Question

Outline the three stages in the extraction of energy from food

Answer 1

first stage - In the first stage, large molecules in food are broken down into smaller units. Proteins are hydrolyzed to their 20 kinds of constituent amino acids, polysaccharides are hydrolyzed to simple sugars such as glucose, and fats are hydrolyzed to glycerol and fatty acids. This stage is strictly a preparation stage; no useful energy is captured in this phase

second stage - In the second stage, these numerous small molecules are degraded to a few simple units that play a central role in metabolism. In fact, most of them sugars, fatty acids, glycerol, and several amino acids are converted into the acetyl unit of acetyl CoA . Some ATP is generated in this stage, but the amount is small compared with that obtained in the third stage.

third stage- In the third stage, ATP is produced from the complete oxidation of the acetyl unit of acetyl CoA. The third stage consists of the citric acid cycle and oxidative phosphorylation, which are the final common pathways in the oxidation of fuel molecules. Acetyl CoA brings acetyl units into the citric acid cycle [also called the tricarboxylic acid (TCA) cycle or Krebs cycle], where they are completely oxidized to CO2. Four pairs of electrons are transferred (three to NAD+ and one to FAD) for each acetyl group that is oxidized. Then, a proton gradient is generated as electrons flow from the reduced forms of these carriers to O2, and this gradient is used to synthesize ATP.