Fermentation

Following Electrons through Cellular Respiration

• Oxygen is the ultimate electron acceptor in cellular respiration.
• NADH is where these electrons come from.
• Glucose is the ultimate, original source of these electrons.
• NAD+ is the electron acceptor in glycolysis.

Figure 9.13, page 158, Campbell's Biology, 5th Edition

Electron Transport

• NAD
  • Picks up electrons in glycolysis, the bridge, and the Krebs cycle.
  • Shuttles the electrons to the electron transport chain via NADH.
• Oxygen
  • The ultimate electron acceptor that drives the whole system.
  • It is converted to water and exhaled.
• So, electrons enter the process in the form of glucose and leave in the form of water.

Part of Figure 9.16, page 161, Campbell's Biology, 5th Edition

To Move the Reaction Forward, Electrons Must Be Removed

• One could say that cellular respiration is all about the ultimate electron acceptor: without this electron acceptor, the reaction will not proceed forward.
• If the ultimate electron acceptor, oxygen, is not available:
  • Glycolysis occurs, and for each glucose we get:
    • 2 ATP
    • 2 pyruvate
    • 2 NADH
  • However, this cannot continue forever—even with an unlimited amount of glucose.
    • To move the reaction forward, electrons must be removed.
    • NAD+ is responsible for removing electrons in glycolysis.
    • So, once all the NAD+ is used up, the reaction will stop.

How to Deal with the NAD+ Crisis

• There are two ways to regenerate NAD+ from NADH:
  1. Alcohol fermentation
  2. Lactic acid fermentation
• Key idea: these processes only occur when there is no oxygen, i.e. in anaerobic conditions.
  • An: no
  • Aerobic: air (oxygen)

1. Alcohol Fermentation

• Pyruvate is converted into alcohol in two steps:
  1. Pyruvate to acetaldehyde
     • CO₂ is produced.
  2. Acetaldehyde to ethanol
     • NADH is converted to NAD+.
• The final product is ethanol (the alcohol people drink).
• Humans cannot do this reaction themselves:
  • We don't have the enzymes.
  • Thus, we can't produce alcohol for free!
• Yeast can do this reaction, and we use them to produce alcohol from a variety of glucose sources: grapes, barley, etc.

Figure 9.17(a), page 163, Campbell's Biology, 5th Edition

2. Lactic Acid Fermentation

• Pyruvate is converted to lactic acid (lactate) in one step.
  • NADH is converted to NAD+.
• Humans can do this, when they are not getting enough oxygen.
  • However, lactic acid builds up in the muscles.
  • This can cause cramping.

Figure 9.17(b), page 163, Campbell's Biology, 5th Edition

Pyruvate Is the Key Molecule

• Pyruvate is the “fork” in the metabolic pathways.
• Which path it takes depends on whether or not oxygen is present:
  • If oxygen is present:
    • It enters the mitochondrion.
    • It goes through the Krebs cycle and the electron transport chain.
    • A large amount of ATP is produced.
  • If oxygen is not present:
    • It is converted into ethanol or lactate (depending on the organism).
    • NAD+ is regenerated.
    • A small amount of ATP is produced.

Figure 9.18, page 163, Campbell's Biology, 5th Edition