Biology - Structure and Functions


  • The basic structural unit of an organ is known as the cell.

  • In 1665, Robert Hooke discovered the cell.

  • A cell is a living organism.

  • A human body has trillions of cells, which vary in shapes and sizes.

  • The organism, which is made up of more than one cell, is known as multicellular organism.

  • The single-celled organisms are known as unicellular organism. E.g. Amoeba.

  • A single-celled organism performs all the essential functions that a multicellular organism performs.

  • Unlike other organisms, Amoeba has no definite shape; so, it keeps on changing its shape.

  • Amoeba has pseudopodia, which means – pseudo means false and podia means feet.

  • Amoeba is a full-fledged organism capable of independent existence.

  • Shape of the cells are normally round, spherical, or elongated.

  • Protoplasm is known as the living substance of the cell.

  • The cells having nuclear substances without nuclear membrane are known as prokaryotic cells. E.g. bacteria and blue green algae.

  • The cells having well organized nucleus with a nuclear membrane are designated as eukaryotic cells. All multicellular organisms are eukaryotic cells.

Cell Structure and Function

  • The basic parts of a cell are cell membrane, cytoplasm, and nucleus.

Human Cell
  • Cell membrane is also known as the plasma membrane.

  • The plasma membrane is porous and allows certain substances or materials move both inward and outward.

  • The central dense round structure in the center is known as nucleus.

  • The jelly-like substance between the nucleus and the cell membrane (as shown in the above image) is known as cytoplasm.

  • Different organelles of cells are also present in the cytoplasm such as Mitochondria, Golgi bodies, Ribosomes, etc.

  • Located in central part, nucleus is almost in spherical shape.

  • Nucleus is separated from the cytoplasm by a porous membrane known as the nuclear membrane.

  • The smaller and spherical structure, found inside the nucleus, is known as nucleolus.

  • Nucleus contains thread-like structures known as chromosomes.

  • Chromosomes carry genes and help in inheriting the characteristics of the parents to the offspring.

  • Gene is a fundamental unit of inheritance in living organisms.

  • The entire constituents of a living cell are known as protoplasm, which include nucleus and cytoplasm.

Plant Cell

  • The cell membrane provides shape to the cells of plants and animals.

  • In plant cell, cell wall is an additional covering over the cell membrane.

Cell Structure
  • An animal cell does not have cell wall.

  • Cell wall gives shape and rigidity to plant cells.

  • Cell wall gives protection, plant cells need protection against varying temperature, high wind speed, atmospheric moisture, etc.

  • Bacterial cell also has a cell wall.

  • Usually, most of the cells are microscopic in size and are not visible to the naked eye.

  • The size of smallest cell is 0.1 to 0.5 micrometer found in bacteria.

  • The size of largest cell is 170 mm × 130 mm, found in the egg of an ostrich.

  • The size of the cells however has no relation with the size of the body of the animal or plant.

  • Some small colored bodies in the cytoplasm of the cells of Tradescantia leaf are known as plastids.

  • Plastids are found in different colors.

  • Some plastids have green pigment and known as chlorophyll.

  • Green colored plastids are known as chloroplasts.

  • Chloroplasts give green color to the leaves.

  • Chlorophyll is essential for the photosynthesis.