D. 20 ATP
Clarify Question
What is the key concept addressed by the question?
· The question asks you to determine the predicted ATP yield of the Krebs cycle per molecule of glucose.
What type of thinking is required?
· You are being asked to dissect, or analyze, the Krebs cycle for ATP and NADH production to determine the total yield of ATP per glucose molecule.
Gather Content
What do you already know about the Krebs cycle and ATP and NADH yield? What other information is related to the question?
· Recall that a key part of cellular respiration is the Krebs cycle, which occurs in the mitochondria. Products from the Krebs cycle include NADH, FADH2, and a few ATP, among other molecules.
· The Krebs cycle must be able to continuously receive acetyl-CoA, a 2-carbon molecule that is converted from a 3-carbon pyruvate (generated during glycolysis) immediately preceding the Krebs cycle. Each 6-carbon glucose molecule is first broken into 2 3-carbon pyruvate molecules during glycolysis, which are in turn oxidized into 2 2-carbon acetyl-CoA molecules with 2 molecules of carbon dioxide produced as well. Each acetyl-CoA molecule is able to enter the Krebs cycle.
· At the beginning of the Krebs cycle each 2-carbon acetyl-CoA molecule joins with a 4-carbon oxaloacetate molecule to form a 6-carbon citrate molecule. In order to take in another acetyl-CoA, the 4-carbon oxaloacetate has to be regenerated, prompting the release of 2 carbon dioxide molecules per pyruvate. Recall that each time a carbon dioxide molecule is produced, bonds are broken and high-energy electrons are released, which are captured by NADH. As it turns out, this kind of bond breaking happens several times per acetyl-CoA during the Krebs cycle, prompting both NADH and FADH2 to be produced.
· How many acetyl-CoA molecules are produced from one glucose molecule? What does that tell you about how many times Krebs will turn per glucose?
Choose Answer
Do you have all necessary information to determine which products would affect cellular respiration?
· As we figured out earlier, the Krebs cycle runs twice per glucose molecule. This is because glucose is broken down into 2 molecules of pyruvate, each of which are converted to acetyl-CoA through pyruvate oxidation. Both acetyl-CoA molecules can enter the Krebs cycle.
· Remember that bond breakage produces high-energy electrons that are captured by NADH. This happens several times each time the Krebs cycle turns, which produces 3 NADH, 1 FADH2, and 1 ATP molecule. Since the Krebs cycle turns twice per glucose molecule, this means that 6 NADH, 2 FADH2, and 2 ATP are made. Since 2.5 ATP are made per NADH and 1.5 ATP are made per FADH2 from ATP synthase, the number of ATP that are made from high-energy electron carriers is 2.5 x 6 NADH + 1.5 x 2 FADH2 = 18 ATP.
· Finally, remember that the Krebs cycle also produces 1 ATP per turn via substrate-level phosphorylation. Since there are 2 turns per glucose, 2 ATP are made. When added to the 18 ATP produced from NADH and FADH2, the total is 20 ATP.
Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
· Answering this question correctly depended on your ability to analyze the Krebs cycle for the different ways that ATP could be produced both from NADH and FADH2 as well as from substrate-level phosphorylation.
· If you got the correct answer, good thinking! If you got an incorrect answer, were you able to accurately tally how many NADH and FADH2 molecules were produced during one turn of the Krebs cycle? Did you realize that the Krebs cycle turns twice per glucose? Were you able to calculate the number of ATP produced from NADH, FADH2 as well as the 2 ATP produced via substrate-level phosphorylation in the Krebs cycle?
Save a GPA. Donate your notes with us.
