Which of the following statements about the sulfur cycle is true?

The sulfur cycle is a vital ecological process that involves the movement of sulfur through different biological and geological pathways, and it plays a critical role in the formation of essential biomolecules in living organisms. The correct statement highlights that sulfur is a crucial component of amino acids, which are the building blocks of proteins. Notably, two of the sulfur-containing amino acids, cysteine and methionine, are integral to protein synthesis in all living organisms, thereby underscoring sulfur's importance in cellular functions.

Understanding the sulfur cycle also involves recognizing that sulfur is found in various forms and is cycled through both terrestrial and aquatic ecosystems. The role of sulfur in biological systems extends beyond just proteins; it is also involved in metabolic processes and the synthesis of coenzymes. Thus, the assertion of its significance in biological functions is foundational to comprehending how ecosystems operate and sustain life through nutrient cycling.