![]() ![]() Each organ performs one or more specific physiological functions. An organ is an anatomically distinct structure of the body composed of two or more tissue types. A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. In humans, as in all organisms, cells perform all functions of life. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells.Ī human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with a variety of tiny functioning units called organelles. Even bacteria, which are extremely small, independently-living organisms, have a cellular structure. Molecules are the chemical building blocks of all body structures.Ī cell is the smallest independently functioning unit of a living organism. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Atoms are made up of subatomic particles such as the proton, electron and neutron. The smallest unit of any of these pure substances (elements) is an atom. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. ![]() To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms and molecules. Figure 1.3 Levels of Structural Organization of the Human Body The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism. ![]()
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