To untrained observers, these seven properties of life may appear to violate the basic laws of physics. Vitalism, the idea that life is endowed with a mystical vital force that violates physical and chemical laws, was once widely advocated. Biological research has consistently rejected vitalism, showing instead that all living systems operate and evolve within the constraints of the basic laws of physics and chemistry. The laws governing energy and its transformations (thermodynamics) are particularly important for understanding life (Cellular Metabolism
). The first law of thermodynamics
is the law of conservation of energy. Energy is neither created nor destroyed, but it can be transformed from one form to another. All aspects of life require energy and its transformation. The energy to support life on earth flows from the fusion reactions in our sun and reaches the earth in the form of light and heat. Sunlight is captured by green plants and cyanobacteria and transformed by photosynthesis into chemical bonds. The energy in chemical bonds is a form of potential energy that can be released when the bond is broken; the energy is used to perform numerous cellular tasks. Energy transformed and stored in plants is then used by the animals that eat the plants, and these animals may in turn provide energy for other animals that eat them.
The second law of thermodynamics
states that physical systems tend to proceed toward a state of greater disorder, or entropy. The energy obtained and stored by plants is subsequently released by a variety of mechanisms and finally dissipated as heat. The high degree of molecular organization found in living cells is attained and maintained only as long as energy fuels the organization. The ultimate fate of materials in the cells is degradation and dissipation of their chemical bond energy as heat. The process of evolution whereby organismal complexity can increase over time may appear at first to violate the second law of thermodynamics, but it does not. Organismal complexity is achieved and maintained only by the constant use and dissipation of energy flowing into the biosphere from the sun. The survival, growth, and reproduction of animals requires energy that comes from breaking complex food molecules into simple organic waste products. The processes by which animals acquire energy through nutrition and respiration command the attention of the many physiological sciences.