Comment on why do you think cells prefer to use carbohydrates as a source of energy before they use fats and proteins.

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.