​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 organismSome 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.
Nucleus is separated from cytoplasm by a nuclear membrane.
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.