Question

The link between glycolysis and Kreb's cycle is (a) Citric acid (b) Acetyl-CoA (c) Succinic acid (d) Oxaloacetic acid

Short answer

(b) Acetyl-CoA

Step-by-step solution

Understand the Metabolic Processes

Glycolysis is the metabolic process that converts glucose into pyruvate, also producing ATP and NADH. This process happens in the cytoplasm of the cell. Then, the process of converting pyruvate into Acetyl-CoA, also known as pyruvate decarboxylation, takes place in the mitochondria. Acetyl-CoA is then used in the Kreb's cycle or citric acid cycle, which also takes place in the mitochondria.

Identify the Link

The question requests the link between glycolysis and the Kreb's cycle. As explained in Step 1, pyruvate, produced by glycolysis, is converted into Acetyl-CoA, which then enters the Kreb's cycle. Therefore, Acetyl-CoA serves as the link between glycolysis and Kreb's cycle.

Choose the correct answer

Based on the analysis done in Step 2, the correct answer should be (b) Acetyl-CoA which is the compound that links glycolysis and the Kreb's cycle.

Key concepts

Glycolysis

Glycolysis is the first step in the process of cellular respiration where a glucose molecule is broken down into two molecules of pyruvate. This process occurs in the cytoplasm of the cell and does not require oxygen, making it anaerobic. During glycolysis, a little energy is released and captured to form two molecules of ATP and NADH.

Here’s a simplified breakdown of the process:

• Glucose Activation: The glucose molecule is phosphorylated using ATP, which prepares it for further breakdown.
• Splitting of Sugar: The activated glucose is split into two three-carbon molecules called glyceraldehyde-3-phosphate (G3P).
• Production of ATP and NADH: Through a series of reactions, the G3P molecules are converted into pyruvate, producing ATP and NADH in the process.

Glycolysis is essential as it sets the stage for further energy-yielding processes such as the Krebs Cycle.

Krebs Cycle

The Krebs Cycle, also known as the citric acid cycle, is a vital component of cellular respiration that occurs in the mitochondria. It follows the glycolysis process and further breaks down the pyruvate that has been converted to Acetyl-CoA. The Krebs Cycle is aerobic, meaning it requires oxygen to proceed and is pivotal in energy production.

Let’s go through the main points:

• Acetyl-CoA Initiation: Acetyl-CoA, derived from pyruvate, enters the cycle by combining with oxaloacetate to form citric acid.
• Chemical Reactions: Citric acid undergoes a series of transformations, releasing carbon dioxide and transferring electrons to electron carriers NAD+ and FAD, forming NADH and FADH2.
• ATP Generation: Energy is directly produced in the form of a single ATP or GTP molecule during one turn of the cycle.
• Regeneration of Oxaloacetate: The process concludes with the regeneration of oxaloacetate, enabling the cycle to continue.

This cycle is fundamental in providing high-energy electron carriers, which will be used in the next step, oxidative phosphorylation, to produce a large amount of ATP.

Acetyl-CoA

Acetyl-CoA plays a crucial role as a central metabolite in cellular respiration. It forms the link between glycolysis and the Krebs Cycle, acting as a key currency in the conversion of pyruvate. After glycolysis, pyruvate enters the mitochondria where it is decarboxylated to form Acetyl-CoA along with a molecule of carbon dioxide.

Important roles of Acetyl-CoA include:

• Energy Production: Acetyl-CoA enters the Krebs Cycle, leading to the generation of electron carriers and ATP.
• Metabolic Pathways: It acts as an important substrate in various biosynthetic pathways including the synthesis of fatty acids.
• Allosteric Regulation: Acetyl-CoA serves as an allosteric activator/inhibitor for different enzymes, regulating metabolism based on cellular energy levels.

Acetyl-CoA is indispensable for linking the energy produced from glucose breakdown to the processes that ultimately release the energy stored in glucose.

Pyruvate Decarboxylation

Pyruvate decarboxylation is a critical step in cellular metabolism that connects glycolysis to the Krebs Cycle. This process takes place in the mitochondria, where pyruvate, produced from glycolysis, is transformed into Acetyl-CoA.

The process involves several key reactions:

• Decarboxylation: Pyruvate loses a carbon atom in the form of carbon dioxide, catalyzed by the enzyme pyruvate dehydrogenase.
• Oxidation: The remaining two-carbon molecule is oxidized, reducing NAD+ to NADH, an electron carrier.
• CoA Addition: Coenzyme A is added to the oxidized molecule, forming Acetyl-CoA.

This step is crucial as it not only produces Acetyl-CoA for entry into the Krebs Cycle but also generates NADH, which is used in the electron transport chain to produce ATP.