Problem 55 - Objective Biology for NEET

Question

The aerobic respiration yields (a) \(8 \mathrm{NADH}_{2}, 2 \mathrm{FADH}_{2}, 2 \mathrm{ATP}\) (b) \(10 \mathrm{NADH}_{2}, 2 \mathrm{FADH}_{2}, 38 \mathrm{ATP}\) (c) \(12 \mathrm{NADH}_{2}, 30 \mathrm{ATP}, \mathrm{H}_{2} \mathrm{O}\) (d) \(10 \mathrm{NADH}_{2}, 2 \mathrm{FADH}_{2}, 2 \mathrm{GTP}, 2 \mathrm{ATP}\)

Step-by-Step Solution

Answer

The correct answer is (b). In aerobic respiration, a single glucose molecule generates up to 38 ATP (2 from Glycolysis, 2 from Citric Acid Cycle, 34 from Electron Transport Chain), 10 NADH (2 from Glycolysis, 2 from Pyruvate Decarboxylation, 6 from Citric Acid Cycle), and 2 FADH2 from the Citric Acid Cycle.

Step 1: Understanding Glycolysis

Glycolysis, the first step of aerobic respiration, generates 2 ATP, 2 NADH and 2 Pyruvate molecules per glucose molecule. This occurs in the cytoplasm of the cell.

Step 2: Understanding Pyruvate Decarboxylation

Each pyruvate generated from Glycolysis goes through Pyruvate Decarboxylation in the mitochondria to generate 1 NADH per pyruvate. In total, 2 NADH are generated as 2 Pyruvate were formed from 1 glucose molecule.

Step 3: Understanding Citric Acid Cycle

Each Acetyl CoA generated from Pyruvate Decarboxylation goes through the Citric Acid Cycle. Each round of this cycle produces 1 ATP, 3 NADH and 1 FADH2. Since 2 Acetyl CoA are generated from 1 glucose molecule, a total of 2 ATP, 6 NADH and 2 FADH2 are produced.

Step 4: Understanding Electron Transport Chain

Electron Transport Chain utilizes the NADH and FADH2 produced in prior steps to produce ATP. It’s estimated that about 30 ATP are produced from 10 NADH and about 4 ATP are produced from 2 FADH2.

Step 5: Total Yield Calculation

By summing up all these molecules, the total yield from 1 glucose molecule in aerobic respiration would be up to 38 ATP (Glycolysis=2 ATP, Citric Acid Cycle= 2 ATP and Electron Transport Chain = 34 ATP), 10 NADH (Glycolysis= 2 NADH, Pyruvate Decarboxylation= 2 NADH and Citric Acid Cycle= 6 NADH), and 2 FADH2 (Citric Acid Cycle= 2 FADH2). Therefore, the correct answer in terms of total yield should be option (b).

Key Concepts

Glycolysis

Glycolysis is a sequence of ten enzyme-catalyzed reactions that convert glucose into pyruvate, with the generation of energy.
Glycolysis occurs in the cytoplasm and starts respiration by splitting a sugar molecule into two three-carbon compounds. These two molecules then go through a series of reactions that result in the production of pyruvate.
For each glucose molecule, glycolysis yields 2 molecules of ATP, 2 molecules of NADH (which carry electrons to the Electron Transport Chain), and 2 molecules of pyruvate, which are then subject to further processes in aerobic respiration like Pyruvate Decarboxylation.

Pyruvate Decarboxylation

Pyruvate Decarboxylation is a link between glycolysis and the Citric Acid Cycle. This process happens within the mitochondria.
Each pyruvate molecule loses a carbon atom, releasing it as carbon dioxide, and is then converted into an acetyl group that combines with coenzyme A (CoA) to form Acetyl CoA.
In addition to the release of carbon dioxide, this reaction produces one molecule of NADH from each pyruvate molecule, so from one glucose molecule, which results in two pyruvates, there are 2 NADH formed. These NADH molecules are later utilized in the Electron Transport Chain.

Citric Acid Cycle

Also known as the Krebs Cycle or the Tricarboxylic Acid (TCA) Cycle, this is a set of chemical reactions used by all aerobic organisms to generate energy.
For each Acetyl CoA molecule that enters the cycle, three molecules of NADH, one molecule of FADH₂, and one GTP/ATP molecule are produced, along with two molecules of carbon dioxide as waste. Since two molecules of Acetyl CoA are generated from one glucose molecule, the cycle must turn twice and hence, yields 6 NADH, 2 FADH₂, and 2 ATP (or GTP) in total.

Electron Transport Chain

The Electron Transport Chain (ETC) is the final stage of aerobic respiration located in the inner mitochondrial membrane.
It comprises a series of electron carriers that transfer electrons from NADH and FADH₂ to molecular oxygen (O₂), producing water. In the process, a proton gradient across the membrane is generated, ultimately leading to the production of ATP through a process called oxidative phosphorylation.
The NADH and FADH₂ that were produced in the earlier stages of respiration donate their electrons to the ETC, and it is in this step that the majority of ATP is produced, with approximately 3 ATPs for each NADH and 2 ATPs for each FADH₂ feeding into the ETC.

ATP yield in respiration

The ATP yield in aerobic respiration is a measure of the energy that an organism extracts from glucose during this process. Efficiency in ATP production is critical for the cell's energy supply.
At the end of the aerobic respiration, up to 38 ATP molecules can be produced from a single molecule of glucose: 2 from glycolysis, 2 from the Citric Acid Cycle, and around 34 from the Electron Transport Chain (30 from the 10 NADH and 4 from the 2 FADH₂), representing a significant energy conversion efficiency.
The actual number of ATP molecules can vary slightly depending on the cell type and conditions, but option (b) in the exercise represents the maximal potential yield under ideal conditions.