7.1Cellular Activities and Thermodynamics: The Meaning of Metabolism
Humans obtain energy for survival by oxidizing nutrients collected from food by respiration, and synthesize body constituents using these nutrients. This mechanism also applies to individual cells. That is, where cells live aerobically, they obtain energy for cellular activity by oxidizing nutrients gained from the exterior environment by respiration. Various intracellular materials are also broken down or transformed. These processes of intracellular substance conversion are collectively referred to as metabolism. Thus, metabolism can be said to have two aspects: material metabolism and energetic metabolism. Metabolism is catalyzed by enzymes (biocatalysts), many of which are present in cells, and each enzyme catalyzes different chemical reactions. What, then, is the difference between dead cells and living cells? Cells that have just died should still have enzymes in them as well as substrates. However, simply combining the necessary components does not make a living cell, because living cells have a flow of energy. Materials taken in by organisms as nutrients are those with a certain level of free energy; cells - which are made of such materials - consist of complex materials with high levels of free energy, and have a complex structure consisting of membranes, granules and the like. Such self-assembly of cells is possible because the free energy of the materials taken in is directed to the synthesis of complex materials, and behind the formation of complex cellular structures lies the degradation of large amounts of nutrients—namely, the incorporation of negentropy (information) contained in them. This is possible only in nonequilibrium systems. For cells, then, life involves achieving a system of nonequilibrium that is sustainable.