Question

If you were to expose cells that are undergoing aerobic cellular respiration to a radioactive oxygen isotope in the form of \(\mathrm{O}_{2}\), which of the following molecules would you expect to be radiolabeled? a. pyruvate b. water c. \(\mathrm{NADH}\) d. \(\mathrm{CO}_{2}\)

Short answer

The radiolabeled molecule in aerobic cellular respiration when exposed to a radioactive oxygen isotope in the form of \(\mathrm{O}_{2}\) would be water. This is because oxygen acts as the final electron acceptor in the electron transport chain and combines with electrons and protons to produce water. Therefore, the correct answer is b. water.

Step-by-step solution

Understand aerobic cellular respiration

Aerobic cellular respiration is a process in which cells use oxygen to break down glucose and produce ATP (adenosine triphosphate), which is used as a cellular energy source. The process consists of three major stages: glycolysis, the citric acid cycle (also known as the Krebs cycle), and the electron transport chain.

Analyze the role of oxygen in aerobic cellular respiration

Oxygen acts as the final electron acceptor in the electron transport chain, which drives the synthesis of ATP. During this process, oxygen combines with electrons and protons (hydrogen ions) to produce water. This is the primary role of oxygen in aerobic cellular respiration.

Identify the molecule that would be radiolabeled with the radioactive oxygen isotope

Since oxygen is primarily incorporated into water during the electron transport chain in aerobic cellular respiration, we should expect the radiolabeled molecule to be water. Thus, the correct answer is: b. water

Key concepts

Glycolysis

Glycolysis is the first stage of aerobic cellular respiration and takes place in the cytoplasm of the cell. During this process, one molecule of glucose, which has six carbon atoms, is broken down into two molecules of pyruvate, each containing three carbon atoms. This transformation entails a series of ten reactions, which are facilitated by different enzymes.

The crucial steps can be simplified into two phases: the energy investment phase and the energy payoff phase. In the investment phase, two ATP molecules are used to modify the glucose molecule, which is eventually split into two three-carbon compounds. In the payoff phase, these compounds are further processed to produce four ATP molecules and two molecules of NADH - a carrier of electrons for later use in the electron transport chain.

Remember, glycolysis doesn't require oxygen and can occur under anaerobic conditions too. However, for aerobic respiration to progress, the pyruvate produced enters the mitochondria for the citric acid cycle.

Citric Acid Cycle

Also known as the Krebs cycle, this is the second stage of aerobic cellular respiration and happens inside the mitochondria. The citric acid cycle is a series of chemical reactions that further breaks down the products of glycolysis. Before it starts, pyruvate is converted into Acetyl-CoA. This reaction produces NADH and releases one molecule of carbon dioxide (CO₂).

Acetyl-CoA then combines with oxaloacetate to form citrate, starting the citric acid cycle. Throughout the cycle, citric acid is broken down and transformed, and through a series of enzyme-driven reactions, two molecules of CO₂ are released, and more NADH and FADH₂ are produced, along with a single molecule of ATP (or GTP) per cycle. The NADH and FADH₂ created here hold high-energy electrons that are crucial for the next stage: the electron transport chain.

Electron Transport Chain

Located in the inner mitochondrial membrane, the electron transport chain (ETC) is where the oxygen you breathe plays a critical role. It's composed of a series of protein complexes and small organic molecules that facilitate the transfer of electrons.

Electrons from NADH and FADH₂, the products of glycolysis and the citric acid cycle, are passed down the transport chain. As they move along, the energy released from these electrons is used by the proteins in the chain to pump protons into the intermembrane space of the mitochondria, creating a gradient.

Ultimately, oxygen acts as the final electron acceptor at the end of the ETC, combining with these electrons and protons to form water. This is why introducing a radioactive oxygen isotope would result in radiolabeled water, as solved in the given exercise.

ATP Synthesis

The culmination of aerobic cellular respiration is the synthesis of ATP. The electron transport chain sets the stage for this final act. As electrons travel through the ETC, protons are pumped into the intermembrane space, creating a proton gradient across the mitochondrial membrane.

ATP synthesis occurs through a process known as chemiosmosis using an enzyme called ATP synthase. As protons flow back into the mitochondrial matrix through ATP synthase, the energy from this movement drives the conversion of adenosine diphosphate (ADP) and inorganic phosphate (Pi) into ATP. This is an example of oxidative phosphorylation.

Each molecule of glucose in aerobic respiration can produce up to approximately 30-32 ATP molecules, highlighting the efficiency of this energy conversion process when compared to anaerobic respiration, which yields only 2 ATP molecules per glucose molecule.