Cell -definition

    •   The word ‘cell’ is taken from the Latin word called ‘cella,’ which refers to a ‘small room’.
    • Cell is the basic unit of life. All cells vary in their size, shape and activity they perform.
    • Cell is called as the structural unit of life as it provides structure to our body.
    • A cell is capable of independently carrying out all necessary activities of life such as nutrition, respiration, excretion, transportation. So, they are known as a basic functional unit of life.
    • Cell is the smallest unit of life on Earth.
    •  Field of biology that studies cells are called Cell biology or cellular biology, or cytology.

    Discovery of cell:

    ·  In 1665, Cells were discovered first by Robert Hooke.

    ·   In 1831  Robert Brown discovered the nucleus in the cell.

    ·   In 1839  first coined the term ‘protoplasm’ for the fluid substance of the cell by Purkinje.

    ·   With the discovery of the electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles

    Cell Theory:

    Cell theory proposed by Schleiden (1838) and Schwann (1839) state  that:

    ·          All living organisms are composed of cells.

    ·         Cell is considered as the fundamental unit of life.

    Cell theory was further explained by Virchow (1855)

    ·         All new cells develop from pre-existing cells

     

     

     

    Kind of Organisms on the Basis of Number of Cells

    Organisms  are divided in two categories on the basis of cells:


    (i) Unicellular Organisms: The organisms that are composed of a single cell and may constitute a whole organism, are named unicellular organisms. For example Amoeba, Paramecium, bacteria, etc.

    (ii) Multicellular Organisms: The organisms which are composed of a collection of cells that perform functions in a coordinated manner, with different cells specialized to perform particular tasks in the body, are termed multicellular organisms.

    Example: Plants, human beings, animals, etc.

    ·         Every multi-cellular organism has come from a single cell due to Cells dividing to produce cells of their own kind.

    Diversity of Cell

    Within the same organism, different cells might be in different shapes and sizes. They are not particularly large, but their forms can be rather diverse. 

    These cells have similar capacities, including collecting and utilizing dietary energy, reacting to their surroundings, and reproducing. 

    The kind of a cell influences its function in part.

    Shape and size of cell

     ·         The size and shape of cells are related to the specific function they perform. 

        Cells like Amoeba have changed shapes. 

        In some cases the  shape  of cell could be more or less fixed and completely different for a particular type of cell; for example, nerve cells have a typical shape

    ·   Plant cells are generally rectangular shapes whereas some animal cells can be cubical, round, round with extensions, or variations of this. 

    Cell organelle  

    A cell organelle is a subcellular structure that has one or more specific jobs to perform in the cell, much like an organ does in the body. 

    ·        Organelles are also known as vesicles within a cell.

    Example of cell organelle

    ·         Nuclei: which store genetic information;

    ·         Mitochondria: which produce chemical energy;

    ·         Ribosomes: which assemble proteins.

    Same organelles are found in all cells, no matter what their function is or what organism they are found in.

    Cell components

    ·         Plasma  membrane,

    ·         nucleus

    ·         cytoplasm


    PLASMA MEMBRANE OR CELL MEMBRANE

    ·         PLASMA MEMBRANE is the outermost covering of any cell that separates the contents of the cell from its external environment.

    ·         The cell membrane also termed as the plasma membrane.

    ·         The plasma membrane permits the entry and exit of some materials in and out of the cell.

    ·         Cell membranes are made up of primarily of fatty-acid-based lipids and proteins.

    ·       Cell membranes separate the cytoplasm  (the contents of the cell) from the external environment. 

    ·        Cell membranes also prevent the movement of some other materials. Therefore

    ·         The cell membrane is known as  selectively permeable membrane

    ·         Diffusion is the spontaneous movement of a substance from a region of high concentration to a region where its concentration is low.

    ·          This feature is found in all cells, both prokaryotic and eukaryotic.

    ·         The cell membrane provides the cell its structure and regulates the materials that enter and leave the cell.

    ·          The process of actively transporting molecules into the cell by engulfing it with its membrane is called endocytosis.

    Nucleus

    ·         The nucleus contains a double-layered covering known as a nuclear membrane.

    ·         The nuclear membrane contains many pores that allow the transfer of material from inside the nucleus to its outside cytoplasm.

    ·         The nucleus also contains chromosomes, they are visible as rod-shaped structures only when the cell is about to divide.

    ·         Chromosomes contain information for the inheritance of characters from parents to the next generation in the form of DNA (Deoxyribose Nucleic Acid) molecules.

    ·         Chromosomes are composed of DNA and protein.

    ·         DNA molecules contain the information that is necessary for constructing and organizing cells.

    ·         Functional segments of DNA are known as genes.

    ·         Chromatin material is visible like an entangled mass of thread-like structures. Whenever the cell is about to divide, the chromatin material gets organized into chromosomes.

     

    ·         The nucleus plays a vital role in cellular reproduction, the process by which a single cell divides and forms two new cells.

    ·       The nucleus determines the cell development and maturity by directing the chemical activities of the cell.

    Cytoplasm

    ·         The cytoplasm contains fluid inside the plasma membrane. It also contains many specialized cell organelles. Each of these organelles performs a specific function for the cell.

    ·         Some important organelles are :

    (i)                 Endoplasmic Reticulum is called (ER) (Channels Network for transport)

    (ii)                Golgi Apparatus (Packaging)

    (iii)              Lysosomes [Suicide bags] (Cleanliness of cell)

    (iv)             Mitochondria (Powerhouse, Energy provider)

    (v)               Plastids

    (vi)              Vacuoles (Storage)