What is a Cell – The Unit of Life Part -I

So in this article i am going to share a topic which is Very importent for any Compitative exam Like NEET, GPAT and other exams,

In this article will learn all about cells, its theory, types and many more..

The cells is generally divided in two parts, First
Prokaryotic cells and 2nd Eukaryotic cells.

In this post we will discuss about Prokaryotic cells.

What is cell – unit of life, defination , cell theory , overview of cell , type of cell and genrally describe prokaryotic cell & Their organelles and functions.

What is Cell CELL, The Unit Of Life

Definition :

A cell is the fundamental structural and functional unit of all living organisms.

All organisms are composed of cells. They include unicellular organisms and multicellular organisms.

Robert Hooke : Discovered cell.

Anton Von Leeuwenhoek : First observed and described a live cell.

The invention of the compound and electron microscopes revealed all the structural details of the cell.


Malthias Schleiden (1838) observed that all plants are composed of different kinds of cells.

Theodore Schwann (1839) reported that cells had a thin outer layer (now known as ‘plasma membrane’). He also found that cell wall is a unique character of plant cells.He proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.

Schleiden & Schwann formulated the cell theory.

Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the cell theory.

Cell theory states that:

  • (i) All living organisms are composed of cells and products of cells.
  • (ii) All cells arise from pre-existing cells.


All cells contain a semi-fluid matrix called cytoplasm. It is the main arena of cellular activities and chemical
reactions. This keeps the cell in ‘living state’.

All cells contain non-membrane bound organelles called Ribosomes. These are found in the cytoplasm, chloroplasts (in plants), mitochondria and on rough ER.

Cells differ in size, shape and activities.

  • Smallest cells: Mycoplasmas (0.3 µm in length)
  • Largest isolated single cell: Egg of ostrich.
  • Longest cells: E.g. Nerve cell.
  • Size of bacteria: 3 to 5 µm.
  • Human RBCs are about 7.0 µm in diameter.

    – Based on the functions, shape of cells may be disc-like, polygonal, columnar, cuboid, thread like, or irregular.

    Types Of Cells :

    Cells are 2 types:-

    1. Prokaryotic cells &
    2. Eukaryotic cells.


    What is a Cell - The Unit of Life Part -I 3

    • They have no membrane bound nucleus and organelles.
    • They include bacteria, blue-green algae, mycoplasma & PPLO (Pleuro Pneumonia Like Organisms).
    • They are generally smaller and multiply more rapidly than the eukaryotic cells.
    • They vary in shape & size. E.g. Bacteria have 4 basic shapes: Bacillus (rod like), Coccus (spherical), Vibrio
      (comma shaped), and Spirillum (spiral).

    Cell organelles in prokaryotic cells

    1. Cell Envelope :-

    It is a chemically complex protective covering.

    It is made of 3 tightly bound layers.

    • Glycocalyx : Outer layer. It differs in composition and thickness among different bacteria. It may be a slime layer (loose sheath) or capsule (thick & tough).
    • Cell wall : Middle layer. Seen in all prokaryotes. It gives shape to the cell and provides a structural support to prevent the bacterium from bursting or collapsing.
    • Plasma membrane : Inner layer. It is semi permeable in nature and interacts with the outside. This is structurally similar to that of the eukaryotes.

    – Based on the types of the cell envelopes and response to Gram staining (developed by Gram), bacteria are 2 types :

  • Gram positive : They take up and retain the gram stain.
  • Gram negative: They do not retain the gram stain.

    2. Mesosome & Chromatophores (Membranous structures)

    Mesosome is formed by the infolding of plasma membrane. It includes vesicles, tubules & lamellae.

    Functions: Mesosome helps.

    • In cell wall formation.
    • In DNA (chromosome) replication.
    • In Distribution of chromosomes to daughter cells.
    • In respiration and secretion processes.
    • To increase the surface area of the plasma membrane and enzymatic content.

    Chromatophores are membranous infoldings in some prokaryotes (e.g. cyanobacteria). They contain pigments.

    3. Nucleoid

    It is formed of non-membranous (naked) circular genomic DNA (single chromosome/ Genetic material) & protein.

    Many bacteria have small circular DNA (plasmid) outside the genomic DNA. It gives some unique phenotypic
    characters (e.g. resistance to antibiotics) to bacteria.

    4. Flagella

    These are thin filamentous extensions from the cell wall of motile bacteria. Their number and arrangement are varied in different bacteria.

    Bacterial flagellum is made of 3 parts

    filament, hook and basal body. The filament is the longest portion and extends from the cell surface to the outside.

    5. Pili and Fimbriae

    These are surface structures that have no role in motility.

    The pili (sing. Pilus) are elongated tubular structures made of a special protein (pilin).

    The fimbriae are small bristle like fibres sprouting out of the cell. In some bacteria, they help to attach the bacteria to rocks in streams and also to the host tissues.

    6. Ribosomes

    In prokaryotes ribosomes are associated with the plasma membrane of the cell.

    They are about 15 nm by 20 nm in size.

    They are made of two sub-units – 50S and 30S units. They together form 70S prokaryotic ribosomes.

    (S= sedimentation coefficient; a measure of density & size).

    Function of Ribosomes –

  • Ribosomes are the site of translation (protein synthesis). Several ribosomes may attach to a single mRNA to form a chain called polyribosomes (polysome). Ribosomes of a polysome translate the mRNA into proteins.

    7. Inclusion Bodies

    These are non-membranous, stored reserve material seen freely in the cytoplasm of prokaryotic cells.

    E.g. phosphate granules, cyanophycean granules and glycogen granules, gas vacuoles etc.

    Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.

    So that was all About Prokaryotic cells. in the next we will discuss about Eukaryotic cells. It is very important for medical students and competitive exams. hope you like this post. Share it with your friends and join our discussion group.

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