The body of all living things, no matter how different in size or appearance, is composed of cells. Viruses are of course an exception. The study of cells shows that - each cell has a high degree of complexity and organisation and can be compared to a chemical factory, which is fully automated, self contained, pollution free and with no labour problems. It continues producing more and more living matter. The moment this process stops we say the cell is dead. Of course, the cell takes up material and energy from its environment. The nonliying substances do not have such a complex structural organization.
Figure: This figure shows in a simple manner how cells combine to form tissues and how tissues combine to form organs.
The lower organisms like ameoba, bacteria etc., are unicellular and consist of just a single cell. In contrast, the body of higher organisms like humans or a neem tree is made up of billions of cells. The cells unite to form tissues. Many tissues come together to form organs, such as heart, liver or intestine in animals (Figure).