Energetic of Bacterial Metabolism
When facultative organisms as examined under the microscopes are grown under aerobic conditions, growth is always more vigorous than that obtained under anaerobic conditions. This is because of the greater amount of phosphate bond energy made available by respiration.
The anaerobic breakdown of the glucose molecule to lactate is accompanied by the phosphorylation of 2 moles of ADP and proceeds with a free energy decline of negative 38,000 calories per mole. This is 16,000 calories less than the calculated negative 56,000 calories free energy change that would be expected from the simple breakdown of glucose to lactate. In the intact cell, a considerably large part of this free energy loss is conserved in the form of ATP. In the breakdown of 1 mole of glucose to the level of lactate, roughly 28 percent, of the energy is thus conserved in the 2 moles of ATP formed.
The anaerobic breakdown of the glucose molecule to lactate is accompanied by the phosphorylation of 2 moles of ADP and proceeds with a free energy decline of negative 38,000 calories per mole. This is 16,000 calories less than the calculated negative 56,000 calories free energy change that would be expected from the simple breakdown of glucose to lactate. In the intact cell, a considerably large part of this free energy loss is conserved in the form of ATP. In the breakdown of 1 mole of glucose to the level of lactate, roughly 28 percent, of the energy is thus conserved in the 2 moles of ATP formed.
In growth under aerobic conditions, the combined processes, done with the help of the microscopes, of glycolysis and oxidative phosphorylation provide a total of 38 moles of ATP. Since the calculated ATP for the complete combustion of glucose is -686,000 calories per mole and since an approximate input of 8000 calories per mole of ATP called for, the efficiency of energy conservation under aerobic conditions is approximately 45 percent.
This energy that is conserved in the form of ATP is used by the cell to perform its various activities. The most important forms of work carried out by the bacterial cell as per observance using the microscopes at the expense of ATP are active transport and the biosynthesis of cellular components from small precursor molecules. Almost all of the energy of bacterial cells is put into biosynthetic work. Their sole mission is to multiply. Since bacteria as viewed using the microscopes, normally live in natural environments over which they have no control and from which they cannot escape, the ability to multiply rapidly fits them to survive. For this task, a large amount of energy is required.
One of the approaches utilized to assess the efficiency of bacterial energy conservation is the measurement of molar growth yields. When growth is limited by the energy source, the total growth obtained in a culture is proportional to the amount of carbohydrate added as observed under the microscopes. When anaerobic organisms employing a fermentative metabolism are grown in a complex medium, the substrate is used almost exclusively for the generation of ATP. Since the amount of ATP produced by various fermentations can be calculated. The growth yield as a function of ATP provides a means of shaping the efficiency with which ATP is used by different organisms. The growth yields for different organisms utilizing a wide variety of substrates and employing different pathways as monitored by means of microscopes are constant and the values of approximately 10 g of cell material per gram-mole of ATP have been obtained under batch conditions of growth. While significantly higher values have been obtained under energy-limited, continuous culture conditions such as 12.4 and 14.0 for E. coli and Klebsiella pneumoniae, respectively, the values are still considerably less than would be expected if all of the ATP used in cellular growth was coupled to biosynthesis. It would accordingly appear as watched under the microscopes that bacteria are inefficient converters of free energy, and there is a large outflow of entropy from the cell. Perhaps this is the price they must pay to cope with the vicissitudes of life outside the laboratory. In order to compete effectively in their natural, nutrient-limited environment, they must be able to react rapidly when nutritional constraints are occasionally relieved.
Control of Energy Metabolism If a microorganism, which can only be viewed using the microscope is to function efficiently, the rate of metabolism along the various branching and anastomosing metabolic pathways must be in harmony in such a way that optimal use is made of the available substrates. For this purpose, the microbial cell has evolved an extremely sophisticated system of controls, some of which normalize its energy-supplying processes. Regulation of the energy-yielding metabolism takes place on different levels the regulation of enzyme production, end product inhibition of enzyme activities, and general metabolic regulation of enzymatic activities by substrate and product levels.
Genetic Regulation In its natural territory, the cell, which can be seen under the microscope, is confronted with a variety of potential energy sources. The survival of a particular species in its highly competitive environment has resulted from the ability of that species to adapt to new experiences in its environment. In so doing, the enzymatic machinery for the degradation of a wide variety of organic compounds is produced per observance using the microscopes. Although the potential for the dissimilation of various substrates is great, the enzymes for such activities are produced or induced only when needed. Controls of this type, induction and repression, are exceedingly common in microorganisms and are examples of genetic regulatory mechanisms. In general, induction exerts effective control of catabolic sequences involving carbon and energy sources, where the synthesis of enzymes catalyzing a particular sequence is turned on or off, depending on the demands for that specific sequence. The classic example of genetic regulation is the utilization of the disaccharide lactose by E. coli.