There are two aspects of bioenergetics that we want to emphasize at the end of this article. These are the dependence of life on photosynthesis and the diversity of energy interconversions in living systems.
Photosynthesis is the only major biological process that uses a source of energy, sunlight, from outside the earth’s environment to convert inorganic molecules to organic molecules, including carbohydrates, proteins, nucleic acids, lipids, and pigments. Green plants and algae are autotrophs; they make their own food. Actually, plants synthesize all the thousands of compounds that they contain from CO2, H2O, and inorganic nitrogen and sulfur compounds absorbed through the roots. The only source of carbon is CO2, which is assimilated through photosynthesis. Most other organisms are heterotrophs; they must take up and catabolize carbohydrates and fats to provide the energy to sustain life. The ultimate source of these compounds is photosynthesis, and the source of energy for their synthesis, sunlight. All heterotrophic organisms are dependent upon photosynthesis for their existence.
Animals also depend on plants for essential organic molecules that they are unable to make. We call some of these molecules vitamins. Several vitamins, including niacin, riboflavin, pyridoxine, and biotin, are key players in catabolic and anabolic metabolism, and deficiencies in these vitamins have severe effects. Also, animals are incapable of synthesizing polyunsaturated fatty acids (fatty acids with more than one double bond). Polyunsaturated fatty acids are essential components of membrane lipids and must be obtained in the diet. So, the next time you have a salad, pay a tribute to photosynthesis.
In photosynthesis, the electromagnetic energy of light is converted to chemical energy in the form of organic molecules. The primary photochemical reactions are electron transfer reactions that create oxidized chlorophylls and reduced acceptors. The reaction center chlorophylls and the acceptors are arranged within the photosynthetic membrane so that the electrons are transferred at least partway across the membrane. Thus, the membrane is charged by the primary electron transport, and electrical work has been done. The electron transport that follows the primary reactions is directly linked to the transmembrane flow of protons into the lumen of the membrane. This proton flow results in the generation of an electrochemical proton gradient. Essentially, part of the light energy is conserved by formation of this gradient as well as by formation of the strong reducing agent NADPH. The flow of protons provides the energy needed for the synthesis of the terminal phosphate anyhydride bond of ATP, an example of the conversion of the osmotic and electrical energy of the proton gradient to chemical bond energy. The syntheses of ATP and NADPH capture some of the light energy. In turn, ATP and NADPH drive the unfavorable reduction of CO2 by H2O to form carbohydrates and O2.
Organisms, especially bacteria, have evolved novel bioenergetic mechanisms that are well suited to their environments. For example, the bacterium Halobacter halo-bium lives in salt marshes and requires NaCl at concentrations that kill other organisms. These halophilic bacteria contain patches of a purple protein, halorhodopsin, on its plasma membrane. Halorhodopsin is a light-driven proton pump and its operation causes protons to be ejected from the cells. The resulting electrochemical proton gradient may be used to drive ATP synthesis or the transport of biochemicals. Given the diversity of the environments in which organisms grow, it is possible that biochemists will uncover new ways in which organisms meet their energetic needs. Perhaps future bioenergeticists will have the opportunity to unravel the mysteries of organisms from planets other than Earth.