Fundamentals of Human Biology and Anatomy

The human body has different structural levels of organization, starting with molecules and macromolecules at the lowest level and increasing in both size and complexity to cells, tissues, organs, and eventually to the systems that make up the whole organism. In the human body, macromolecules such as polypeptides (proteins and enzymes), polynucleotides, and polysaccharides are as important as small molecules (H2O, CO2, O2, N2) that constitute or interact with them. Between the molecules and the cells are the organelles that are the components of the cells, and without the formation of the cells, they do not have the properties of a living entity. Cells are the smallest independent units of life because all life depends on the many chemical activities of cells. Some of the basic functions of the cells are growth, metabolism, and reproduction.