According to the first law of thermodynamics, energy can be transformed (changed from one form to another), but it can neither be created nor destroyed. This is sometimes called the law of conservation of energy. As a result of energy transformations, according to the second law of thermodynamics, the universe and its parts (including living systems) become increasingly disorganized. The term entropy is used to describe the degree of disorganization of a system. Energy transformations thus increase the amount of entropy of a system. Only energy that is in an organized state—called free energy—can be used to do work. Since entropy increases in every energy transformation, the amount of free energy available to do work decreases. As a result of the increased entropy described by the second law, systems tend to go from states of higher free energy to states of lower free energy.
The chemical bonding of atoms into molecules obeys the laws of thermodynamics. A complex organic molecule such as glucose has more free energy (less entropy) than six separate molecules each of carbon dioxide and water. Therefore, in order to convert carbon dioxide and water to glucose, energy must be added. Plants perform this feat using energy from the sun in the process of photosynthesis (fig. 4.12).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.