Question

Which of the following molecules has the highest amount of energy that will be collected during its further metabolism in cellular respiration? C6H1206 pyruvate CO2 02 ATP

Answer 1

The answer is C₆H₁₂O₆.

During cellular resipration, glucose (one of the primary isomers with molecular formula C₆H₁₂O₆) is metabolised in a series of steps to yield energy. The first step of this process is glycolysis, which is a series of processes that result in the production of 2 molecules of pyruvate, 2 ATP and 2 NADH. In the next step, called link reaction, each pyruvate molecule is converted to a molecule of acetyl-CoA with the release of one molecule of NADH and CO₂ each. The acetyl-CoA then enters the Kreb's cycle or the citric acid cycle. The yield of one turn of the citric acid cycle is 1 GTP (which is equivalent to 1 ATP), 3 NADH, 1 FADH₂ and 2 CO₂ . NADH and FADH₂, which are carriers of electrons between reactions, are then taken through a process called oxidative phosphorylation in the mitochondria, where a group of proteins called the electron transport chain pass on the electrons in a chain of redox reactions. The energy released in these redox reactions is used to generate a proton gradient in the mitochondria, which is then utilized by an enzyme called ATP synthase to produce ATP, the primary energy currency of the cell. At the end of the electron transport chain, oxygen molecules act as electron acceptors and together with protons produce a molecule of water.

1 molecule of NADH results in the production of 3 ATP molecules and 1 molecule of FADH₂ results in the production of 2 ATP molecules.

So, in total, 1 molecule of pyruvate eventually results in the production of 1 GTP + 3 NADH + 1 FADH₂ = (1 + 3 x 3 + 1 x 2) ATP = 12 ATP molecules

1 glucose molecule results in the production of 2 pyruvate molecules, so from the Kreb's cycle alone, glucose produces 2 x 12 = 24 ATP molecules. Added to this are the 2 ATP and 2 NADH (equivalent to 2 x 3 = 6 ATP molecules) produced during glycolysis and 2 NADH molecules produced during the Link reaction (1 NADH molecule per pyruvate molecule) which are equivalent to 6 ATP molecules. So, in total, 1 molecule of glucose results in 38 molecules of ATP.

Clearly, 1 molecule of glucose (C₆H₁₂O₆) yields more energy than 1 molecule of pyruvate.

CO₂ is a byproduct of cellular respiration and cannot be metabolized further to yield energy.

As explained above, O₂ acts as an electron acceptor at the end of the electron transport chain, a process that does not involve any release of energy.

ATP itself is a molecule that stores chemical energy in the cell that is produced as a product of cellular respiration. It is not metabolized during respiration.

Therefore, C₆H₁₂O₆ has the highest amount of energy that will be collected during its further metabolism in cellular respiration.