The Molecular Wheel: Krebs Cycle Explained

If the cell needs to harvest high-energy electrons for the next stage of respiration, then it must break down the remains of glucose. If the krebs cycle processes Acetyl-CoA to release CO2 and charge NADH and FADH2, then its main purpose is producing these electron carriers.

If the krebs cycle harvests energy from carbon bonds, then it needs a way to transport that energy to the inner membrane. If NADH and FADH2 are the specific coenzymes that pick up and carry these electrons, then they act as the shuttles for the system.

If electrons are passed down a chain, then something must eventually catch them so the chain doesn't get "clogged." If oxygen has a high affinity for electrons and combines with them and protons to form water, then oxygen is the final acceptor.

If the Electron Transport Chain is essentially a pump, then it needs electricity to run. If that "electricity" comes in the form of electrons stripped from carbon during the krebs cycle, then that cycle is the source of the pump's power.

If NADH drops its electrons at the very first protein complex in the chain, then it powers more proton pumping. If FADH2 drops its electrons further down the line at the second complex, then it powers less pumping. If less pumping means less ATP, then NADH produces more than FADH2.

If glycolysis occurs in the cytoplasm, then the products must move into a specialized organelle for aerobic respiration. If the enzymes for the krebs cycle are located only in the mitochondrial matrix, then the statement that it occurs in the cytoplasm is false.

If a biochemical pathway is named after its first stable intermediate product, then we must identify that product. If citric acid is the first molecule formed when Acetyl-CoA joins the cycle, then the krebs cycle and the Citric Acid Cycle are two names for the same process.

If one glucose molecule is split into two pyruvate molecules during glycolysis, then there are two "units" to process. If each pyruvate is converted into one Acetyl-CoA, then the krebs cycle must complete two full rotations to account for the original glucose.

If the ETC uses the energy from the krebs cycle's electrons, then it powers a proton pump. If protons flow back through ATP synthase, then ATP is made. If oxygen picks up the spent electrons and protons, then water is created. Glucose splitting happens earlier in glycolysis.

If the cell needs to maximize the production of ATP following the krebs cycle, then it needs a large surface area for transport proteins. If the inner mitochondrial membrane is folded into finger-like shapes, then those shapes are called cristae.

If we track the energy output of one rotation of the cycle, then we see one molecule of GTP (which is converted to ATP) being made. Since it takes two turns per glucose, the total is 2, but a single turn of the krebs cycle produces exactly 1 ATP.

If "direct" means the molecule is a reactant in the cycle, then we must look at the steps. If the cycle uses water and enzymes to rearrange carbons but does not use O2, then it is not direct. However, if it stops without oxygen because the ETC backs up, it is only "indirectly" aerobic.

If glycolysis and the krebs cycle each provide a small amount of ATP (2 each), then the majority must come from the ETC. If the ETC produces roughly 32 ATP, then the total logical sum for a single glucose molecule in a standard eukaryotic cell is approximately 36.

If these two stages of respiration happen in different parts of the mitochondria, then we must distinguish them by location. If the fluid-filled center is the site of carbon oxidation, then that is where the krebs cycle takes place.

If the krebs cycle needs "empty" carriers (NAD+) to accept electrons, then the ETC must empty the "full" carriers (NADH). If the ETC stops, then all carriers stay full. If there are no empty carriers available, then the krebs cycle has nowhere to put electrons and must stop.

If the cycle breaks down carbon chains, then it must release carbon as waste. If it captures energy from those bonds, then it produces ATP and high-energy electron carriers. If CO2, ATP, NADH, and FADH2 are all results of these reactions, then they are the correct outputs.

If the cycle is a continuous loop, then there must be a starting molecule that gets recycled. If a two-carbon Acetyl-CoA enters the matrix, then it must bond with a four-carbon oxaloacetate to form a six-carbon citric acid, starting the circuit.

If the cell is fully oxidizing the remains of glucose, then the carbon-carbon bonds must be broken. If those carbons are combined with oxygen and released as a gaseous byproduct, then the krebs cycle is the stage where this carbon dioxide waste is generated.

If the energy from the krebs cycle has been converted into a proton gradient, then the cell needs a motor to turn that gradient into chemical energy. If a specific enzyme acts as a turbine to bond ADP and phosphate, then that enzyme is ATP synthase.

If a 2-carbon and 4-carbon join, then a 6-carbon molecule is made. If one CO2 is lost, it becomes 5-carbon. If another CO2 is lost, it becomes 4-carbon to restart the cycle. Therefore, the transitions 6 to 5, 5 to 4, and 4 to 6 are all correct.