Welcome to My AP Biology Thoughts podcast, my name is Adrienne Li and I am your host for episode #76 called Unit 3 Cellular Energetics: Anaerobic Respiration. Today we will be discussing what anaerobic respiration is, the two types, examples, and its connection to the rest of unit 3. 

To start off, let me define what anaerobic respiration is. It is the process that regenerates NAD+ for glycolysis so it can produce ATP and its key characteristic is that it occurs when oxygen is not available. This distinguishes anaerobic from aerobic respiration, which occurs when oxygen is available. Also, it’s important to stress that its main purpose is to regenerate NAD+ which is a big misconception that many people have. 

Diving deeper into anaerobic respiration, there are two types. One is alcohol fermentation which occurs in plants, fungi, and bacteria. It starts out with glycolysis where glucose is converted into pyruvate, but since oxygen is not present, it converts into CO2 and acetaldehyde. Then, energy from NADH is applied to acetaldehyde and converts into ethanol and NAD+, thus regenerating the NAD+ that is necessary for glycolysis to occur and produce 2 ATP. A real life example is seen in yeast, where they use alcoholic fermentation which allows bread dough to rise. This is because CO2 is a waste product of alcoholic fermentation which causes gas bubbles to form. The other type of anaerobic respiration is lactic acid fermentation which occurs in animals. Similar to alcohol fermentation, it uses glucose to create pyruvate but then it is converted to lactate instead of acetaldehyde and lactic acid is produced instead of CO2. During this process, NADH is oxidized into NAD+ which is used in glycolysis to produce 2 ATP. This occurs when we exercise which we feel through the burning sensation during a tough workout. The burning sensation is lactic acid build up, which occurs when our respiratory and cardiovascular systems cannot transport oxygen to our muscles fast enough. Therefore, they resort to lactic acid fermentation to immediately produce ATP.

To connect anaerobic respiration to the rest of the unit, let’s zoom out a bit to the bigger picture of cellular energetics. Keep in mind, anaerobic respiration does produce energy, that’s just not its main purpose. With the regeneration of NAD+, it is used in glycolysis to produce 2 ATP. Other cellular pathways that exist include the krebs cycle and chemiosmosis, however they don’t occur in anaerobic respiration because oxygen isn’t present. They do however occur in aerobic respiration because oxygen is present and acts as the final electron acceptor. This aids in the formation of a proton gradient along with the energy of the electrons from NADH and FADH2 which actively transports H+ into the intermembrane space. From there, chemiosmosis occurs where H+ flows from high to low concentration and back into the matrix through ATP synthase, which then creates ATP. This is a brief explanation of aerobic respiration but it shows the different processes that occur in both due to the presence or absence of the final electron acceptor oxygen. So that about sums up anaerobic respiration and its connection to other cellular energetic...