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6.2 Anaerobic Cellular Respiration
Unit C
Module 5 ~ Lesson 6
Anaerobic Respiration
Anaerobic respiration does not completely break down glucose and yields only two ATP molecules for every glucose molecule. Multicellular organisms, including humans, can only operate in oxygen-free conditions for a limited amount of time. Not enough energy is created in anaerobic respiration for multicellular organisms to carry out metabolic processes. Organisms that only rely on anaerobic respiration for an energy source are typically unicellular (bacteria and yeasts). Glycolysis proceeds even in the absence of oxygen. If no oxygen is present at the end of glycolysis, alternalte pathways are available to some organisms. Two such pathways are lactate fermentation and ethanol fermentation.
Lactate fermentation
Some single-celled organisms, such as yeast, and some animals cells, such as muscle cells carry out lactate fermentation in the absence of oxygen. When the products of glycolysis have exceeded the oxygen supply, pyruvate molecules build up faster than they can be used. For example, during a strenuous workout, your muscle cells will convert pyruvate to lactic acid (lactate) by oxidizing NADH. NAD+ returns to be used again in glycolysis. This buildup of lactate can cause muscle cramps and fatigue. When oxygen is available again, the lactate is converted into pyruvate, which moves to the mitochondria to take part in aerobic respiration once again. 
Ethanol fermentation
Some organisms function anaerobically and aerobically. These organisms, like yeast and bacteria, convert pyruvate to ethanol and CO2(g) by oxidizing NADH. NAD+ returns to be used again in glycolysis. 
Read
Read "Anareobic Cellular Respiration Uses a Different Final Electron-Acceptor" and "Fermentation" on pages 189 to 193 of your textbook.Self-Check
Fermentation occurs in the absence of oxygen.
The products of fermentation are used to produce beer, vinegar, buttermilk, yogurt, cheese, and more. Two particularly interesting products, because of their potential to replace fossil fuels, are biogas and biofuels.
The NAD+ is required for the process of glycolysis to continue to occur in the cells.
