Anaerobic Respiration

• The equation for anaerobic respiration is glucose + pyruvate → lactate or ethanol + carbon dioxide
• Anaerobic respiration is the process by which cells produce energy without oxygen.
• Anaerobic respiration occurs when oxygen levels are low, such as during intense exercise.
• Bacteria and archaebacteria are organisms that can survive even in environments that are deprived of oxygen
• Final electron acceptors in Anaerobic Respiration

  sulfate, nitrate, or carbon dioxide
• Archaebacteria like methanogens use CO2 as their final electron acceptor which results in the production of methane as its by-product
• When the cell runs out of NAD+ the process of glycolysis will stop. Therefore, NAD+ must be regenerated from NADH. To regenerate NAD+ in the absence of oxygen, pyruvate molecules must undergo lactic acid or alcoholic fermentation.
• Lactate is produced in lactic acid fermentation while ethanol is produced in alcoholic fermentation.
• Lactic acid fermentation


  This anaerobic pathway is common to obligate anaerobes, which cannot tolerate the presence of oxygen.
• Type of fermentation that happens in our muscle cells during extraneous physical activities

  Lactic acid fermentation
• Type of fermentation where pyruvate is converted to lactate through lactate dehydrogenase

  Lactic acid fermentation
• The hydride group from NADH attaches to one of the carbon molecules of pyruvate. Another hydrogen molecule is also used in the process which attaches to the oxygen in the pyruvate molecule. This process produces lactate and oxidizes NADH to NAD+ which will be reused.
• Varieties of bacteria that undergo lactic acid fermentation

  homofermentative LAB and heterofermentative LAB
• Homofermentative lactic acid bacteria

  organism that utilizes glucose molecules to produce lactic acid alone. These organisms are used as starters of dairy products that help improve the products’ flavor.
• Lactobacillus delbrueckii and Streptococcus salivarius are used in the production of yogurt while Lactobacillus helveticus is used in cheese-making.
• Heterofermentative lactic acid bacteria


  organisms that produce ethanol and carbon dioxide along with lactic acid as their by-products.
• Heterofermentative lactic acid fermentation


  These organisms, however, are rarely used as starters

  of dairy products.
• Heterofermentative LAB includes Lactobacillus brevis, Lactobacillus

  fermentum, Lactobacillus plantarum, and species of Leuconostoc spp. These bacteria differ in shape and in the environmental conditions that promote their optimum growth.
• Alcoholic fermentation


  a process by which organisms ferment glucose and produce ethanol as its by-product.
• Alcoholic fermentation


  This process is used in beer and winemaking with the help of an

  organism called yeast.
• In step 1, the carboxyl group in pyruvate undergoes decarboxylation and is replaced by a hydrogen molecule. This is catalyzed by pyruvate decarboxylase and assisted by thiamine pyrophosphate which results in the formation of acetaldehyde. The carboxyl group that is removed from the pyruvate molecule is released as CO2.
• In step 2, the hydride group of NADH is transferred to the carbon

  molecule of acetaldehyde. Since the carbon molecule has a double bond with oxygen, it has to be weakened to allow the transfer. This process is catalyzed by alcohol dehydrogenase.
• Alcohol dehydrogenase contains a zinc ion that weakens the double bond between carbon and oxygen that promotes the transfer of the hydride group from NADH. Another hydrogen molecule reacts with the oxygen in the acetaldehyde. Consequently, ethanol is produced and NADH is oxidized to NAD+.
• Anaerobic respiration is considered to be less efficient in producing energy compared to aerobic respiration since it does not utilize oxygen as the final electron acceptor and less energy is released in comparison.
• In lactic acid fermentation, the products are lactate and NAD+ as a result of the oxidation of NADH.
• On the other hand, ethanol and NAD+ are the products of alcoholic fermentation.
• Fermentation in bread is described as the conversion of sugar by yeast to produce ethanol and carbon dioxide.
• In cheese-making, several species of Lactococcus and Lactobacillus are used as starters to ferment lactose sugar into lactic acid. This process decreases the pH of cheese which prolongs its shelf-life and enhances its flavor as sharp and fresh.
• In winemaking, yeast is added to grape juice.
• Yeast converts the sugar content of grape juice into ethanol and carbon dioxide.
• In yogurt, lactose has to be converted first into galactose and glucose then these will be fermented by the bacteria present in the sugar into lactic acid.
• The products of anaerobic respiration are 2 ATP, 2 NADH, and 2 pyruvate molecules. When pyruvate molecules undergo fermentation the products are either ethanol or lactate.