Question

What stage of metabolism involves the conversion of small molecules to \(\mathrm{CO}_{2}, \mathrm{H}_{2} \mathrm{O}\), and energy?

Short answer

Stage 3, known as the citric acid cycle and oxidative phosphorylation.

Step-by-step solution

- Understand the given molecules

Identify the small molecules acting as reactants. Typically, these are carbohydrates, fats, and proteins.

- Identify the metabolic stages

Review the three main stages of metabolism: Stage 1 involves digestive breakdown into smaller units. Stage 2 processes these molecules into acetyl-CoA. Stage 3 involves the conversion of acetyl-CoA to \(\text{CO}_2, \text{H}_2 \text{O}\), and energy in the mitochondria.

- Link the molecules to the correct stage

Analyze that \(\text{CO}_2, \text{H}_2 \text{O}\) and energy production specifically occurs in Stage 3, known as the citric acid cycle (Krebs cycle) and oxidative phosphorylation.

Key concepts

Metabolism Stages

Metabolism is a complex process that converts food into energy. It involves three main stages. Stage 1 is digestion, where food is broken down into smaller molecules like glucose from carbohydrates, fatty acids from fats, and amino acids from proteins. Stage 2 involves transforming these smaller molecules into a compound known as acetyl-CoA. This stage prepares the molecules for the next step. Stage 3 happens inside the mitochondria, where acetyl-CoA is further processed to produce carbon dioxide (CO2), water (H2O), and energy. These stages are crucial for understanding how the body utilizes nutrients.

Think of metabolism stages as a series of steps in an energy factory. Each step plays an essential role in producing the energy required by our bodies to perform various functions.

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle, is a key part of metabolism that takes place in the mitochondria. Once acetyl-CoA enters the cycle, it undergoes a series of chemical reactions. These reactions help release stored energy from nutrients.
During the citric acid cycle, acetyl-CoA combines with a molecule called oxaloacetate to form citric acid. Citric acid then undergoes multiple transformations, releasing CO2 and transferring high-energy electrons to molecules called NAD+ and FAD. These electrons are crucial because they drive the next stage of energy production: oxidative phosphorylation. The cycle completes when oxaloacetate is regenerated, ready to combine with another acetyl-CoA.
This cycle ensures a constant supply of energy by breaking down acetyl-CoA and harvesting its stored energy.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of energy production in cells, occurring in the inner membrane of mitochondria. This process uses high-energy electrons from the citric acid cycle to produce ATP, the energy currency of cells.
These electrons are carried by NADH and FADH2 to the electron transport chain (ETC). As electrons move through the ETC, they create a flow that pumps protons across the mitochondrial membrane, generating a proton gradient. The energy from this gradient is used by ATP synthase to convert ADP into ATP.
Oxidative phosphorylation is key because it produces the majority of ATP, which cells use for work like muscle contraction and molecule synthesis. Without this stage, cells would not have enough energy to survive.

Acetyl-CoA Conversion

Acetyl-CoA conversion is a bridge between the breakdown of nutrients and their use in energy production. Once smaller molecules are broken down in digestion, they need to be converted into a usable form. This is where acetyl-CoA comes in.
In cells, glucose is converted to pyruvate through glycolysis. Pyruvate then enters mitochondria and changes into acetyl-CoA. Similarly, fatty acids undergo beta-oxidation to become acetyl-CoA. Even proteins can be broken down into amino acids and then into acetyl-CoA.
Acetyl-CoA is vital because it enters the citric acid cycle, linking nutrient breakdown to energy extraction.

Energy Production

Energy production in the body is a sophisticated process that starts with the intake of food and ends with the creation of ATP. This multi-stage process ensures that cells have a steady supply of energy.
First, macronutrients from food, like carbs, fats, and proteins, are broken down into smaller molecules. Then, these molecules turn into acetyl-CoA. Acetyl-CoA enters the citric acid cycle, where it releases CO2 and transfers electrons to NAD+ and FAD. Finally, oxidative phosphorylation uses these electrons to produce ATP.
This entire process is efficient and essential for maintaining bodily functions, from cellular repair to movement. Understanding each stage helps grasp how the body turns food into fuel.