The Primary Energy‐Transformations in Biological Systems
Authors: Lehninger Albert L. 
Published: August 2012

In this paper I shall review the main outlines of current research on the molecular aspects of the primary energy‐coupling mechanisms in cells, those carried out by energy‐transducing membranes. They include the capture of solar energy by the chloroplast membranes of green plants, used to generate carbohydrates and molecular oxygen from carbon dioxide and water, and the counterpart of photosynthesis, the process of respiration in heterotrophic organisms, in which reduced organic products generated by photosynthesis are oxidized at the expense of dioxygen to form carbon dioxide and water. — Although the cycling of dioxygen, carbon dioxide, and organic matter between the plant and animal worlds is well known, it is not generally appreciated that the magnitude of biological energy flux in these cycles is huge compared to the total energy flux in man‐made devices. The approximate turnover of energy in the biosphere is of the order of 20 x 1020 kjoules per annum. In 1950 the total energy turnover in all man‐made systems was only about 3 to 4 percent of that of the biosphere. However, by 1970 energy flow through man‐made systems had increased almost 3 fold. These data indicate that the increasing rate of combustion of fossil fuels by man is beginning to perturb the biological energy cycle. A major consequence is that the concentration of carbon dioxide in the atmosphere has been increasing at a significant rate, at a time when there is also a decrease, at least in some parts of the world, in the counterbalancing utilization of CO2 by green plants, due to deforestation. The „greenhouse effect”︁ of increased atmospheric CO2 may not only change the earth's climate, but also may influence the rate of photosynthesis. — It is also not generally appreciated that energy flow in the biosphere leads to production of enormous amounts of organic matter potentially useful in furnishing man's energy requirements. For every human being on the earth, the plant world produces by photosynthesis about 70 kilograms of cellulose per day. Thus the plant world could in principle furnish us with huge amounts of biomass in the form of cellulose, starch, protein, and other plant products, which can be used not only for food, but also for fuel. There is therefore, a very large potential for world biomass production and utilization that modern technology can and will exploit. However, this paper is more concerned with the longer‐term aspects of bioenergetics, particularly the primary energy‐conserving devices of energy‐transducing membranes that participate in photosynthesis and in animal respiration. We should think of these membrane systems as prototypes or models for future development of new kinds of energy‐conserving devices of high efficiency, capable of utilizing solar energy without at the same time causing significant imbalances in the cycles of oxygen and carbon dioxide in the biosphere. Solar energy is virtually limitless: the amount striking the surface of the earth over a period of only 2.7 days is ample to provide at a very high level all the energy and food needs of the entire population of the earth for a period of one year. — In this spirit I shall outline the properties of the energy‐transducing membranes of two intracellular organelles: chloroplasts and mitochondria, the major power plants of eukaryotic cells.