Steps in aerobic respiration and fermentation
Aerobic respiration is the metabolism of glucose or fatty acids that are completely oxidized by organisms with oxygen to carbon dioxide (CO₂) and water (H₂O). Aerobic respiration occurs in bacteria cytoplasm. Mitochondria are the main site of eukaryotes. Aerobic respiration is divided into three phases. The first stage is glycolysis which occurs in cytoplasm; the second stage is citric acid cycle, also known as the tricarboxylic acid cycle, which reacts at mitochondrial matrix; and the third stage is oxidative phosphorylation in the inner mitochondrial membrane (electron transport chain and ATPase).
Fermentation is the incomplete oxidation and decomposition of organic substances such as sugars (plants, animals, fungi and some bacteria) in an anaerobic environment with the help of enzyme catalysis. Glycolysis is still the first stage. The second stage also occurs in cytoplasm, and pyruvate is broken down into alcohol, carbon dioxide, lactic acid. Bubbles are often seen in glass jars for home brewing, and jar walls are slightly heated.
Energy and Electron Acceptors
The biggest difference between them is the way and efficiency of energy production. Aerobic respiration is the primary way eukaryotes and aerobic bacteria obtain energy. Most of energy is from oxidative phosphorylation in mitochondrial inner membrane. Electrons fall from high potential energy state (NADH and FADH₂) to low potential energy state (oxygen atoms) along the respiratory chain or electron transport chain. Decreased potential energy is transformed into a transmembrane proton gradient that couples energy in organic matter with ATP production. The following process is like a generator in a dam: protons flow through ATPase to synthesize ATP. About 40% of energy stored in glucose is converted to ATP, and the remaining is dissipated as heat.
40% may not seem efficient enough, but it's actually about 15 times more than fermentation. In fermentation, glycolysis is the only step for energy generation. There is no electron transport chain and ATPase. 2 ATP molecules is made by the substrate level phosphorylation. The other steps do not make ATP, and there is some heat dissipation. Their significance is to consume the high-energy compound NADH produced by glycolysis and to re-produce NAD⁺. Otherwise, NAD⁺ would be depleted by glycolysis to inhibit fermentation. For most eukaryotes, almost all the energy for life activities is provided by aerobic respiration. Thus, fermentation only helps them survive temporarily, and is not a strategy in the long term.
Glucose + 2NAD⁺ + 2ADP + 2Pi → 2 Pyruvate + 2ATP + 2NADH + 2H₂O + 2H⁺
Pyruvate + NADH + H⁺ → Lactic Acid +NAD⁺
Products of aerobic respiration and fermentation
Harmless CO₂ and H₂O are the end products of aerobic respiration. However lactic acid or alcohol from fermentation is toxic. After strenuous exercise, lactic acid accumulates in animal muscles to causes soreness. Some bacteria use lactic acid to lower pH in their surroundings to inhibit other bacteria. Alcohol denatures proteins, so if the lack of oxygen lasts too long, both plants and fungi will die.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 32ATP
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP
C₆H₁₂O₆ → 2C₃H₆O₃(Lactic acid) + 2ATP