Summarize the inputs and outputs of the citric acid cycle (Krebs cycle): 3. What goes in? b. What comes out? gain of ATP per glucose? Has any ATP been produced? If so, what is the net C. citric acid cycle take place in eukaryotes? d Where does the
The Correct Answer and Explanation is:
The citric acid cycle, also known as the Krebs cycle, is a key metabolic pathway that occurs in the mitochondria of eukaryotic cells. It plays a crucial role in cellular respiration by oxidizing acetyl-CoA derived from carbohydrates, fats, and proteins, to produce energy in the form of ATP, NADH, and FADH2.
Inputs:
- Acetyl-CoA: This molecule, which is derived from pyruvate (produced in glycolysis) or fatty acids, is the primary input to the citric acid cycle. Each acetyl-CoA combines with oxaloacetate to form citrate at the start of the cycle.
- NAD+ and FAD: These are electron carriers that will be reduced during the cycle, becoming NADH and FADH2.
- ADP: This molecule is involved in the production of ATP by substrate-level phosphorylation.
Outputs:
- CO2: Two molecules of carbon dioxide are released for every round of the cycle. This is the byproduct of the decarboxylation reactions.
- NADH and FADH2: These reduced electron carriers are crucial for the electron transport chain, where they will ultimately generate more ATP through oxidative phosphorylation.
- ATP: A small amount of ATP is directly produced via substrate-level phosphorylation.
- Oxaloacetate: This molecule is regenerated at the end of the cycle and is ready to combine with another acetyl-CoA molecule to start the next round of the cycle.
ATP Gain:
For each glucose molecule (which is split into two molecules of pyruvate during glycolysis), two rounds of the citric acid cycle occur. In each round:
- 3 NADH are produced (each generating about 3 ATP via the electron transport chain),
- 1 FADH2 is produced (each generating about 2 ATP),
- 1 ATP (or GTP) is produced via substrate-level phosphorylation.
Thus, the total ATP production per glucose molecule (2 cycles) is approximately 2 ATP directly and 6 ATP from NADH and 2 ATP from FADH2, totaling about 8 ATP per cycle.
