The most probable answer for this question would be that almost every life process requires specialized cells in multicellular organisms. To simply put it, cells of multicellular organisms are specialized in a way that they are all grouped into their respective tissues and these tissues are all grouped into their respective organs and these organs are all grouped together into their respective systems and these systems make up the multicellular organisms. These systems have their own functions in maintaining and sustaining the life that the organisms has. The organs have their own functions as well, thus specialized cells are mostly needed in respiration, digestion, circulation, movement, excretion, reproduction, immunity, coordination, and synthesis.
Lipids would be the answer that you are looking for. When talking food energy it's best to speak in terms of calories. Therefore, lipids or fats offer the greatest amount of heat or energy.
