Background Information
Anaerobic Pathways
Glycolysis allows organisms to obtain energy from nutrients in the absence of oxygen.However, step 6 (G3P to BPB) of the glycolytic pathway reduces NAD+ to NADH. As entioned earlier, cells possess a limited supply of NAD+. If glycolysis continues without a mechanism to oxidize NADH back into NAD+, step 6 will be blocked and glycolysis will come to a halt. In oxidative respiration, the ETC oxidizes NADH to NAD+, allowing glycolysis to continue. Organisms have evolved several ways of recycling NAD+ and allow glycolysis to continue when oxygen is not available.
One method involves transferring the hydrogen atoms of NADH to certain organic molecules instead of the electron transport chain. This process is called fermentation. Bacteria have evolved dozens of different forms of fermentation, but eukaryotes primarily use two methods: ethanol fermentation and lactate (lactic acid) fermentation.
Ethanol Fermentation
In ethanol fermentation, NADH passes its hydrogen atoms to acetaldehyde, a compound formed when a carbon dioxide molecule is removed from pyruvate by the enzyme pyruvate decarboxylase. This forms ethanol, the alcohol used in alcoholic beverages. This process allows NAD+ to be recycled and glycolysis to continue. The two ATP molecules produced satisfy the organism’s energy needs, and the ethanol and carbon dioxide are released as waste products. Humans have learned ways of making use of these metabolic
wastes. Ethanol fermentation carried out by yeast (a variety of single-celled fungi) is of great historical, economic, and cultural importance. Breads and pastries, wine, beer, liquor, and soy sauce are all products of
fermentation.
Bread is leavened by mixing live yeast cells with starches (in flour) and water. The yeast cells ferment the glucose from the starch and release carbon dioxide and ethanol. Small bubbles of carbon dioxide gas cause the bread to rise (or leaven) and the ethanol evaporates away when the bread is baked. In beer making and winemaking, yeast cells ferment the sugars found in carbohydrate-rich fruit juices, such as grape juice. The mixture bubbles as the yeast cells release carbon dioxide gas and ethanol during fermentation. In winemaking, fermentation ends when the concentration of ethanol reaches approximately 12%. At this point, the yeast cells die as a result of ethanol accumulation and the product is ready to be consumed as a beverage. Flooded plants undergo ethanol fermentation in the roots and may die if oxygen is not returned to the roots. This is why it is important not to overwater houseplants.
Lactate (Lactic Acid) Fermentation
Under normal conditions, animals such as humans catabolize glucose by aerobic respiration. However, during strenuous exercise, muscle cells respire glucose faster than oxygen can be supplied. Under such conditions, oxidative respiration slows down and lactate fermentation begins. In lactate fermentation, NADH produced in glycolysis transfers its hydrogen atoms to pyruvate in the cytoplasm of the cell, regenerating NAD+ and allowing glycolysis to continue. This results in a change of pyruvate into lactate. The accumulation of lactate molecules in muscle tissue causes stiffness, soreness, and fatigue. Lactate is transported through the bloodstream from the muscles to the liver. When vigorous exercise ceases, lactate is oxidized back to pyruvate, which then goes through the Krebs cycle and oxidative phosphorylation.
The extra oxygen required to catabolize lactate to CO2 and H2O (through the aerobic pathway) is referred to as oxygen debt. Panting after bouts of strenuous exercise is the body’s way of “paying” the oxygen debt.
Glycolysis allows organisms to obtain energy from nutrients in the absence of oxygen.However, step 6 (G3P to BPB) of the glycolytic pathway reduces NAD+ to NADH. As entioned earlier, cells possess a limited supply of NAD+. If glycolysis continues without a mechanism to oxidize NADH back into NAD+, step 6 will be blocked and glycolysis will come to a halt. In oxidative respiration, the ETC oxidizes NADH to NAD+, allowing glycolysis to continue. Organisms have evolved several ways of recycling NAD+ and allow glycolysis to continue when oxygen is not available.
One method involves transferring the hydrogen atoms of NADH to certain organic molecules instead of the electron transport chain. This process is called fermentation. Bacteria have evolved dozens of different forms of fermentation, but eukaryotes primarily use two methods: ethanol fermentation and lactate (lactic acid) fermentation.
Ethanol Fermentation
In ethanol fermentation, NADH passes its hydrogen atoms to acetaldehyde, a compound formed when a carbon dioxide molecule is removed from pyruvate by the enzyme pyruvate decarboxylase. This forms ethanol, the alcohol used in alcoholic beverages. This process allows NAD+ to be recycled and glycolysis to continue. The two ATP molecules produced satisfy the organism’s energy needs, and the ethanol and carbon dioxide are released as waste products. Humans have learned ways of making use of these metabolic
wastes. Ethanol fermentation carried out by yeast (a variety of single-celled fungi) is of great historical, economic, and cultural importance. Breads and pastries, wine, beer, liquor, and soy sauce are all products of
fermentation.
Bread is leavened by mixing live yeast cells with starches (in flour) and water. The yeast cells ferment the glucose from the starch and release carbon dioxide and ethanol. Small bubbles of carbon dioxide gas cause the bread to rise (or leaven) and the ethanol evaporates away when the bread is baked. In beer making and winemaking, yeast cells ferment the sugars found in carbohydrate-rich fruit juices, such as grape juice. The mixture bubbles as the yeast cells release carbon dioxide gas and ethanol during fermentation. In winemaking, fermentation ends when the concentration of ethanol reaches approximately 12%. At this point, the yeast cells die as a result of ethanol accumulation and the product is ready to be consumed as a beverage. Flooded plants undergo ethanol fermentation in the roots and may die if oxygen is not returned to the roots. This is why it is important not to overwater houseplants.
Lactate (Lactic Acid) Fermentation
Under normal conditions, animals such as humans catabolize glucose by aerobic respiration. However, during strenuous exercise, muscle cells respire glucose faster than oxygen can be supplied. Under such conditions, oxidative respiration slows down and lactate fermentation begins. In lactate fermentation, NADH produced in glycolysis transfers its hydrogen atoms to pyruvate in the cytoplasm of the cell, regenerating NAD+ and allowing glycolysis to continue. This results in a change of pyruvate into lactate. The accumulation of lactate molecules in muscle tissue causes stiffness, soreness, and fatigue. Lactate is transported through the bloodstream from the muscles to the liver. When vigorous exercise ceases, lactate is oxidized back to pyruvate, which then goes through the Krebs cycle and oxidative phosphorylation.
The extra oxygen required to catabolize lactate to CO2 and H2O (through the aerobic pathway) is referred to as oxygen debt. Panting after bouts of strenuous exercise is the body’s way of “paying” the oxygen debt.