The Carbon Source

Biomass is typically 50% carbon on a dry weight basis, an indication of how important it is. Since organic substances are at the same general oxidation level as organic cell constituents, they do not have to undergo a primary reduction to serve as sources of cell carbon. They also serve as an energy source. Consequently, much of this carbon enters the pathways of energy-yielding metabolism and is eventually secreted from the cell as C02 (the major product of energy-yielding respiratory metabolism or as a mixture of C02 and organic compounds, the typical end-products of fermentation metabolism). Many microorganisms can use a single organic compound to supply both carbon and energy needs. Others need a variable number of additional organic compounds as nutrients. These additional organic nutrients are called growth factors and have a purely biosynthetic function, being required as precursors of certain organic cell constituents that the organism is unable to synthesize. Most microorganisms that depend on organic carbon sources also require C02 as a nutrient in very small amounts.111 In the fermentation of beet molasses to ethanol and glycerol, it was found that by manipulating several fermentation parameters, the ethanol yield (90.6%) and concentration (8.5% v/v) remained essentially the same, while the glycerol concentration went from 8.3g/lto 11.9 g/1. The C02 formation, however, was reduced! With glycerol levels over 12 g/1, the ethanol yield and concentration reduced with the C02-formation near normal again.'51 In fermentations, the carbon source on a unit of weight basis may be the least expensive raw material, however, quite often represents the largest single cost for raw material due to the levels required. Facultative organisms incorporate roughly 10% of substrate carbon in cell material, when metabolizing anaerobically, but 50-55 % of substrate carbon is converted to cells with fully aerobic metabolism. Hence, if 80 grams per liter of dry weight of cells are required in an aerobic fermentation, then the carbon required in that fermentation equals (80/2) (100/50) = 80 grams of carbon. If this is supplied as the hexose glucose, with molecular weight 180 and carbon weight 72, then (80) (180)/72 = 200 gram per liter of glucose are required.

Carbohydrates are excellent sources of carbon, oxygen, hydrogen, and metabolic energy. They are frequently present in the media in concentrations higher than other nutrients and are generally used in the range of 0.2-25%. The availability of the carbohydrate to the microorganism normally depends upon the complexity of the molecule. It generally may be ranked as:

hexose > disaccharides > pentoses > polysaccharides

Carbohydrates have the chemical structure of either polyhydroxyaldehydes or polyhydroxyketones. In general, they can be divided into three broad classes: monosaccharides, disaccharides and polysaccharides. Carbohydrates have a central role in biological energetics, the production of ATP. The progressive breakdown of polysaccharides and disaccharides to simpler sugars is a major source of energy-rich compounds.111 During catabolism, glucose, as an example, is converted to carbon dioxide, water and energy. Enzymes catalyze the conversion from complex to simpler sugars. Three major interrelated pathways control carbohydrate metabolism:

- The Embden-Meyerhof pathway (EMP)

- The Krebs or tricarboxylic acid cycle (TCA)

- The pentose-phosphate pathway (PPP)

In the EMP, glucose is anaerobically converted to pyruvic acid and on to either ethanol or lactic acid. From pyruvic acid it may also enter the oxidative TCA pathway. Per mole of glucose broken down, a net gain of 2 moles of ATP is being obtained in the EMP. The EMP is also the entrance for glucose, fructose, and galactose into the aerobic metabolic pathways, such as the TCA-cycle, In cells containing the additional aerobic pathways, the NADH2 that forms in the EMP where glyceraldehyde-3-phosphate is converted into 3-phosphoglyceric acid, enters the oxidative phosphorylation scheme and results in ATP generation.[3] In fermentative organisms the pyruvic acid formed in the EMP pathway may be the precursor to many products, such as ethanol, lactic acid, butyric acid (butanol), acetone and isopropanol.[1]

The TCA-cycle functions to convert pyruvic and lactic acids, the end products of anaerobic glycolysis (EMP), to C02 and H20. It also is a common channel for the ultimate oxidation of fatty acids and the carbon skeletons of many amino acids. The overall reaction is:

2C3H4O3 + 502 + 30 ADP + 30 P; 6C02 + 4H20 + 30 ATP

for pyruvic acid as the starting material.[3] Obviously, the EMP-pathway and TCA-cycle are the major sources of ATP energy, while they also provide intermediates for lipid and amino acid synthesis.

The PPP handles pentoses and is important for nucleotide (ribose-5-phosphate) and fatty acid biosynthesis (NADPH2). The Entner-Doudoroff pathway catabolizes glucose into pyruvate and glyceraldehyde-3-phosphate. It is important primarily in Gram negative prokaryotes.'61

The yeast Saccharomyces cerevisiae will ferment glucose, fructose and sucrose without any difficulties, as long as the minimal nutritional requirements of niacin (for NAD), inorganic phosphorus (for phosphate groups in 1, 3-diphosphoglyceric acid and ATP) and magnesium (catalyzes, with hexokinase and phosphofructokinase, the conversion of glucose to glucose-6-phosphate and fructose-6-phosphate to fructose-1,6-diphosphate) are met. Table 2 lists some of the important biological molecules involved in catabolism and anabolism.tll[3J S. cerevisiae ferments galactose and maltose occasionally, but slowly; inulin very poorly; raffinose only to the extent of one third and melibiose and lactose it will not ferment. S. cerevisiae follows the Embden-Meyerhof pathway and produces beside ethanol, 2 moles of ATP per mole of glucose.

Table 2. Fundamental Biological Molecules[1][3]

Simple molecule

Constituent atoms

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