Environmental Engineering Reference
In-Depth Information
modes of locomotion, and complex nervous systems to
control these processes. For them the energetic advan-
tages of oxidation over anaerobic fermentation are clear.
Lactic acid fermentation liberates 195 kJ for each mole-
cule of glucose, alcoholic fermentation yields 232 kJ,
but a complete oxidation of that sugar releases 2.8 MJ, a
12-14-fold gain. Three kinds of nutrients can be metab-
olized to yield energy: carbohydrates, lipids, and proteins
(but proteins are used only if the other nutrients are in
short supply). Energy released by their oxidation is par-
tially conserved in ATP. As in photosynthesis, ATP is
the principal energy carrier, the key link between cellular
catabolism (degradation of nutrient substrates) and anab-
olism (biosynthesis of complex substances), locomotion
(muscle contraction) and active transport of metabolites
against the concentration gradient.
The biochemistry of these sequential, enzymatically
catalyzed reactions is well understood (de Duve 1984;
Nelson and Cox 2000). Glycolysis of glucose or glyco-
gen takes place in the cytoplasm of all heterotrophs, and
it produces, following the Embden-Meyerhof-Parnas
pathway, pyruvic acid (fig. 4.1). Nicotinamide adenine
dinucleotide (NAD) is the electron carrier (NADH), and
pyruvic acid is the precursor compound for anaerobic
respiration (the pathway that ends in lactic acid), alcohol
fermentation (producing ethanol and CO 2 ), and aerobic
respiration, the tricarboxylic acid (citric acid, Krebs) cycle
(fig. 4.1). This cycle, taking place inside the mitochon-
dria, converts a variety of organics (fatty acids and amino
acids) to CO 2 and transfers the released electrons down
the electron transport chain, producing large amounts of
ATP and reducing oxygen to water. The maximum
energy gain is 38 mol of ATP for each mole of glucose
broken down in prokaryotic cells, an overall free energy
change of about 2.8 MJ. With 31 kJ/mol available
from each ATP transformation to ADP, the overall effi-
ciency of the whole sequence would be about 42%.
In eukaryotic cells the net ATP gain is a bit smaller—
two moles are needed to move NADH from the
cytoplasm—but because the free energy of the com-
pound may be up to 50 kJ/mol in mammalian cells,
the overall efficiency may be over 60% (a value of 31
kJ/mol is valid only for unimolar concentrations, neutral
pH, and 25 C). Respiration of fatty acids yields a maxi-
mum of 44 ATP/mol, but since oxidized compounds
have higher energy contents than glucose (around 3.4
MJ/mol), the peak efficiencies are about 60%. Only two
molecules of ATP are gained during the breakdown of
glucose to lactic acid (the overall free energy is 197
kJ), and the process has efficiency of about 30%; in verte-
brates it can be sustained only briefly. Only invertebrates
living in oxygen-poor environments evolved longer-
lasting low-efficiency anaerobic pathways leading to ala-
nine, succinate, and propionate.
The intensity of ATP generation is stunning (Broda
1975). A 60-kg man consuming daily about 12 MJ
(@700 g) of food in carbohydrates would make and use
no less than 70 kg of ATP (assuming production of 36
molecules of ATP for every digested hexose molecule),
more than his total weight. This rate, roughly 3 g ATP
for each gram of dry body mass, is minuscule compared
to intensities achieved by respiring bacteria. Azotobacter,
breaking down carbohydrates while fixing large amounts
of N 2 , produces 7000 g of ATP for each gram of its dry
mass. While solar luminosity is immense (390 YW), so is
the star's mass (1 : 99 10 33 g). Consequently, the Sun's
power intensity averages about 200 nW/g, but the daily
metabolism of schoolchildren proceeds at a rate of 3
mW/g of body weight, 15,000 times the power intensity
of the Sun, and respiring Azotobacter reaches up to 100
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