Final answer:
Proteins are essential for muscle development, act as enzymes and immune system components, and can provide energy when needed. They do not typically serve as insulation.
Step-by-step explanation:
Proteins serve a multitude of functions in living organisms. Structures such as muscles are comprised largely of proteins, which provide strength and the capacity for contraction. Proteins also play a critical role as enzymes, acting as catalysts to speed up various chemical reactions necessary for life. Beyond these roles, proteins are vital in the immune system, where they function as antibodies to identify and neutralize foreign invaders like pathogens.
In addition to these functions, proteins can also serve as transport molecules, such as hemoglobin, which carries oxygen in the blood. Though providing energy is not their primary role, proteins can be metabolized to provide energy in the absence of carbohydrates and fats. However, insulation is not typically a function of proteins; this is more often a role filled by body fat.