Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 541

beyond 5 mmol/l over 90% of the glucose is converted to lactate. The metabolism of gluta-
mine is altered in tumor cells and proliferating normal cells.
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.

10. 11. OVERVIEW OF AUTOTROPHIC METABOLI SM

Cell growth requires two basic atoms in large quantity: carbon and hydrogen. The function
or transfer of hydrogen/proton in cells also leads us to classify hydrogen as electron donor.
While cell can obtain hydrogen from organic (organotroph) or inorganic (lithotroph)
compounds, there are two sources for carbon as well: CO 2 or organic compounds. While
the source of hydrogen may not be as critical, the source of carbon can make a significant
difference in cell metabolism. So far we have been concerned primarily with heterotrophic
growth (e.g. organic molecules serve as carbon-energy sources). However, autotrophs obtain
nearly all their carbon from CO 2 . Table 10.7 shows a summary of the major characteristics
of heterotrophs and autotrophs. Most autotrophs (either photoautotrophs or chemoauto-
trophs) fix or capture CO 2 by a reaction catalyzed by the enzyme ribulose bisphosphate
carboxylase, which converts ribulose-l,5-diphosphate plus CO 2 and H 2 O into two molecules
of glyceric acid-3-phosphate. This is the key step in the Calvin cycle (or Calvin e Benson cycle).
This cycle is summarized in Fig. 10.36 and provides the building blocks for autotrophic
growth.
Energy for autotrophic growth can be supplied by light (photoautotroph) or chemicals
(chemoautotroph). Here, we consider the special case of photoautotrophic growth.
Photosynthesis takes place in two phases. The overall reaction is

light

2 (10.54)
The first phase of photosynthesis is known as the light phase. Light energy is captured and
converted into biochemical energy in the form of ATP and reducing agents, such as NADPH.
In this process, hydrogen atoms are removed from water molecules and are used to reduce

6 CO

2 þ 6

H O ! C H O

6 þ

6O

2 6 12

TABLE 10.7 Summary of Heterotrophic and Autotrophic Metabolisms

Classification Carbon source Energy source Examples

Photoautotrophs CO 2 Light Green plant, algae, cyanobacteria,

photosynthetic bacteria

Photoheterotrophs Organic compounds Light Nonsulfur purple bacteria

Chemoautotrophs CO 2 Oxidation-reduction reaction Nitrifying bacteria; hydrogen,

sulfur, and iron bacteria

Chemoheterotrophs Organic compounds Oxidation-reduction reaction All animals, most microorganisms