Metabolic Pathways of Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a series of metabolic pathways within cells that involve the breakdown of glucose and other organic molecules to produce energy in the form of ATP (adenosine triphosphate).

Glycolysis: Breaking Down Glucose to Pyruvate

Glycolysis is the first stage of cellular respiration, taking place in the cytoplasm. It involves the breakdown of one molecule of glucose into two molecules of pyruvate.

ATP Requirement in Glycolysis

• Energy Investment Phase: ATP is required for the initial phosphorylation of glucose and intermediates in the glycolytic pathway.
• Energy Pay-off Stage: The energy investment phase is followed by the energy pay-off stage, where ATP is generated through substrate-level phosphorylation.

Net ATP Gain

Glycolysis results in a net gain of ATP molecules.

Aerobic Conditions: Conversion of Pyruvate to Acetyl Coenzyme A

In aerobic conditions, pyruvate generated during glycolysis is further processed. It is converted into an acetyl group that combines with coenzyme A (CoA), forming acetyl coenzyme A (acetyl-CoA).

Citric Acid Cycle (Krebs Cycle): Generating ATP and Carbon Dioxide

1. Formation of Citrate

Acetyl-CoA combines with oxaloacetate to form citrate, initiating the citric acid cycle. This cycle takes place in the mitochondria's matrix.

2. Enzyme-Controlled Steps

During a series of enzyme-controlled steps, citrate is gradually converted back into oxaloacetate, resulting in the generation of ATP and the release of carbon dioxide (CO2).

---

Metabolic Pathways of Cellular Respiration

Role of Dehydrogenase Enzymes and NADH

1. Dehydrogenase Enzymes

Dehydrogenase enzymes play a crucial role in cellular respiration by removing hydrogen ions (H+) and electrons (e-) from specific organic molecules.

2. Formation of NADH: Explanation

Dehydrogenase enzymes pass these removed hydrogen ions and electrons to the coenzyme NAD (nicotinamide adenine dinucleotide), forming NADH. This process occurs in both glycolysis and the citric acid cycle.

3. Electron Transport Chain

Hydrogen ions and electrons carried by NADH are then transferred to the electron transport chain, located on the inner mitochondrial membrane.

Summary

Cellular respiration is a vital metabolic process that involves the breakdown of glucose and other organic molecules to produce ATP, the cell's primary energy source. Glycolysis initiates this process by breaking down glucose into pyruvate, with ATP being both consumed and produced during different stages. In aerobic conditions, pyruvate is further processed to form acetyl-CoA, which enters the citric acid cycle. The citric acid cycle generates ATP and releases carbon dioxide through a series of enzyme-controlled reactions. Dehydrogenase enzymes play a crucial role by transferring hydrogen ions and electrons to NADH, which then feeds these electrons into the electron transport chain on the inner mitochondrial membrane. This chain is a key player in the production of additional ATP, ultimately providing the cell with the energy needed for its functions.