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Cellular respiration is the process by which cells extract energy stored in food and transfer that energy to molecules of ATP. The process is highly exergonic (releases a lot of energy) and occurs in the mitochondria of cells. 

There are two types of cellular respiration: 

  • ❌ Anaerobic (when oxygen is NOT present) 
  • ✅ Aerobic (when oxygen is present) 

In this article, we will only discuss anaerobic respiration. Check out this article to learn about aerobic respiration. 

 Anaerobic respiration originated millions of years ago when there was no free oxygen in the earth's atmosphere. Even today it is the sole means by which anaerobic bacteria release energy from food. The two steps of anaerobic respiration are as follows:

  • Glycolysis 
  • Alcohol or Lactic Acid Fermentation

1️⃣ Glycolysis 

Glycolysis is a ten-step process 😱 that breaks down 1 molecule of glucose (a six-carbon molecule) into 2 three-carbon molecules called pyruvates and releases 4 molecules of ATP. For the AP Biology exam, you do not need to memorize the steps of glycolysis. Here are some important things to remember for the process of glycolysis:

  • The simplified equation is: 2 ATP + 1 Glucose ➡ 2 Pyruvate + 4 ATP
  • Occurs in the cytoplasm
  • Releases ATP without using oxygen
  • ATP is produced by substrate level phosphorylation
  • The enzyme that catalyzes the third step is an allosteric enzyme. This means that glycolysis can be stopped if the cell doesn't need any more ATP. 

2️⃣ Alcohol Fermentation
Alcohol fermentation or simply fermentation is the process by which certain cells convert pyruvate from glycolysis into ethyl alcohol and carbon dioxide in the absence of oxygen and, in the process, oxidize NADH back to NAD 🔄.
Fun fact: The bread-baking industry depends on the ability of yeast to carry out fermentation and produce carbon dioxide, which causes bread to rise. The beer, liquor, and wine  industry depends on yeast to ferment sugar into ethyl alcohol. 

2️⃣ Lactic Acid Fermentation
During lactic acid fermentation, pyruvate from glycolysis is reduced to form lactic acid or lactate. In the process, NADH gets oxidized back to NAD 🔄. Here are some common ways this process is used:

  • This is the process that the dairy industry uses to produce yogurt and cheese. 
  •  Human skeletal muscles 💪 carry out lactic acid fermentation when the blood cannot supply adequate oxygen to muscles during strenuous exercise🏊‍♂️🏋️‍♀️. Lactic acid in the muscle causes fatigue and burning. The lactic acid continues to build up until the blood can supply the muscles with adequate oxygen to repay the oxygen debt. 

Notice that alcohol and lactic acid fermentation are both labeled as step in the process of anaerobic respiration. This is meant to show that after glycolysis, the pyruvate can go through alcohol OR lactic acid fermentation but never both. 

Overall, it is important to understand that all of these processes function towards one goal: to produce ATP. By understanding the process of anaerobic respiration, you'll be able to learn processes like aerobic respiration and photosynthesis with ease. If you're ever confused, come chat with us at Fiveable. Good luck! 😊

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