12.9 - EXERCISES

In this task, you are asked to differentiate several biological concepts:
- Respiration and Combustion: Respiration is a biological process in living organisms that converts food into energy, while combustion is a chemical process that burns fuel to release energy. Respiration is controlled and gradual, utilizing enzymes, whereas combustion is rapid and uncontrolled
- Glycolysis and Krebs’ Cycle: Glycolysis is the first step of cellular respiration where glucose is broken down into pyruvate in the cytoplasm, while the Krebs Cycle takes place in the mitochondria, where pyruvate is further broken down to produce energy carriers like NADH and FADH2. Glycolysis produces a small amount of ATP, but the Krebs Cycle generates more energy-rich electron carriers.
- Aerobic Respiration and Fermentation: Aerobic respiration occurs in the presence of oxygen and produces a large amount of ATP through the complete oxidation of glucose, while fermentation occurs in the absence of oxygen, producing much less ATP and resulting in byproducts like lactic acid or alcohol.

Respiratory substrates are substances that can be broken down during respiration to release energy. The most common respiratory substrate is glucose, derived from carbohydrates. When glucose is oxidized during the process of respiration, it provides the necessary energy for cellular activities.

Glycolysis can be represented schematically as a series of steps where glucose is broken down into two molecules of pyruvate. This process is divided into two phases: the energy investment phase, where ATP is consumed, and the energy payoff phase, where ATP is produced. The main enzymes involved, such as hexokinase, phosphofructokinase, and pyruvate kinase, play crucial roles at various steps.

Aerobic respiration consists of several key steps: glycolysis, the Krebs Cycle, and the Electron Transport Chain. Glycolysis happens in the cytoplasm, converting glucose into pyruvate. Then pyruvate enters the mitochondria where it is converted into acetyl CoA, which enters the Krebs Cycle. Finally, the electrons from NADH and FADH2 are transferred through the Electron Transport Chain to produce ATP in the inner mitochondrial membrane. This process requires oxygen and results in the complete oxidation of glucose.

The Krebs Cycle (or Citric Acid Cycle) can be schematically represented as a series of biochemical reactions that begins with acetyl CoA combining with oxaloacetate to form citrate. Through a series of transformations, carbon dioxide is released, and energy carriers (NADH and FADH2) are generated. The cycle regenerates oxaloacetate so it can continue the cycle with new acetyl CoA.

The Electron Transport System (ETS) is a series of complexes located in the inner mitochondrial membrane. It facilitates the transfer of electrons from NADH and FADH2 to oxygen, which ultimately forms water. As electrons move through the complexes, protons are pumped into the intermembrane space, creating a proton gradient that powers ATP synthesis through ATP synthase. This process is known as oxidative phosphorylation.

The calculation of net gain of ATP from aerobic respiration is based on several assumptions: that the pathways function sequentially without any interfering reactions, that NADH produced in glycolysis can enter the mitochondria, and that only glucose serves as the respiratory substrate without other metabolites interfering.

The respiratory pathway is termed amphibolic because it involves both catabolic and anabolic processes. While it breaks down substrates such as carbohydrates to release energy, it also provides intermediates that can be used to synthesize necessary biological molecules like fats and amino acids during periods of anabolism.

The Respiratory Quotient (RQ) is defined as the ratio of carbon dioxide produced to oxygen consumed during respiration. It gives insight into which substrate is being metabolized. For fats, the RQ is generally less than 1 (about 0.7), indicating that less CO2 is produced relative to O2 consumed during fatty acid metabolism.

Oxidative phosphorylation is the metabolic pathway in aerobic respiration where ATP is produced from ADP and inorganic phosphate using the energy derived from the transfer of electrons in the electron transport chain. Oxygen acts as the final electron acceptor, enabling this process.

The step-wise release of energy during respiration allows cells to efficiently capture as much energy as possible rather than releasing it all at once as heat. This controlled process ensures that energy can be stored in the form of ATP, which is the energy currency used for various cellular activities.