the ATP-generating strategies involve metabolic pathways consisting of multiple discrete enzyme-cat­alyzed steps. These steps operate in a specific se­quence to convert an initial substrate or substrates into the end product or products of the pathway, ac­companied by the formation of ATP. Several central

FIG. 6-8 Colonies of Campylobacter jejuni af­ter 48 hours of incubation on Campy blood agar in an atmosphere with reduced oxygen concentration.

164 Chapter 6 cellular metabolism

FERMENTATION

PATHWAY END PRODUCTS

Homolactic acid Lactic acid

Heterolactic acid Lactic acid + ethanol + C0₂

Ethanolic Ethanol + C0₂

Propionic acid Propionic acid + C0₂

Mixed acid Ethanol + acetic acid + lactic

acid + succinic acid + formic

acid + H₂ + C0₂

Butanediol Butanediol + C0₂

Butyric acid Butyric acid + butanol + acetone

+ co₂

Fermentation does not require an inorganic sub­stance to utilize an organic substrate for generating ATP as a form of usable cellular energy and materi­als for growth; fermentation does not require air; ATP is generated exclusively by substrate level phosphorylation.

Microorganisms can be grouped into categories based on their requirement for or intolerance of oxy­gen. Aerobes grow in the presence of air that con­tains molecular oxygen. They generally grow only by

respiratory metabolism. Other microorganisms, called microaerophiles, grow only at reduced con­centrations of molecular oxygen. Such organisms re­quire oxygen for growth but only at concentrations (about 5%) lower than atmospheric levels (20%). Generally, microaerophilic organisms will not grow in air. Some microaerophiles prefer to grow at ele­vated carbon dioxide concentrations (5% to 10%) and are called capnophiles. Campylobacter jejuni, a bac­terium that causes gastroenteritis and diarrhea, is capnophilic (FIG. 6-8).

Facultative anaerobes can grow in the presence or absence of air. Many facultative anaerobes, such as Escherichia coli, switch between respiration and fer­mentation, depending on the availability of molecu­lar oxygen; they usually carry out fermentative me­tabolism in the absence of oxygen and respiration in the presence of oxygen. Other facultative anaerobes, such as streptococci, carry out only a fermentative metabolism whether or not oxygen is present.

Other bacteria are anaerobes and grow only in the absence of air. Obligate anaerobes, such as Clostrid­ium species, can only carry out fermentative metabo­lism. Only a few groups of bacteria and protozoa are obligate anaerobes. Some of these, such as Clostrid­ium botulinum, the bacterium that causes botulism, are very sensitive to oxygen and even a brief expo­sure to oxygen will kill them.

metabolic pathways have key roles in the metabolism of microbial cells. The reactions that lead to ATP gen­eration involve various intermediary metabolites linked together in a series of small steps to form uni­fied metabolic pathways. The intermediary metabo­lites and the cellular energy are used to build the molecules of new cells for growth and reproduction.

Respiration

A respiration pathway has three distinct phases (FIG. 6-9). These three phases for the respiratory metabo­lism of glucose are: phase i, glycolysis; phase 2, Krebs cycle; and phase 3, electron transport chain. The overall respiration pathway results in the forma­tion of carbon dioxide from the organic substrate

FIG. 6-9 Respiration occurs in three distinct metabolic pathways. In the first pathway, called glycolysis, carbohydrates are broken down to pyruvate. Then in the Krebs cycle the pyruvate is further metabolized to carbon dioxide. Reduced coenzymes formed during gly­colysis and the Krebs cycle are used to generate a protonmotive force for chemiosmotic generation of ATP in the third and final phase of respiration.

In the third and final stage of respiration, reduced coenzyme molecules formed during glycolysis and the Krebs cycle are reoxidized and ATP is generated by chemiosmosis. This reoxidation of reduced coen­zymes involves the transport of electrons through a series of membrane-bound carriers to the terminal electron acceptor. This transfer of electrons is called electron transport and the carriers are referred to as the electron transport chain. The process of electron transport through the membrane-embedded carriers also results in the expulsion of hydrogen ions (pro­tons) across the membrane. Because substances move by diffusion from regions of high to low concentra­tion, the difference of proton concentration across the membrane exerts a force. This protonmotive force, caused by the concentration gradient of protons across the membrane, is used to generate ATP by chemiosmosis.

Respiration involves three distinct phases: the cata­bolic breakdown of a substrate by glycolysis with the generation of some ATP and reduced coenzyme (NADH), the complete oxidation of the intermediary metabolite to produce CO, in the Krebs cycle with the generation of additional ATP and reduced coen­zymes (NADH + FADH), and the reoxidation of the reduced coenzymes involving an electron transport chain with the resultant generation of ATP by chemiosmosis.

Glycolysis

Glycolysis is the initial stage of all the main path­ways of carbohydrate metabolism. It occurs within the cytoplasm in both prokaryotic and eukaryotic f cells. The most common pathway of glycolysis is the Embden-Meyerhof pathway (FIG. 6-10). In this pathway the 6-carbon molecule glucose is first con-

166 CHAPTER 6 CELLUtAR METABOLISM

molecule. In aerobic respiration, water is also pro­duced as a result of the reduction of oxygen. The clas­sic equation that summarizes aerobic respiration us­ing glucose as substrate is:

FIG. 6-10 The Embden-Meyerhof pathway of glycolysis is a central metabolic pathway in various eukaryotic and prokaryotic cells for the conversion of carbohydrates to pyruvate t and the formation of ATP. Each red ball represents a carbon atom. In the Embden-Meyer- hof pathway a molecule of glucose is converted to two molecules of pyruvate, with the net production of two molecules of ATP and two molecules of reduced coenzyme NADH. In the initial step in the Embden-Meyerhof pathway, glucose is converted to glucose-6-phos- phate. ATP is used here rather than produced. The reactive glucose-6-phosphate is then modified and converted to fructose-1,6-bisphosphate. Again, ATP is consumed rather than produced so that now the net ATP balance is minus two. Fructose-1,6-bisphosphate is en­zymatically split into two phosphate-containing molecules: glyceraldehyde-3-phosphate (PGA) and dihydroxyacetone phosphate (DHAP). DHAP is converted into PGA so that now there are two molecules of PGA, and beyond this point in the pathway each step осcurs twice for each molecule of glucose metabolized. Next, the PGA molecule loses 2 elec­trons—becoming more oxidized—and the electrons are transferred to NAD⁺ to form NADH. Also in this reaction an additional phosphate group is added to form 1,3-bisphosphoglycerate. This is a substrate level phosphorylation reaction in which the generation of ATP does not depend on chemiosmosis. At this point in the glycolytic pathway there is an exact balance between the amount of ATP consumed and the amount of ATP generated.

Now net ATP production begins. The 1,3-bisphosphoglycerate is used to convert ADPto ATP, thus balancing the original energy investment of ATP. The resulting molecule is rearranged to produce phosphoenolpyruvate (PEP). PEP is transformed into pyruvate, the end product of the glycolytic pathway. It is in this last reaction that a phosphate group is I added to ADP to form additional ATP, yielding the net gain of two ATP molecules for each glucose molecule degraded via this pathway.