When barium hydroxide, ba(oh)2, mixes with water, the solution turns cold.
what is an indicator of a chemical change in the scenario?
formation of a gas
formation of a precipitate
change in energy
change in color

Water has a solvent capacity in wich happens all the metabolic reactions

Carbohydrates are used as a source of quick energy, because it´s used a very small amount of ATP to degrade carbohydrates compare to the lipids that need more ATP to be degraded.   In addition to that, the carbohydrates are easly found in nature, easier than lipids.  They are at the base of the trophic chain, present in the plant, fruits and vegetables.