Answer:
This can be explained based on structure, and on metabolisms of carbohydrate.
Explanation:
Carbohydrate has a relatively simple structural composition than the either protein or fatty acids. The C, H and Oxygen molecules are of relatively fewer in number and few chains than fats and protein.Thus cells will spend less energy to break bonds(catabolisims) among these molecules during cellular respiration pathways to trap the energy.
In addition glucose the end products of carbohydrate did not need any processing before it enters glycolysis, Kreb's Cycle and oxidative phosphorylation pathways to generate energy for the cells.
Conversely, for protein to be used it has to be first deaminated(removal of amino acid) by the liver before it enters glycolysis,while fats needs to be broken down and undergo beta oxidation with the long chains removed before it can form acetyl CoA. Therefore cells will prefer few steps, less endergonic pathways of glucose than longer more endergonic amino acids and fatty acid pathways.
Furthermore, glucose can be used in cellular respiration to produce energy either aerobically or anaerobically, while fats can only be used anaerobically. Therefore, since cells usually prefer to thrive in aerobic conditions they breakdown glucose easily during this period, and when lack of oxygen occurs they switched to anaerobic, Thus, the versatility of glucose to oxygen concentration makes glucose a better choice. Besides if fats was used anaerobically, ketone bodies build up which may be toxic to the cells.
