Problem 8 - Biology Concepts and Applications

Question

In the third stage of aerobic respiration, _________ is the final acceptor of electrons from glucose. a. water b. hydrogen c. oxygen d. NADH

Step-by-Step Solution

Answer

Oxygen is the final electron acceptor in aerobic respiration.

Step 1: Identify the Stages of Aerobic Respiration

Aerobic respiration consists of three main stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation (electron transport chain).

Step 2: Understand the Role of Electron Transport Chain

In the electron transport chain, the electrons that originate from glucose are transferred through a series of proteins embedded in the inner mitochondrial membrane. The primary purpose is to create a proton gradient to produce ATP.

Step 3: Determine the Final Electron Acceptor

Electrons pass through the chain and eventually must be accepted by a final molecule to complete the electron transport process. This molecule, crucial for the conversion of ATP, is present at the end of the chain.

Step 4: Identify Oxygen's Role

Oxygen acts as the final electron acceptor in the electron transport chain. It combines with electrons and protons to form water as a byproduct, completing the chain and allowing for continuous flow of electrons.

Key Concepts

Electron Transport Chain

The Electron Transport Chain is a vital part of aerobic respiration that occurs in the inner membrane of mitochondria. Here, electrons harvested from earlier stages are passed among a series of proteins. This process represents the third and final stage of aerobic respiration. The goal is to transfer electrons effectively to create a proton gradient, driving the synthesis of ATP, the energy currency of the cell.

Electrons released during Glycolysis and the Citric Acid Cycle are carried to the Electron Transport Chain by carrier molecules like NADH and FADH2. As electrons move along the chain, they lose energy, which is used to pump protons across the inner mitochondrial membrane into the intermembrane space.

• This movement creates a buildup of protons (a proton gradient) in the intermembrane space, equivalent to a battery that's full of potential energy.
• Finally, when protons flow back into the mitochondrial matrix through ATP synthase, the enzyme uses this flow to synthesize ATP from ADP and inorganic phosphate.

Oxygen plays a critical role here; it acts as the final electron acceptor. Once it receives the electrons, it combines with protons to form water, preventing a bottleneck and ensuring the chain can continue to function efficiently.

Glycolysis

Glycolysis is the first stage of aerobic respiration and occurs in the cytoplasm of the cell. It begins the process of extracting energy by breaking down glucose, a six-carbon sugar, into two molecules of pyruvate. This process can occur with or without the presence of oxygen.

During Glycolysis:

• A singular glucose molecule, a six-carbon compound, is split into two three-carbon compounds known as pyruvates.
• This breakdown releases a modest amount of energy, which is captured in the form of 2 ATP molecules per glucose molecule.
• Additionally, glycolysis produces 2 NADH molecules, which are critical for transporting electrons to the Electron Transport Chain later in the aerobic process.

Despite yielding only a small amount of ATP, glycolysis is crucial because it lays the groundwork for the aerobic processes that follow in the mitochondria, including the Citric Acid Cycle and Electron Transport Chain.

Citric Acid Cycle

The Citric Acid Cycle, also known as the Krebs Cycle, represents the second stage of aerobic respiration and takes place in the mitochondrial matrix. This cycle is essential for oxidizing the three-carbon pyruvate produced from glycolysis into carbon dioxide, while also transferring high-energy electrons to carrier molecules.

Here's how it works:

• The pyruvate from glycolysis is converted into acetyl-CoA, which then enters the cycle.
• The cycle completes two turns for every glucose molecule, since each glucose yields two pyruvate molecules.
• During each turn, acetyl-CoA is broken down into two molecules of CO2, and along the way, electrons are captured by NAD+ and FAD to form NADH and FADH2, respectively. These carrier molecules are crucial as they ferry the high-energy electrons to the Electron Transport Chain.
• Additionally, one molecule of ATP (or an equivalent GTP) is produced per cycle turn.

The efficiency of the Citric Acid Cycle in generating electron carriers underscores its importance, as it directly feeds into the Electron Transport Chain where the main production of ATP occurs.