Class 8 Science



Cell

Cell: The basic structural unit of a living organism is called cell. All living beings are made up of cells. Some organisms are made up of a single cell or of a few numbers of cells. Some other organisms are made up of numerous cells.

Discovery of Cell: Robert Hooke coined the term ‘cell’ in 1665. Using a self-made microscope, he observed cells in a thin slice of cork.

Over the next 175 years, research led to the formation of the cell theory, first proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838 and formalized by the German researcher Rudolf Virchow in 1858.


Cell Theory

  • The cell is the basic structural and functional unit of life; all organisms are composed of cells.
  • All cells are produced by the division of preexisting cells (in other words, through reproduction). Each cell contains genetic material that is passed down during this process.
  • All basic chemical and physiological functions - for example, repair, growth, movement, immunity, communication, and digestion - are carried out inside of cells.
  • The activities of cells depends on the activities of subcellular structures within the cell (these subcellular structures include organelles, the plasma membrane, and, if present, the nucleus)

Variety in cell number, shape and size:

Variety in Number: Living beings show a large variation in number of cells. Large organisms; like humans, elephants, tiger, etc. are made up of trillions of cells but amoeba and bacteria are made up of single cell. On the basis of number of cells, organisms can be divided into two groups, viz. unicellular and multicellular organisms.


  • Unicellular Organism: An organism which is made up of a single cell is called unicellular organism, e.g. bacteria, protozoa, yeast, etc.
  • Multicellular Organism: An organism which is made up of more than one cell is called multicellular organism, e.g. humans, elephants, tiger, banyan, snail, etc.

Tissue: A group of specialized cells made for a specific task is called a tissue. While a single cell is responsible for all the functions in a unicellular organism, different tissues perform different functions in a multicellular organism.

Variation in Shape

Cells show wide variations in terms of shape. A particular shape of a cell facilitates the particular function performed by that cell. Some examples are as follows:


  • Amoeba is a unicellular organism. It is of irregular shape. The shape of amoeba keeps on changing because of formation of pseudopodia. Similarly, white blood cells (WBCs) in our body are of irregular shape.
  • Cells are generally round, spherical or elongated.
  • Smooth muscle cells are spindle-shaped. They are swollen in the middle and pointed at both ends.
  • Red Blood Cells are concave and round. RBCs can easily pass through narrow capillaries because of concave shape.
  • Cells of striated muscle are cylindrical in shape and are very long.
  • Nerve cells (neurons) appear like a star with long tail.

Size of Cells: While most of the cells are small in size, cells show a wide variation in size as well. The smallest cell is 0.1 to 0.5 micron and it is a bacterium. Ostrich egg is the largest living cell and it measures 170 mm x 130 mm.