MODERN BOTANY - P. RAVEN - 1990

SECTION II. ENERGY AND LIVING CELLS

CHAPTER 6. RESPIRATION

Anaerobic Pathways

In Eukaryotic Cells (as in most Bacteria), Pyruvate is produced under aerobic conditions and completely oxidized to carbon dioxide and Water. However, in the absence of oxygen, pyruvate is not the end product of Glycolysis because the NADH₂ generated during The oxidation of glyceraldehyde-3-phosphate must be reoxidized to NAD. Without this step, glycolysis would halt, as The Cell would be depleted of NAD to serve as an electron acceptor. During this oxygen-free, or anaerobic, process in many bacteria, Fungi, Protozoa, and animal cells, lactic acid is produced; hence, this pathway is known as Lactic acid Fermentation. For instance, lactic acid accumulates in Muscle cells during strenuous activity, such as when a sprinter runs short distances. In Muscles, The conversion of glucose to lactate (lactic acid) induces an "oxygen debt." Lactate lowers the pH within the muscles, thereby diminishing the contractile capacity of the muscle fibrils and triggering the sensation of muscle fatigue. Lactate is transported via the Blood to the Liver, where it is converted back into pyruvate and subsequently into glucose and Glycogen.

In Yeast and many plant cells under anaerobic conditions, pyruvate is broken down into ethanol and CO₂. Under these circumstances, NADH₂ is reoxidized by transferring its electrons (and protons) to pyruvate. This process is preceded by the removal of CO₂ (decarboxylation), yielding ethanol and carbon dioxide instead of lactate (Fig. 6-17). Because the primary product of glycolysis under these conditions is alcohol, the process is termed Alcoholic Fermentation.

Yeasts are present as a whitish "bloom" on the Skin of grapes. When glucose-rich juice from grapes or other fruits is collected and stored in hermetically sealed casks, these yeast cells transform the fruit juice into wine by fermenting the glucose into ethanol. Nevertheless, yeasts, like All living organisms, have a definite threshold of alcohol tolerance; once a critical concentration is reached (approximately 12%), the yeasts cease to function.

Thermodynamically, lactic acid and alcoholic fermentations are comparable. In both cases, NADH is reoxidized, and the net energy yield is limited to only two ATP molecules. The overall equation for the glucose fermentation process is as follows:

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Fig. 6-17. A. The reaction sequence by which pyruvate is converted to ethanol under anaerobic conditions. In the first step, CO₂ is released. In the second step, NADH₂ is oxidized while acetaldehyde is reduced. Most of the energy from the glucose remains trapped in the alcohol, which serves as the primary end product of this reaction chain. Nevertheless, the regeneration of NAD allows glycolysis to continue with a modest, yet occasionally vital, yield of ATP. B. The Significance of anaerobic glycolysis. Yeast cells, which form a powdery bloom on grapes, mix with the juice extracted during pressing. Storing this mixture under anaerobic conditions causes the yeasts to ferment the glucose in the grape juice into alcohol. In modern winemaking, pure yeast cultures are added to relatively sterile grape juice for fermentation rather than relying on wild yeasts naturally inhabiting the grapes.

Alcoholic fermentation releases approximately 7% of the total energy stored in a glucose molecule, amounting to 52 kcal; roughly 93% remains locked within the two alcohol molecules. Of the 52 kcal, only 14.6 are conserved in the form of two ATP molecules. Consequently, in terms of energy yield, anaerobic fermentation is relatively inefficient.