Page 756 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactions and Kinetics 711

Diatoms: Photosynthetic microorganisms of the Chrysophyta Endogenous respiration: (1) The digestion of an organism’s
group that contains silica. own cellular matter by their own metabolic processes
DNA: (1) Deoxyribonucleic acid. (2) A linear polymer made for energy production (Stewart et al., 1959, p. 312).
up of deoxyribonucleotide repeating units (com- The rate of autodigestion is a constant proportion of
posed of the sugar 2-deoxyribose, phosphate, and the cell matter present. (2) The basal energy require-
a purine or pyrimidine base) linked by the phos- ment of bacteria, i.e., motion and enzyme activation
phate groups joining at the 3 position of one sugar that results in the metabolism of certain components
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and the 5 position of the next sugar; DNA contains of protoplasm. As endogenous metabolism proceeds,
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the genetic code (Rittman et al., 1990, p. 24). (3) the bacteria reach a point at which they can no longer
Each molecule of DNA consists of two strands sustain life and die. A portion of the dead bacteria
coiled around each other to form a double helix, a undergo lysing with a release of the nutrients that
structure like a spiral ladder. Each rung of the other bacteria can utilize. A residual organic fraction
ladder consists of a pair of chemical groups called remains, which is insoluble and not metabolizable by
bases (of which there are four types), which com- other bacteria, and is believed to be polysaccharide
bine in specific pairs so that the sequence on one (McKinney, 1963). (3) Those processes occurring
strand of the double helix is complementary to that when the organism is deprived of exogenous energy
on the other. It is the specific sequence of bases that (Sherrard, 1977, p. 1969). (4) Thermodynamically,
constitutes the genetic information (Apple Corpor- endogenous decay is the metabolism of the biomass
ation, 2005). itself to sustain life (Droste, 1998, p. 410). (5) The
DNA: Double-stranded helix-shaped molecule that contains digestion of cellular tissue by the metabolic
all the genetic information required for cell repro- processes of an organism for energy production
duction and contains in coded form all the genetic (p. 312); and most organisms, whether aerobic or
information needed to carry out cell functions. The anaerobic, maintain endogenous respiration during
unique genetic coding is in the particular sequence of growth and that the rate of autodigestion is a constant
four nitrogen bases, adenine, guanine, cytosine, and proportion of the cell material present and is inde-
thymine. The information contained in the DNA is pendent of substrate concentration (Stewart et al.,
‘‘read’’ and carried to the ribosomes for the produc- 1959, p. 320).
tion of proteins by RNA (Rittman and McCarty, Enthalpy: Defined as, H ¼ E þ PV, in which E ¼ internal
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2001, p. 11). energy (kJ), P ¼ pressure (kPa), V ¼ volume (m ).
DPN: Diphosphopyridine nucleotine, a coenzyme that facili- Enthalpy is the heat content of the substance.
tates electron transfer, or hydrogen transfer (McKin- Entropy: A measure of the ‘‘disorder’’ of a system in kJ. For
ney, 1962, p. 54). DPNH is the same with a proton example, a gas if released from a pressurized cylin-
added. der of volume V(1) to one that is under vacuum that
Electron acceptor: (1) Matter reduced. (2) The final electron also has a volume V(1) will expand spontaneously
acceptor (Rittman and McCarty, 2001, p. 46) as a from V(1) to 2V(1) with increase in entropy.
part of a complex biochemical pathway. Enzyme: (1) A polymer of amino acids, i.e., a protein that
Electron carrier: Molecule that moves electrons from one catalyzes reactions (Rittman et al., 1990, p. 24).
compound to another (Rittman and McCarty, 2001, (2) Each protein enzyme exhibits a high degree of
p. 46). specificity, with specific sites to which the substrate
Electron donor: (1) Matter oxidized. (2) Primary electron molecule becomes bound, forming an enzyme sub-
donor. The initial compound that donates an electron strate complex, E S. The enzyme allows an exother-
(Rittman and McCarty, 2001, p. 46) in a complex mic reaction to proceed by overcoming an energy
biochemical pathway. barrier. The reaction velocity is affected by tempera-
Embden–Meyerhoff pathway: (1) A biochemical pathway ture in accordance with the Arrhenius equation,
that degrades glucose to pyruvate (Prescott et al., log k ¼ log A E(activation)=(2.303 RT), in which
1993, p. G8). (2) Commonly called the glycolytic k ¼ rate constant, A ¼ constant (mols=L), R ¼ gas
pathway, in which one molecule of glucose, a six- constant 1.987 cal=degree=mol), T ¼ temperature (K)
carbon compound, produces two molecules of pyru- (Gaudy and Gaudy, 1980, pp. 115–131). (3) Coupled
vate ion, a three-carbon compound, with a net gain biochemical reactions are catalyzed by enzymes; the
of two molecules of ATP per molecule of glucose enzyme provides a pathway that allows the reaction
reacted. The pathway is anaerobic (Campbell, 1991, to occur (Rawn, 1989, p. 272). (4) The number
p. 294). See also citric acid cycle. of known enzymes is >1500 (Bailey and Ollis,
Endergonic: A reaction in which the standard free energy of 1977, p. 78).
reaction is positive, DG R 8 > 0 and consequently, the Enzyme–substrate complex: (1) Enzymes bring substrates
equilibrium constant, K eq , is less than one, i.e., together at a special place on their surface called
K eq < 1. The reaction is not spontaneous and requires the ‘‘active site.’’ The enzyme may interact with
an input of energy. See also, exergonic. the substrate in two ways (in general): (a) by the