Cracking the AP Biology Exam


Cellular Energetics


Some organisms can’t undergo aerobic respiration. They’re anaerobic. They can’t use oxygen to make ATP. We just learned that oxygen is important because it’s the final electron acceptor in the electron transport chain.

How do anaerobic organisms derive energy? Since glycolysis is an anaerobic process, they can make 2 ATP from this stage. However, instead of carrying out the other stages of aerobic respiration (the Krebs cycle, electron transport chain, and oxidative phosphorylation), these organisms carry out a process called fermentation. Under anaerobic conditions, pyruvic acid is converted to either lactic acid or ethyl alcohol (or ethanol) and carbon dioxide.

For the AP Biology Exam, you should remember the two pathways anaerobic organisms undergo: glycolysis and fermentation. Unfortunately, anaerobic respiration is not very efficient. It only results in a gain of 2 ATP for each molecule of glucose broken down.

As you can see from the chart below, there are two basic end products in anaerobic respiration. In both pathways, the NADH formed during glycolysis reduces (adds hydrogen to) pyruvate.

What types of organisms undergo fermentation?

  • Yeast cells and some bacteria make ethanol and carbon dioxide.
  • Other bacteria produce lactic acid.


Although human beings are aerobic organisms, they can actually carry out fermentation in their muscle cells. Have you ever had a cramp? If so, that cramp was the consequence of anaerobic respiration.

When you exercise, your muscles require a lot of energy. To get this energy, they convert enormous amounts of glucose to ATP. But as you continue to exercise, your body doesn’t get enough oxygen to keep up with the demand in your muscles. This creates an oxygen debt. What do your muscle cells do? They switch over to anaerobic respiration. Pyruvic acid produced from glycolysis is converted to lactic acid. As a consequence, the lactic acid causes your muscles to ache.