Question

After glucose is fully oxidized by glycolysis, pyruvate processing, and the citric acid cycle, where is most of its energy stored?

Short answer

After glucose is fully oxidized by glycolysis, pyruvate processing, and the citric acid cycle, most of its energy is stored in ATP molecules. A total of 33 ATP molecules are generated for one glucose molecule being oxidized, making ATP the primary storage molecule for glucose's energy.

Step-by-step solution

Understand the glucose oxidation process

Glucose is broken down through three major pathways - glycolysis, pyruvate processing, and the citric acid cycle (also known as the Krebs cycle or TCA cycle). These processes convert glucose into various molecules, such as pyruvate, Acetyl CoA, and ATP, which store energy in their chemical bonds.

Identify the molecules storing energy during glucose oxidation

The primary molecules storing energy during the glucose oxidation process include: 1. ATP 2. NADH 3. FADH2 Note that while ATP directly provides energy for various cellular processes, NADH and FADH2 donate electrons to the electron transport chain in the mitochondria, which results in additional ATP synthesis through oxidative phosphorylation.

Determine the number of each molecule produced during the oxidation process

The glucose breakdown results in the net production of molecules storing energy: 1. Glycolysis produces 2 ATP, 2 NADH 2. Pyruvate processing generates 2 NADH (one per pyruvate) 3. Citric acid cycle yields 2 ATP, 6 NADH, and 2 FADH2

Include the role of oxidative phosphorylation in energy storage

During oxidative phosphorylation, the electron transport chain utilizes the electrons from NADH and FADH2 to create a proton gradient across the inner mitochondrial membrane. This gradient drives the synthesis of additional ATP molecules through ATP synthase. NADH generates approximately 2.5 ATP per molecule, while FADH2 produces approximately 1.5 ATP per molecule.

Calculate the total ATP yield

To find out where most of the energy is stored during glucose oxidation, calculate the total ATP yield: 1. Glycolysis: 2 ATP (direct) + 2 NADH * 2.5 ATP/NADH = 7 ATP 2. Pyruvate processing: 2 NADH * 2.5 ATP/NADH = 5 ATP 3. Citric acid cycle: 2 ATP (direct) + 6 NADH * 2.5 ATP/NADH + 2 FADH2 * 1.5 ATP/FADH2 = 21 ATP Total ATP yield: 7+5+21 = 33 ATP

Conclusion

After glucose is fully oxidized by glycolysis, pyruvate processing, and the citric acid cycle, its energy is primarily stored in ATP molecules. A total of 33 ATP molecules are generated for one glucose molecule being oxidized, so the majority of glucose's energy is stored in ATP.

Key concepts

Glycolysis

Glycolysis is the first step in the process of glucose oxidation. It occurs in the cytoplasm of the cell and does not require oxygen to proceed, making it anaerobic. During glycolysis, one molecule of glucose, which is a six-carbon molecule, is split into two three-carbon molecules known as pyruvate. This process involves a series of ten enzyme-catalyzed reactions.

• Produces 2 ATP directly by substrate-level phosphorylation.
• Generates 2 NADH by transferring electrons to NAD⁺.
• Does not require oxygen, so it can occur in both aerobic and anaerobic conditions.

Despite the small amount of ATP produced directly, glycolysis initiates the pathway that will yield much more ATP later.

Pyruvate Processing

Following glycolysis, pyruvate undergoes a transition into acetyl-CoA before entering the citric acid cycle. This occurs in the mitochondria and is known as pyruvate processing or pyruvate decarboxylation. During this process

• Each pyruvate loses one carbon as carbon dioxide (CO₂), resulting in a two-carbon acetyl group.
• This two-carbon acetyl group combines with coenzyme A, forming acetyl-CoA.
• 2 NADH are produced (one from each pyruvate) as electrons are transferred to NAD⁺.

So, pyruvate processing plays a crucial role in oxidizing pyruvate and funneling the carbon atoms of glucose into the citric acid cycle.

Citric Acid Cycle

Once acetyl-CoA is formed, it enters the citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle. This cycle takes place in the mitochondrial matrix. Each cycle turn further oxidizes the carbons, releasing them as CO₂

• For each acetyl-CoA, 2 CO₂ are released.
• 1 ATP (or GTP) is formed directly by substrate-level phosphorylation.
• 3 NADH and 1 FADH₂ are produced per acetyl-CoA.

Since each glucose molecule generates two acetyl-CoA, the cycle runs twice per glucose, doubling the energy carriers produced. The cycle's main purpose is to gather high-energy electrons in the form of NADH and FADH₂.

ATP

Adenosine triphosphate (ATP) is the energy currency of the cell. It stores and supplies the energy needed for many biochemical cellular processes. During glucose oxidation, ATP is produced both directly and indirectly.

• Glycolysis yields 2 ATP directly.
• The citric acid cycle produces 2 additional ATP.
• Most ATP is made indirectly through oxidative phosphorylation.

The breakdown of one glucose molecule results in a total of about 30-36 ATP molecules when you consider all processes involved.

NADH

Nicotinamide adenine dinucleotide (NADH) is a crucial energy carrier produced during glycolysis, pyruvate processing, and the citric acid cycle. NADH contributes significantly to energy production

• Each glycolysis reaction produces 2 NADH.
• Pyruvate processing results in 2 NADH (one for each pyruvate).
• The citric acid cycle generates 6 NADH per glucose.

NADH holds electrons that are later donated to the electron transport chain in oxidative phosphorylation, contributing considerably to ATP synthesis.

FADH2

Flavin adenine dinucleotide (FADH₂) is another important electron carrier generated during glucose breakdown, specifically in the citric acid cycle.

• Each citric acid cycle run produces 1 FADH₂ per acetyl-CoA.
• Given 2 acetyl-CoA per glucose, you get 2 FADH₂ overall.

Although FADH₂ produces less ATP compared to NADH—about 1.5 ATP per FADH₂, its contribution to the electron transport chain is vital for overall ATP production.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage in glucose oxidation, occurring in the mitochondria's inner membrane. It is where most of the ATP from glucose is produced, using the electrons carried by NADH and FADH₂.

• The electron transport chain creates a proton gradient across the inner mitochondrial membrane by passing electrons from NADH and FADH₂ through proteins.
• This gradient powers ATP synthase, an enzyme that synthesizes ATP from ADP and inorganic phosphate.
• NADH produces about 2.5 ATP, while FADH₂ produces approximately 1.5 ATP.

Oxidative phosphorylation maximizes the energy extracted from glucose, making it a pivotal process in cellular respiration.