Introduction to Cells: Science Cbse / Ncert Revision Notes


Cell is the structural and functional unit of all living organisms. All living organisms are made up of cells.A group of cells that have similar structure and function constitute a tissue. Tissues aggregate to form an organ. Organs together constitute an organ system. Organ systems aggregate to form an organism.
A microscope is an instrument used to see objects too small for the naked eye. An English scientist, Robert Hooke was the first person to discover the existence of cells with the help of a microscope in 1665.
Introduction to Cells, Science Cbse / Ncert Revision Notes, Class 8

Types of microscopes

  • Compound microscope is the microscope designed to magnify objects that can be brought close to the device.
  • Scanning electron microscope is used to examine the external parts of various organisms.
  • The transmission electron microscope is used to view the internal structure of a cell and its organelles.

Types of organisms

Organisms that are made up of a single cell and perform all their vital activities, like reproduction, locomotion and digestion, are called unicellular organisms.

Organisms that are made up of more than one cell are called multicellular organisms. Most life that can be seen with the naked eye is multi-cellular, as are all animals and plants.

Amoeba is an example of unicellular organism performing different activities. It acquires its food by special structures called as pseudopodia. They use pseudopodia for locomotion also. It gives rise to new individual by binary fission.

Paramecium is a single cell, built in such a way that it performs all its vital activities, like reproduction, locomotion, digestion, and so on.

Shape of cells

Cells differ in their shape. Cells can be round, spherical, elongated, pointed and even branched.
  • 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.

Size of cells

The size of the cell has no relation with the size of the organism. The single cell which can be seen with a naked eye is hen’s egg. The largest cell is the egg of an ostrich measuring 170 millimetres by 130 millimetres. The smallest cell is a bacterial cell measuring 0.1 to 0.5 micro-metres..

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