The study of bioenergetics leads us into a world of novelty and greater significance and has found new encouragement in industry. The biogas generation by anaerobic fermentation has also led to new interest in research in the light of bioenergetics.

The study of energy relations for each chemical step in the living system may be an item of bioenergetics. The energy change can be calculated in terms of calories or joules per mole. This is applicable for catabolic processes, for example, the anaerobic or glycolytic paths or oxidative phosphorylation. The anabolic paths are equally fitting, e. g., the carbon fixation or the photosynthesis and nitrogen fixation by the symbiotic organisms [1].

The accounting and balancing of free energy change of certain reac­tions may lead to some fruitful conclusions. When glucose is oxidized in a bomb calorimeter (an almost one-step reaction),

C6 H12O6 + 6O2 ^ 6CO2 + 6H2O — 686,000 cal (pH 7.0)

but when equivalent CO2 is produced in a biological system (through a multistep reaction),

C6H12O6 + 6O2 + 38ADP + 38H3PO4 ^ 6CO2 + 38ATP + 44H2O

-382,000 cal (pH 7.0)

A noteworthy departure is the conservation of -304,000 cal/mol of glu­cose and gain of 38 moles of ATP, energy-rich (bond) compounds, i. e., 800 cal/mol of ATP. It also means 50,666 cal of energy are wasted if, on aver­age, 1 mole of carbon dioxide produced chemically is wasted in the form of heat, an inferior quality of energy.

A simple calculation will reveal that each nutrient has some specified energy or calorific values. This can be compared to the different energy

TABLE 1.2 Comparison of Some Common Fuels Fuel Kcal Thermal efficiency (%) Effective heat (kcal) Gobar gas, m3 4713 60 2828 Kerosene, L 9122 50 4561 Firewood, kg 4708 17 800 Dry cow dung, kg 2092 11 230 Charcoal, kg 6930 28 1940 Soft coke, kg 6292 28 1762 LPG (butane), kg 10,882 60 6529 Coal gas, m3 4004 60 2402 Electricity, kWh (hot plate) 860 70 602 Source: Permission from KVTC, Mumbai.

values of different fuels, i. e., coal, kerosene, firewood, and so forth (see Table 1.2). Taking glucose as a model carbohydrate,

C6H12O6 + 6O2 ^ 6CO2 + 6H2O — 686,000 cal
(molecular weight, MW = 180 g).

686,000 cal
180g

3800 cal/g

and taking palmitic acid as model fatty acid,

C16H32O2 + 23O2 ^ 16CO2 + 16H2O — 2,338,000 cal (MW = 256 g)

2,338,000 cal
256 g

Similarly, in amino acids, peptides show roughly the same value as that of carbohydrates. In biological systems (measurement through metabolic cage), it has been found that the biological energy values are slightly higher than those shown theoretically. This is more so by “specific dynamic action.” When mixed foods particularly protein are taken, the total calorific value is enhanced. The exact reasons are not yet clear. Let us concentrate on a few examples in the following:

In ethanol fermentation (pH 7.0),

C6H12O6 ^ 2[C2H5OH + CO2] — 56,000 cal In lactic fermentation,

C6H12O6 ^ 2[CH3CHOHCOOH] — 47,000 cal

But in lactic fermentation from polysaccharide,

(Glucosyl)n ^ 2[CH3CHOHCOOH] + (Glucosyl)n—1 — 52,000 cal
CH3CHOHCOOH + 3O2 ^ 3CO2 + 3H2O — 319,500 cal

If glucose is the starting point (as is the case of ethanol fermentation), then 2 moles of ATP are invested and finally 2 X 2 moles of ATP are regenerated and the net gain of ATP remains 2 (see Fig. 1.1). But if glyco­gen is the starting point, then only 1 mole is invested in the formation of fructose 1,6-diphosphate.

Hence, net gain in ATP is 4 — 1 = 3. Twice a mole of reduced Co I is produced by the conversion of 3 phosphoglyceraldehyde to 1,3 diphos- phoglycerate.

ATP + H2O ^ ADP + H3PO4 — 8000 cal

But AF of formation of ATP = +12,000 cal.

The energy conservation or efficiency factor can be calculated in two different ways:

1. How much potential energy-rich chemical compounds are now gained?

a. Ethanol fermentation: —16,000/—56,000, about 29%

b. Lactic fermentation: —24, 000/—52,000, about 46%

2. How much energy of reaction has been utilized as heat of formation of the energy-rich compounds?

a. Ethanol fermentation: 24,000/—56,000, about 43%

b.

Lactic fermentation: 36,000/—52,000, about 69%

1, 3-diphosphoglyceric acid ◄— 3-PhosPhoglyceraldehydes and

Dihydroxy acetone p——

2 (p) Glyceric acid—- ► ( p) Enolpyruvic acid ^ 2ATP ^ Pyruvic acid

Figure 1.1 Anaerobic part of biological oxidation.

