Question

Which of these statements about the oxygen in the air we breathe is true? a. It functions as the final electron acceptor of the electron-transport chain. b. It combines with hydrogen to form water. c. It combines with carbon to form \(\mathrm{CO}_{2}\). d. Both \(a\) and \(b\) are true. e. Both \(a\) and \(c\) are true.

Short answer

The correct answer is option d (Both statements a and b are true). Oxygen functions as the final electron acceptor of the electron transport chain and combines with hydrogen to form water during this process.

Step-by-step solution

Understanding cellular respiration

Cellular respiration is the process by which cells convert nutrients, mainly glucose, into energy (ATP) needed for cell functions. This process consists of three main stages: glycolysis, the citric acid (Krebs) cycle, and the electron transport chain. Oxygen plays a critical role in the electron transport chain as the final electron acceptor.

Examining the role of oxygen in the electron transport chain

In the electron transport chain, the electrons that were obtained from the breakdown of glucose in the previous stages of cellular respiration are transported through a series of protein complexes in the inner mitochondrial membrane. During this process, protons (H+) are pumped across the membrane, creating a gradient. This gradient is later used to generate ATP through the process of oxidative phosphorylation. At the end of the electron transport chain, oxygen picks up electrons and combines with protons to form water. Thus, oxygen acts as the final electron acceptor in the electron transport chain and, through this process, generates water.

Examining the formation of carbon dioxide

Oxygen does not directly combine with carbon in respiration to form carbon dioxide (CO2). Carbon dioxide is produced as a byproduct of the citric acid (Krebs) cycle during cellular respiration. In this stage, the breakdown of glucose continues, and carbon, originally derived from glucose, is released in the form of CO2.

Evaluating the correctness of each statement

a. True. Oxygen functions as the final electron acceptor of the electron transport chain. b. True. Oxygen combines with hydrogen to form water during the electron transport chain. c. False. Oxygen does not combine with carbon to form CO2 directly during respiration. d. True. Both statements a and b are true. e. False. Statement a is true, but statement c is false.

Selecting the correct answer

Based on the analysis of each statement, the correct answer is option d (Both statements a and b are true).

Key concepts

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes located in the inner membrane of the mitochondria, the powerhouses of the cell. During this stage of cellular respiration, electrons are carried from molecules like NADH and FADH₂, harvested in earlier stages, to the ETC.

As electrons pass from one complex to the next, protons—also known as hydrogen ions (H⁺)—are pumped from the mitochondrial matrix into the intermembrane space. This transfer creates a proton gradient across the inner mitodrial membrane. It's like a dam holding back water; in this case, the 'water' is the built-up potential energy of the separated protons.

Function of Oxygen in the ETC

Oxygen's role is vital at the terminal stage of the ETC, where it acts as the ultimate electron acceptor. It takes on these electrons and pairs them with the protons that have been pumped across the membrane, creating water as a byproduct. This reaction is critical because it removes low-energy electrons from the system, allowing the chain to continue functioning.

Oxidative Phosphorylation

Oxidative phosphorylation is the process that occurs after the electron transport chain, responsible for most of the ATP generation in cellular respiration. This stage takes advantage of the proton gradient created by the ETC.

Through a process called chemiosmosis, protons flow back into the mitochondrial matrix through an enzyme called ATP synthase. The flow of protons drives the enzymatic addition of a phosphate group to ADP, forming the energy-rich molecule ATP.

ATP: The Energy Currency

ATP acts as the energy currency for cells, powering various cellular activities. With every 'turn' of ATP synthase, ATP is produced from ADP and inorganic phosphate. This step is where the 'phosphorylation' part of 'oxidative phosphorylation' gets its name—oxygen is needed to keep the whole process 'oxidizing' or going.

Krebs Cycle

The Krebs cycle, also known as the citric acid cycle, is the central metabolic pathway that underpins aerobic respiration. It happens within the mitochondrial matrix, reliant on the products of glycolysis: pyruvate, which has been converted into acetyl-CoA.

During this cycle, acetyl-CoA combines with oxaloacetate to form citrate, which then goes through a series of transformations. These reactions release carbon dioxide and reduce NAD⁺ and FAD to NADH and FADH₂, respectively, which carry electrons to the ETC.

Byproducts and Energy Yield

The cycle doesn't produce ATP directly but generates the reduced carriers NADH and FADH₂ that play a key role in driving the ATP-generating electron transport chain. Furthermore, two carbon atoms enter the cycle from each acetyl-CoA and two carbon dioxide molecules are released, highlighting the cycle's role in producing the CO₂ we exhale.