• 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 multicellular 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) Lysosome
(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 membrance.
• Nucleolus: Nucleus contains a still smaller round body known as nucleoulus.
• Chromosomes: Nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of character from the parents to the offsprings
• 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.