The percentage efficiency figures raise doubt about the interpreta­tion. Such efficiency is never achieved by a man-made machine but biological systems can. If we accept the lower figures with a margin, we are conserving no less than 25% of our expenditure in the form of provident fund energy, even under sudden stress, i. e., anaerobic conditions.

Let us look at the situation when a reduced coenzyme is regenerated or oxidized (brief and simplified):

-ATP — ATP — ATP + і O2

NADH (H+)——- > FAD——- > Cytochrome——- ► Cytochrome—— ► H2O

Stoichiometrically,

CoIH(H+) + 1 O2 + 3ADP + 3H2PO4 ^ CoI++ 3ATP + 4H2O

Similarly in the oxidative part, through the tricarboxylic acid cycle, the major aspects may be represented as in Fig. 1.2.

From alpha ketogluterate to succinate, 1 mole of energy-rich phos­phate in the form of guanosine triphosphate (GTP) is gained. Succinate to fumarate mediated by FAD coenzymes generates two equivalents of ATP. In the rest of the events, 4 sets of reduced Co I, when regenerated, give rise to 4 X 3 = 12 equivalents of ATP. In the entire sequence of events, from pyruvate plus oxaloacetate into citrate/isocitrate and finally back to oxaloacetate, a total of 15 equivalents of energy-rich phosphate bonds (ATP) are gained.

In combining the anaerobic part, 2 additional moles of reduced Co I will be reoxidized and 6 ATP equivalents will be regenerated. Starting from glucose-6-P all the way to CO2 and H2O, we see that 2 + 6 + (2 X 15) = 38 equivalents of ATP are gained. The balance of the equation has been

Oxaloactate————— ► Citrate/Isocitrate

Co I

‘ —CO2, Co I

Fumarate -4————————— Succinate GTP

FAD (=2ATP)

Figure 1.2 Tricarboxylic acid cycle (oxidative pathway).

cited earlier. An oxidative pathway is considered to be more effective from a biochemical energetic viewpoint.

One anabolic example of photosynthesis is briefly discussed. Theoretically, reversal of this known reaction should fit well for photo­synthesis:

C6H12O6 + 6O2 > 6(CO2 + H2O) — 686,000 cal

But in fact, we find a slightly different figure. The entire reaction may be symbolically represented as

2H2O + 2NADP+ -—-> 2NADPH (H+) + O2

2 Chloroplast 4 ‘ 2

3CO2 + 9ATP + 5H2O Triosephosphate + 9ADP

+ 6NADPH (H+) + 8H3PO4 + 6NADP+

But the actual stoichiometric presentation shows

n(CO2 + H2O) > ( CH2O)n + nO2 + n(113,000 cal)

almost 22,000 cal higher than expected; fortunately, however, the ender- gonic reaction derives its energy from light energy. These figures are jus­tified because the part of the reaction occurring in the absence of light needs a large excess of energy-rich compounds (ATP). The deficiency of ATP is, however, taken care of by two linked reactions:

Cyclic photophosphorylation:

nADP + nH3PO4 hv > nATP + nH2O

Noncyclic photophosphorylation:

4Feox + 2ADP + 2H3PO4 + 4H2O — h> 4Fered + 2ATP + O2 + 2H2O + 4H+

or 2Co IIred + 2ATP + O2 + 2H2O + 2H+

The deficiency of 1 mole of ATP per mole of CO2 fixed is provided by cyclic photophosphorylation. The other anabolic process is the nitrogen fixa­tion, which is also highly energy consuming.

The heat of formation of NH3 by a chemical pathway can only be determined indirectly. By the Haber process, high pressure and tem­perature is needed and the yield remains very low. So the input in energy in the technological process remains in large excess than the the­oretical heat of formation of NH3.

Nitrogen fixation can take place in nature in two major ways. Molecular nitrogen is converted to oxides of nitrogen in the atmosphere

by electrical discharge and gets into soil by rainwater in the form of nitrites and nitrates. These are reduced to ammonia by the biological nitrogen fixation of symbiotic organisms or by blue-green algae.

In Escherichia coli and Bacillus subtilis, NO 3 is reduced to NH3

[NO—3 ^ NO2—1 ^ N2O2—2 ^ NH2OH ^ NH3]

and an oxidation reduction potential of 0.96 V (pH 7.0) is utilized by these systems to convert other materials to a more oxidized state.

3

NH3 + 2"O2 ^ NO2 + H2O + H+ — 36,500 cal
NO— + 2-O2 ^ NO— — 17,500 cal

2e— 2e— 2e—

N = N———- » HN = NH———- » H2NNH2———- » 2NH3

Via Mo-protein complex

Hydrogen is made available from reduced coenzymes, and the energy
is available from ATP produced by the oxidation of general metabolites.

In some systems, H2 becomes the by-product, and this could be an ideal fuel or it can be used in a suitable chemical cell for the production of energy.