Explosions that Destroy Houses Traced to Methane from Landfill

Methane is produced at sites where organic matter landfills have been blown off their foundations be-
decomposes and creates anaerobic conditions. Sani- cause methane from a nearby landfill seeped through
tary landfills represent such sites, and methane pro- the ground into their basements. The methane was
duction at landfills is so extensive that it may be ignited by the pilot lights of the water heater or fur-
trapped and used as a fuel. In some cases, however, nace systems. The explosions moved the houses sev-
such methane production represents a serious prob- eral feet away from their original locations. These in-
lem, particularly for nearby houses. Several houses in cidents serve as important reminders to keep houses
Kentucky and other states that were located near a safe distance from landfills.

The production of methane by methanogens has several practical consequences. Methane is found in natural gas. It is a flammable gas. Methane seeping from landfills, where it is produced by methanogens degrading the waste deposits, sometimes enters nearby houses and causes explosions. Some sewage treatment facilities collect the methane that is formed

during anaerobic decomposition of wastes and use it as a source of fuel for generating heat and electric­ity. Some communities are supplied with a portion of their natural gas (methane) from this metabolic process. In the future, methane produced by mi­croorganisms may be used as a fuel for auto­mobiles.

SUMMARY

Process of Metabolism Within a Cell (pp. 158-164)

• Cells exhibit various strategies for converting chemi­
cal and light energy into the energy stored within
ATP, the central currency of energy of the cell. These
processes of cellular metabolism also transform start­
ing materials into the organic chemicals that make up
the cell's structural and functional components.

Role of Enzymes (pp. 158-159)

• Enzymes are proteins that act as biological catalysts
to accelerate the rates of chemical reactions by lower­
ing the activation energy necessary for the reaction to
occur. Different enzymes are needed to catalyze dif­
ferent reactions, each cell having thousands of en­
zymes, each enzyme binding only specific substrates
to its active site.

Coenzymes and Oxidation-reduction Reactions (pp. 159-160)

• An oxidation-reduction or redox reaction involves
the transfer of an electron from a donor, which is oxi­
dized, to an acceptor, which is reduced. All redox re­
actions must be balanced. Cellular metabolism gener­
ates reducing power to convert substrate materials
into the more reduced molecules of the cell by cou­
pling oxidation reactions with the reduction of coen­
zymes.

ATP and Cellular Energy (pp. 160-161)

• All cells carry out metabolic reactions that transfer
energy to ATP. ATP requires energy-releasing meta­
bolic reactions for its formation. ATP's stored energy
is used to drive energy-requiring biosynthetic reac­
tions. Breaking an ATP high-energy bond yields 7300

calories per mole of energy. Substrate level phosphor­ylation is the generation of ATP from ADP + P, cou­pled to energetically favorable reactions. The forma­tion of ATP can also be driven by a protonmotive force in chemiosmosis. Metabolic Pathways and Carbon Flow (pp. 161-162)

• The metabolic pathways utilized in ATP generation
involve various intermediary metabolites linked to­
gether in a series of small steps to form unified bio­
chemical pathways. In a catabolic pathway, larger
molecules are split into smaller ones. Cells form rela­
tively small molecules that can act as the basis for the
carbon skeletons of larger macromolecules that are
synthesized in anabolic (biosynthetic) pathways.

Autotrophic and Heterotrophic Metabolism (pp. 162-164)

• The synthesis of ATP can be achieved autotrophi-
cally—through the oxidation of inorganic substrates
or through the conversion of light energy to chemical
energy—or may be generated heterotrophically
through the utilization of organic substrates.

Metabolic Pathways (pp. 164-187)

Respiration (pp. 165-171)

• The Embden-Meyerhof pathway of glycolysis con­verts the 6-carbon molecule glucose into two mole­cules of the 3-carbon molecule pyruvate, plus two molecules of reduced coenzyme and two molecules of ATP.

• Glycolysis is the first step in all pathways of carbohy­drate metabolism. Its product, pyruvate, feeds into

788 CHAPTER 6 CELLULAR METABOLISM

the Krebs cycle and is converted to carbon dioxide with a net production of four ATP molecules. The Krebs cycle is not always completed; its intermediary products are siphoned out of the cycle and so must be continuously resynthesized.

• During oxidative phosphorylation, electrons from NADH and FADH₂ are transferred through an elec­tron transport chain, which includes a series of oxida­tion-reduction reactions of membrane-bound carrier molecules and the reduction of a terminal electron ac­ceptor. Chemiosmosis provides the energy for ATP production as a result of this process.

• An external electron acceptor is required to complete respiratory metabolic pathways. In aerobic respira­tion, oxygen is the terminal electron acceptor. Aerobic respiration is an efficient generator of ATP that comes from chemiosmosis.

• Protonmotive force is the potential energy gradient across a membrane established when protons are pumped across the membrane. Energy released when protons move back across the membrane by diffusion is coupled with the energy-requiring conversion of ADP to ATP. Generation of ATP using protonmotive force is called chemiosmosis.

Lipid and Protein Catabolism (pp. 171-173)

• Lipases break down fats into their fatty acid and glyc­erol components, which are further metabolized in the cell. Fatty acids are catabolized by beta-oxidation in which carbon fragments of long chains of fatty acids are removed two at a time and acetyl-CoA is formed. ATP is generated chemiosmotically by the re-oxidation of reduced coenzymes.

• Proteases break down proteins into short polypep­tides and amino acids. Amino acids are enzymatically deaminated, producing carboxylic acid, which can enter either the glycolytic pathway or the Krebs cycle. ATP is generated as a result of the protonmotive force across the plasma membrane of prokaryotes or the mitochondrial membranes of eukaryotes.

