BODY ORGANIZATION OF ORGANISMS Organisms develop from a zygote formed by the fertilization of an egg by a sperm. A living organism can carry out all its biological functions, survive independently, grow and develop during its life span and reproduce to form offspring. An organism includes six groups of components (in order of size): atoms, molecules, cells, tissues, organs and systems. These components are closely related to each other for the survival of the organism. Anatomy deals with the structures that constitute the organism, while physiology deals with the function of these structures. 1. Atoms - Molecules - Compounds Atoms are the simplest structural elements of living things, and more than 100 elements have been identified in the world. Each element has a unique atomic structure. The most frequent elements found in living things are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. Two atoms of the same element generally associate to form a molecule. For instance, two hydrogen atoms associate to form a hydrogen molecule. Compounds differ from molecules in that their formation results from the combination of two or more atoms from different elements. The most well-known compound is water, formed from the association of two hydrogen atoms and one oxygen atom. Carbon dioxide is formed from the association of two oxygen atoms and a carbon atom.
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