Cell Structure and Functions - Revision Notes

CBSE Class 08 Science 
Revision Notes
Chapter – 8
Cell — Structure and Functions
  • Cell: The smallest structural and functional unit of an organism, which is typically microscopic and consists of cytoplasm and a nucleus enclosed in a membrane.
  • All organisms are made of smaller parts called organs.
  • Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.
  • Cells were first observed in cork by Robert Hooke in 1665.
  • Cells exhibit variety of shapes and sizes.
  • Number of cells also varies from organism to organism.
  • Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example.
  • Some organisms are single-celled, while others contain large number of cells.
  • The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multi cellular organisms.
  • The cell has three main parts, (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, and (iii) the nucleus.
     
  • Cell membrane: The basic component of a cell. The cytoplasm and nucleus are enclosed within cell membrane. There is an outer thick layer in cells of plants called cell wall.
  • Cytoplasm: The jelly-like substance present between the cell membrane and the nucleus. Various organelles present in the cytoplasm are:
    (i) Mitochondria
    (ii) Plastids (present only in plant cell)
    (iii) Endoplasmic reticulum (ER)
    (iv) Ribosome
    (v) Lysosomes
    (vi) Vacuole
    (vii) Golgi body
    (viii) Centrosome (present only in animal cell).
     
  • Nucleus: Nucleus is separated from cytoplasm by a nuclear membrane. It is generally spherical in the centre of the cell.
  • Nuclear membrane: Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane.
  • Nucleolus: Nucleus contains a still smaller round body known as nucleolus.
  • Chromosomes: Nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of character from the parents to the off springs
  • Cells without well organised nucleus, i.e. lacking nuclear membrane, are called prokaryotic cells.
  • Plant cells differ from animal cells in having an additional layer around the cell membrane termed cell wall.
  • Coloured bodies called plastids are found in the plant cells only. Green plastids containing chlorophyll are called chloroplasts.
  • Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells.
  • Size of cells: The size may be as small as a millionths of a metre or may be as large as a few centimetres. Size of the cells has no relation with the size of the body of the animal or plant. It is related to its function.