Slide 1 If oxygen is not present - pyruvate is reduced to lactic acid or ethanol. This process is known as fermentation and it is an anaerobic process. During fermentation, NAD+ is regenerated. That is the goal of the fermentation pathways - to regenerate the cell's supply of NAD+ so glycolysis can continue to run. The ethanol and lactic acid that are produced are waste products to the cell. This is very important to understand - the goal of fermentation is to regenerate the cell's supply of NAD+ - not to make ethanol or lactic acid. In fact, these waste products are toxic to the cells that make them.
Slide 2 This image from the tutorial shows the basic lactic acid and ethanol fermentation pathways. We will look at lactic acid fermentation first. Glycolysis is located on the right and we see NAD+ being consumed in glycolysis to produce NADH. During lactic acid fermentation, pyruvate is reduced to lactic acid by taking electrons from NADH. Remember, oxiation reducation reactions are always paired - as pyruvate gains electrons - NADH loses electrons and is regenerated to NAD+. The lactate that is produced is a waste product to the cell but the NAD+ that is regenerated can go back to glycolysis so glycolysis can continue to run.
Slide 3 Alchohol fermentation is similar except the production of ethanol occurs in two steps. First, pyruvate loses a carbon molecule in the form of CO2 and is converted to acetaldehyde. Then the acetylaldehyde is reduced to ethanol by accepting electrons from NADH. Just as in lactic acid fermentation, NAD+ has been regenerated and can go back to glycolysis and ethanol is a waste product to the cell. Notice - in neither of these pathways is there any ATP being produced. The only ATP produced by a fermenting organism is the net gain of 2 ATP molecules produced during glycolysis.
Slide 4 Lactic acid fermentation is done by some fungi, some bacteria like the Lactobacillus acidophilus in yogurt, and sometimes by our muscles. Normally our muscles use cellular respiration like the rest of our bodies, using O2 supplied via our lungs and blood. However, under greater exertion when the oxygen supplied by the lungs and blood system can't get there fast enough to keep up with the musclesneeds, our muscles can switch over and do lactic acid fermentation. Not for long however - keep running with insufficient oxygen and you will collapse! In the process of lactic acid fermentation, the 3-carbon pyruvic acid molecules are reduced to lactic acid. It is the presence of lactic acid in yogurt that gives it its sour taste, lactic acid produced by the bacteria and/or fungi in many cheeses that gives those cheeses their characteristic flavors, and it is the presence of lactic acid in our muscles that makes them burn when we work out vigorously.
Slide 5 Alcohol fermentation is done by yeast and some kinds of bacteria. The waste products of this process are ethanol and carbon dioxide (CO2). Humans have long taken advantage of this process in making bread, beer, and wine. In bread making, it is the CO2 which causes the bread to rise, and the ethanol evaporating that gives bread its wonderful smell while baking. It is the CO2 produced by the process of fermentation that makes beer bubbly. Remember - ethanol is a waste product to the yeast that produce it. But it is a waste product with significant free energy. When we consume ethanol - we are consuming a high energy molecule and because we are efficient at conserving excess calories in the form of fat - it is quite easy to get a beer belly from this waste product.
Slide 6 To summarize what we know about fermentation first, it is an anaerobic process - no oxygen is used Second, the purpose of fermentation is to regenerate the cell's supply of NAD+ Third, lactate and ethanol are waste products! And finally, there is no energy gain for the cell during fermentation - a fermenting organism has a net gain of 2 ATP/glucose from glycolysis.