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Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Diversity of Metabolism in Procaryotes (page 1)

(This chapter has 8 pages)

© Kenneth Todar, PhD


A lot of hoopla is made about microbial diversity. The unicellular eucaryotes (protista) exhibit a fair amount of structural diversity, but the procaryotes (bacteria and archaea) lack this distinction. There are but a few basic morphologies, the possibilities of motility and resting cells (spores), and a major differential stain (the Gram stain) that differentiates procaryotes microscopically. So what is all the hoopla about regarding procaryotes? It is about biochemical or metabolic diversity, especially as it relates to energy-generating metabolism and biosynthesis of secondary metabolites. The procaryotes, as a group, conduct all the same types of basic metabolism as eucaryotes, but, in addition, there are several types of energy-generating metabolism among the procaryotes that are non existent in eucaryotic cells or organisms. The diversity of procaryotes is expressed by their great variation in modes of energy generation and metabolism, and this feature allows procaryotes to flourish in all habitats suitable for life on earth.

Even within a procaryotic species, there may be great versatility in metabolism. Consider Escherichia coli. The bacterium can produce energy for growth by fermentation or respiration. It can respire aerobically using O2 as a final electron acceptor, or it can respire under anaerobic conditions, using NO3 or fumarate as a terminal electron acceptor. E. coli can use glucose or lactose as a sole carbon source for growth, with the metabolic ability to transform the sugar into all the necessary amino acids, vitamins and nucleotides that make up cells. A relative of E. coli, Rhodospirillum rubrum, has all the heterotrophic capabilities as E. coli, plus the ability to grow by photoautotrophic, photoheterotrophic or lithotrophic means. It does require one growth factor, however; biotin must be added to its growth media.

Fundamentally, most eucaryotes produce energy (ATP) through alcohol fermentation (e.g. yeast), lactic acid fermentation (e.g. muscle cells, neutrophils), aerobic respiration (e.g. molds, protozoa, animals) or oxygenic photosynthesis (e.g. algae, plants). These modes of energy-generating metabolism exist among procaryotes, in addition to all the following types of energy production which are virtually non existent in eucaryotes.

Unique fermentations proceeding through the Embden-Meyerhof pathway

Other fermentation pathways such as the phosphoketolase (heterolactic) and Entner-Doudoroff pathways

Anaerobic respiration: respiration that uses substances other than O2 as a final electron acceptor

Lithotrophy: use of inorganic substances as sources of energy

Photoheterotrophy: use of organic compounds as a carbon source during bacterial photosynthesis

Anoxygenic photosynthesis: photophosphorylation in the absence of O2

Methanogenesis: an ancient type of archaean metabolism that uses H2 as an energy source and produces methane

Light-driven nonphotosynthetic photophosphorylation: unique archaean metabolism that converts light energy into chemical energy

In addition, among autotrophic procaryotes, there are three ways to fix CO2, two of which are unknown among eucaryotes, the CODH (acetyl CoA pathway) and the reverse TCA cycle.

Energy-Generating Metabolism The term metabolism refers to the sum of the biochemical reactions required for energy generation AND the use of energy to synthesize cell material from small molecules in the environment. Hence, metabolism has an energy-generating component, called catabolism, and an energy-consuming, biosynthetic component, called anabolism. Catabolic reactions or sequences produce energy as ATP, which can be utilized in anabolic reactions to build cell material from nutrients in the environment. The relationship between catabolism and anabolism is illustrated in Figure 1 below.
Figure 1. The relationship between catabolism and anabolism in a cell. During catabolism, energy is changed from one form to another, and keeping with the laws of thermodynamics, such energy transformations are never completely efficient, i.e., some energy is lost in the form of heat. The efficiency of a catabolic sequence of reactions is the amount of energy made available to the cell (for anabolism) divided by the total amount of energy released during the reactions.


During catabolism, useful energy is temporarily conserved in the "high energy bond" of ATP - adenosine triphosphate. No matter what form of energy a cell uses as its primary source, the energy is ultimately transformed and conserved as ATP - the universal currency of energy exchange in biological systems. When energy is required during anabolism, it may be spent as the high energy bond of ATP which has a value of about 8 kcal per mole. Hence, the conversion of ADP to ATP requires 8 kcal of energy, and the hydrolysis of ATP to ADP releases 8 kcal.

Figure 2. The structure of ATP. ATP is derived from the nucleotide adenosine monophosphate (AMP) or adenylic acid, to which two additional phosphate groups are attached through pyrophosphate bonds (~P). These two bonds are energy rich in the sense that their hydrolysis yields a great deal more energy than a corresponding covalent bond. ATP acts as a coenzyme in energetic coupling reactions wherein one or both of the terminal phosphate groups is removed from the ATP molecule with the bond energy being used to transfer part of the ATP  to another molecule to activate its role in metabolism. For example, Glucose + ATP -----> Glucose-P + ADP  or  Amino Acid + ATP ----->AMP-Amino Acid + PPi.

Because of the central role of ATP in energy-generating metabolism, expect to see its involvement as a coenzyme in most energy-producing processes in cells.

chapter continued

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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