What role do proteins play in biological systems?

Proteins serve a myriad of essential functions in biological systems, which justifies the choice D as the correct answer. They are pivotal in various roles:

1. Energy Storage: While carbohydrates and fats are often the primary sources of energy in biological organisms, proteins can also serve as a secondary energy source when necessary. In times of metabolic need, the body can break down proteins into amino acids, which can then be converted into energy.

2. Structural Support: Proteins contribute significantly to the structure and integrity of cells and tissues. For instance, collagen, a structural protein, forms a major component of connective tissues, providing strength and support. Other proteins, like keratin, contribute to the structure of hair, nails, and the outer layers of skin.

3. Catalysts for Biochemical Reactions: Enzymes, which are specialized proteins, act as catalysts in biochemical reactions. They speed up reactions by lowering the activation energy needed, allowing metabolic processes to occur efficiently and rapidly. This is crucial for sustaining life, as it enables cells to carry out necessary functions without taking an impractical amount of time.

The answer encompasses all these roles, thus validating the choice that proteins are fundamental to energy storage, structural integrity, and