Fermentation (pp. 173-179)

• In fermentation, organic substrate molecules are used to generate ATP by substrate level phosphorylation. Organic molecules formed as products of fermenta­tive metabolism serve as terminal electron acceptors. The amount of ATP is limited to that formed during glycolysis, yielding far less ATP per substrate mole­cule than respiration.

• All fermentation pathways are anaerobic. A complete fermentation pathway begins with a substrate, in­cludes glycolysis and reoxidation of the coenzyme, and terminates in the formation of end products.

• Ethanolic or alcoholic fermentation converts pyru­vate to ethanol and carbon dioxide by yeasts, such as Saccharomyces cerevisiae. It is used to produce beer, wine, and distilled liquor.

• Lactic acid fermentation produces lactic acid as an end product. Homolactic fermentation uses the Embden-Meyerhof pathway of glycolysis and pro­duces only lactic acid. It is used in the production of dairy products such as cheese and yogurt. It is carried out by streptococci and lactobacilli. Hetero-

lactic fermentation produces ethanol and carbon dioxide and is carried out by Leuconostoc and Lacto­bacillus species.

• Propionic acid fermentation is carried out by propi­onic acid bacteria and produces propionic acid and carbon dioxide. This pathway is used in the produc­tion of Swiss cheese, giving it the characteristic holes and flavor.

• Mixed-acid fermentation yields ethanol, acetic acid, formic acid, hydrogen, and carbon dioxide. This path­way is carried out by members of the Enterobacteri-aceae, including E. coli. It can be detected by the I Methyl Red test.

• In the butanediol fermentation pathway, Klebsiella species produce butanediol. An intermediary metab­olite in this pathway, acetoin, can be detected by the Voges-Proskauer test, which distinguishes E. coli from Enterobacter aerogenes for water quality testing.

• The butanol fermentation pathway is carried out by members of the genus Clostridium; the end products of this pathway can be acetone and carbon dioxide, propanol and carbon dioxide, butyrate or butanol.

Photosynthetic Metabolism (pp. 179-183)

• Photoautotrophs, which include the photosynthetic bacteria, algae, and green plants, use light as their en­ergy source and carbon dioxide as their carbon source. In oxygenic photosynthesis the electrons of I water reduce carbon dioxide, and oxygen gas is given off. Chlorophyll and bacteriochlorophyll are the light-trapping pigments.

• Photoheterotrophs use light as the energy source and C0₂ and organic compounds as the carbon source. Green and purple nonsulfur bacteria are photo­heterotrophs.

• In photosynthetic microorganisms the flow of elec­trons—initiated when a chlorophyll molecule is ener- I getically excited by absorbing light energy—estab­lishes a protonmotive force across a membrane dur- I ing the process of photophosphorylation.

• Photosystems are light-trapping pigments organized into clusters of 200 to 300 molecules. These pigments harvest light energy in their chemical bonds, causing electrons to become excited and reach a higher energy level. That energy is released and transferred to a neighboring pigment when the electron returns to a lower energy level.

• Electron transport systems consist of a series of mole­cules that alternately accept and donate electrons from and to their neighboring molecules. The elec­trons are those expelled from a photosystem. These transfers accomplish oxidation-reduction reactions with the release of energy.

• The photosynthetic metabolism of cyanobacteria, al­gae, and plants releases oxygen atoms split from wa­ter into the atmosphere.

• The Calvin cycle is carried out by most autotrophic microorganisms in which carbon dioxide is reduced to form organic matter. This requires reducing power in the form of reduced coenzyme NADPH and ATP. The product of this pathway is glyceraldehyde 3-phosphate.

SUMMARY 189

Chemoautotrophic Metabolism (pp. 183-184)

• Chemoautotrophs (chemolithotrophs) are bacteria that can combine inorganic substances such as sulfur or nitrogen with oxygen to generate ATP for cellular energy via aerobic respiration. Such bacteria play im­portant roles in mineral cycling, for example, the con­version of ammonia to nitrite and nitrate, and hydro­gen sufite to sufate.

• Chemoautotrophic microorganisms couple the oxida­tion of an inorganic compound with the reduction of a suitable coenzyme. They use chemiosmosis to gen­erate ATP. Important mineral cycling reactions are the result of chemoautotrophic metabolism.

• Regardless of the mode of metabolism the strategies are the same: synthesize ATP, reduce coenzyme (NADPH) and small precursor molecules to serve as the building blocks of macromolecules, and then use the energy, reducing power, and precursor molecules to synthesize the macromolecular constituents of the organism.

Nitrogen Fixation (pp. 184-185)

• Nitrogen fixation is the conversion of atmospheric ni­trogen into reduced nitrogen-containing compounds such as ammonia. Nitrogen fixation is carried out only by members of a few bacterial genera.

• Nitrogenase is the enzyme that converts molecular nitrogen into reduced nitrogen compounds.

• Some nitrogen fixing bacteria live in symbiotic asso­ciation with plants in specialized structures called nodules. Others have structures called heterocysts where nitrogen fixation occurs.

Methanogenesis (pp. 185-187)

• Methanogens are strictly anaerobic archaebacteria
that produce methane as a result of anaerobic respira­
tion. The oxidation-reduction reactions of the coen­
zymes in the metabolism of methanogens establishes
an electron chain through which electrons move. This
movement of electrons is coupled with the moving of
protons across a membrane, and the protonmotive
force is used for ATP synthesis by chemiosmosis.

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