells like nerve cells have a fixed shape (non-motile) that suits their function of transmitting nerve impulses.

Unicellular and multicellular organisms
Unicellular organisms are the organisms in which a single cell performs all the functions like nutrition, respiration, excretion and reproduction.
    •  These are also called as acellular organisms.
        e.g. Amoeba, Chlamydomonas, Paramecium and Bacteria possess single cells constituting  the whole organism.

     
                                                                             

Multicellular organisms are the organisms which possess many cells to perform different functions.
    •  Multicellular organisms represent themselves as a member of a group of cells or as an individual.
        e.g. Fungi, plants and animals have many cells that group together to form tissues.
    •  Division of labour in multicellular organims increases the efficiency of the organism for its better survival

Level of organisation
   
      


    •  A cell is a basic unit of life.
    •  A group of cells that have similar structure and function constitute a tissue. e.g.  A group of hepatic cells form hepatic tissue.
    •  A group of tissues aggregate to form an organ. e.g. hepatic tissues aggregate to form the organ, liver.
    •  A group of organs together constitute an organ system. e.g. Buccal cavity, oesophagus, stomach, intestine collectively form the organ system, digestive system.
    •  A group of organ systems aggregate to form an organism. e.g. Digestive system, respiratory system, circulatory system, muscular system, nervous system reproductive system etc. work together to form a complete organism.

Prokaryotic and eukaryotic cells
Prokaryotic cells are the cells which have indefinite nucleus called as nucleiod. A single chromosome represents the genetic material. Genetic material is not covered by a nuclear membrane e.g Bacteria.
      
     

Eukaryotic cells are the cells which possess a definite nucleus. e.g. Plant cell. Eukaryotic cells have nucleus with a distinct nuclear membrane. Chromosomes are considered to be vehicles of inheritance and are seen in nucleoplasm. Eukaryotes can be unicellular such as trypanosoma, euglena and paramecium. They can be multi-cellular such as fungi, plants and animals.

Structure of a prokaryotic cell
    •  A typical bacterial cell is a prokaryotic cell which possesses following characters.
    •  The cell wall is a non-living layer composed of polysaccharides and proteins.
    •  The plasma membrane is a living membrane made up of lipids. It is selectively permeable. Therefore, it transports ions, nutrients and wastes into and out of the cell.
    •  The cytoplasm is a clear, thick, jelly-like material that forms the medium for all cellular functions.
    •  The cytoplasm of bacteria contains the nucleoid, ribosome and flagella.
    •  Nucleoid is a single circular DNA molecule considered as the genetic material.
    •  Ribosome transfers genetic messages into proteins.
    •  Flagella are extracellular appendages which help in locomotion.
    •  Prokaryotes do not have vacuoles.

Differences between a prokaryotic and a eukaryotic cell

           PROKARYOTIC CELL            EUKARYOTIC CELL Very small cells each measuring from 1µm to 10µm. Large cells but microscopic measuring from 5µm to 100µm. Absence of specific nucleus. Genetic material is dispersed as nucleoid. Presence of nucleus with well defined nuclear membrane.  A single chromosome is present. Chromosomes are many in number. Membrane bound cell-oganelles are absent. Membrane bound cell organelles are present.

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