The study of energy transfer in living organisms. Almost all the energy used on Earth comes directly or indirectly from the sun. The way in which it is converted in cells depends on the principles and laws of thermodynamics. The first law concerns the relationship between work, heat, and internal energy; it is equivalent to the statement that energy can be converted from one form into another, but cannot be created or destroyed. Thus, when light energy falls on a plant some is reflected, some absorbed as heat, and some converted into chemical energy in glucose by *photosynthesis
, but the total amount of energy is constant. In fact energy transfers in living organisms tend to be inefficient. About 2% of the energy in incident radiation is converted into chemical energy. Heterotrophic organisms obtain energy from the nutri-ents they ingest; about 10-20% of the energy is passed on at each link in the *food chain
, which tends to have three or four links only.
The second law of thermodynamics concerns the way in which energy is transferred or converted. It states that any conversion of energy from one form into another involves some dissipation of energy as unavailable heat energy. The availability of energy in a system is determined by its entropy, which is a measure of randomness or disorder. In any process the total entropy increases (an alternative statement of the second law), and there is a constant degradation of energy in the universe into energy that is unavailable for work. The growth of organisms is characterized by decreases of entropy in the sense that disordered systems (e.g. C02 and 02 gas) are converted into ordered chemical structures. However, the total entropy of the organism and its surroundings always increases in any change. All chemical reactions are driven by a decrease of free energy.