Final answer:
In stage I of metabolic pathways, macromolecules are broken down into their individual building blocks or monomer units.
Step-by-step explanation:
In stage I of metabolic pathways, macromolecules are broken down into their individual building blocks or monomer units. For example, carbohydrates are broken down into simple sugars, fats are broken down into fatty acids and glycerol, and proteins are broken down into amino acids.
These monomer units are then further broken down in stage II through different reaction pathways to produce a common end product called Acetyl-coenzyme A, which can be used in stage III to produce even more ATP.
In summary, stage I of metabolic pathways involves the breakdown of macromolecules into their monomer units.