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| Living Beings are Organised |
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| Organisation refers to a kind of structure or system or situation where the smaller components become arranged into larger ones in a hierarchical order of complexity, to co-ordinate with one another in performing common functions or achieving common goals. |
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| In the living system, we can see a hierarchy of organisation from the atomic level to the organismic level. The following levels of organisation can be recognised in the living system. |
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| Atomic Level |
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| All living organisms are basically composed of atoms of some chemical elements, particularly carbon, hydrogen, oxygen and nitrogen. |
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| Molecular Level |
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| Atoms of chemical elements combine with one another to form molecules. A molecule may have atoms of the same element
(like O2, H2, N2) or of different elements
(like H2O, CO2, CH4 etc). |
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| Cellular Level |
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| The molecules of various chemical substances in a specific proportion become organised into protoplasm - the physical basis of life. The protoplasm gets organised into cells, which represents the structural and functional unit of life. A living organism may have a single cell (unicellular) or many cells (multicellular) in its body. |
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| Tissue Level |
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| In multicellular organisms, cells having a common origin, group to form tissue. The cells in a tissue may be similar in structure to common functions (as in epithelium) or dissimilar in structure performing different functions (as in blood). There are different types of tissues meant for different functions. |
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| Organ Level |
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| Different types of tissue may together become organised into an organ for some specific function. E.g., a gland is an organ meant for secretion. In a gland there are four tissues - |
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 Epithelium (for secretion) |
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Muscular tissue (for contraction of the gland) |
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Connective tissue (for binding the tissues in the gland) |
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Nervous tissue (for stimulating the release of secretion) |
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| Organ System Level |
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| A group of organs become organized into an organ system for coordinating a major function. Different organs in the digestive system carry out functions related to nutrition in an animal. |
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| Organismic Level |
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| Many organ systems in coordination with one another form the organismic (or individual) level. A frog, for example, has various organ systems in its body like the integumentary system, digestive system, respiratory system, circulatory system, excretory system, reproductive system and nervous system. |
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| fig. 1.1 Levels of Organisation |
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