Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 545

Reducing power can be used to generate ATP through the electron transport chain. If oxygen
is the final electron acceptor for this reducing power, the process is called aerobic respiration.If
another electron acceptor is used in conjunction with the electron transport chain, then the
process is called anaerobic respiration. Cells that obtain energy without using the electron
transport chain use fermentation. Substrate-level phosphorylation supplies ATP. The end prod-
ucts of fermentative metabolism (e.g. ethanol, acetone-butanol, and lactic acid) are important
commercially and are formed in response to the cell's need to balance consumption and the
production of reducing power.
For most animal cells, glucose and glutamine are the major carbon and energy sources.
Both nutrients are required; glucose provides pentose sugars via the PP pathway, glucos-
amine-6-phosphate and the widely used precursor glyceraldehyde-3-phosphate. While other
sugars such as fructose or galactose can be used in place of glucose, glucose is more rapidly
utilized. Glutamine is required for the synthesis of purines and the formation of guanine
nucleotides. Glutamine is also the primary source of nitrogen in the cell, via transamidation
and transamination reactions. Both carbon sources are required, although asparagine may
replace glutamine in some cells. Glutamine enters the cell and may be deamidated to gluta-
mate in the cytosol or in the mitochondria.
Many normal cells produce glutamine, but cells which grow in culture show high rates of
glutamine consumption. Glutamine metabolism occurs primarily in the TCA cycle, where
glutamine enters as a -ketoglutarate. More than half the CO 2 production by normal diploid
fibroblasts is derived from glutamine.
Autotrophic organisms use CO 2 as their carbon source and rely on the Calvin (or Calvin e
Benson) cycle to incorporate (or fix) carbon from CO 2 into cellular material. Energy is
obtained either through light (photoautotroph) or oxidation of inorganic chemicals (chemoauto-
troph). Figure 10.36 summarizes the major metabolic pathways and their interrelationship for
autotrophs.
Further Reading
Alberts, B., others, 1998. Essential Cell Biology, Garland Publishing, Inc., New York.
Alberts, B., Bray, D., Johnson, A., Lewis, J., Raff, M., Roberts, K., Walter, P., 1998. Essential Cell Biology: An Introduction
to the Molecular Biology of the Cell, Garland Publication, Inc., New York.
Black, I.G., 1996. Microbiology: Principles and Applications, 3rd ed., Prentice Hall, Upper Saddle River, NJ.
Cook, P.R., 1999. The Organization of Replication and Transcription, Science 284, 1790 e 1795.
Crueger, W., Crueger, A., 1990. Biotechnology. A Textbook of Industrial Microbiology (T. D. Brock. ed., English edition),
2nd ed., Sinauer Associates, Inc., Sunderland, MA.
Eigenbrodt, E., Fister, P., Reinacher, M., 1991. In: Wang, D., Ho, C.S. (Eds.), Animal Cell Bioreactors, Butterworth-
Heinemann Press.
Elgard, L., Molinari, M., Helenius, A., 1999. Setting the Standards: Quality Control in the Secretory Pathway, Science
286, 1882 e 1888.
Harder, A. Roels, J.A., 1982. Application of simple structured models in bioengineering, Adv. Biochem. Eng., 21,
55 e 107.
Kapanidis, A.N., Margeat, E., Laurence, T.A., Doose, S., Ho, S.O., Mukhopadhyay, J., Kortkhonjia, E., Mekler, V.,
Ebright, R.H., Weiss, S., 2005. Retention of transcription initiation factor sigma70 in transcription elongation:
single-molecule analysis. Mol. Cell 20 (3) 347 e 56.
Kelly, M.T., Hoover, T.R., 1999. Bacterial Enhancers Function at a Distance, Asm News 65, 484 e 489.
Kolter, R., Losick, R., 1998. One for All and All for One, Science 280, 226 e 227.