In step ©, the five-carbon compound formed in step © also releases a CO2 molecule and a hydrogen atom, forming a four-carbon compound. Again, NAD+ is reduced to NADH. Notice that in this step a molecule of ATP is also synthesized from ADP.

In step ©, the four-carbon compound formed in step © releases a hydrogen atom to form another four-carbon compound. This time, the hydrogen atom is used to reduce FAD to FADH2. FAD, or flavin adenine dinucleotide, is a molecule very similar to NAD+. Like NAD+, FAD accepts electrons during redox reactions.

In step ©, the four-carbon compound formed in step © releases a hydrogen atom to regenerate oxaloacetic acid, which keeps the Krebs cycle operating. The electron in the hydrogen atom reduces NAD+ to NADH.

Recall that in glycolysis one glucose molecule produces two pyru-vic acid molecules, which can then form two molecules of acetyl CoA. Thus, one glucose molecule is completely broken down in two turns of the Krebs cycle. These two turns produce four CO2 molecules, two ATP molecules, and hydrogen atoms that are used to make six NADH and two FADH2 molecules. The CO2 diffuses out of the cells and is given off as waste. The ATP can be used for energy. But note that each glucose molecule yields only two molecules of ATP through the Krebs cycle—the same number as in glycolysis.

The bulk of the energy released by the oxidation of glucose still has not been transferred to ATP. Glycolysis of one glucose molecule produces two NADH molecules, and the conversion of the two resulting molecules of pyruvic acid to acetyl CoA produces two more. Adding the six NADH molecules from the Krebs cycle gives a total of 10 NADH molecules for every glucose molecule that is oxidized. These 10 NADH molecules and the two FADH2 molecules from the Krebs cycle drive the next stage of aerobic respiration. That is where most of the energy transfer from glucose to ATP actually occurs.

Comparing CO2 Production

Materials disposable gloves, lab apron, safety goggles, 250 mL flask, 100 mL graduated cylinder, phenolphthalein solution, pipet, drinking straw, water, clock, sodium hydroxide solution


1. Put on your disposable gloves, lab apron, and safety goggles.

2. Add 50 mL of water and four drops of phenolphthalein to the flask.

3. Use the straw to gently blow into the solution for 1 minute. Add the sodium hydroxide one drop at a time, and gently swirl the flask. Record the number of drops you use.

4. When the liquid turns pink, stop adding drops.

5. Empty and rinse your flask as your teacher directs, and repeat step 2. Walk vigorously for 2 minutes, and repeat steps 3 and 4.

Analysis Which trial produced the most carbon dioxide? Which trial used the most energy?

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