Advanced Placement (AP) Biology Practice Exam 2025 – Comprehensive All-in-One Resource for Exam Success!

A hormone is defined as a regulatory substance produced by specific glands and transported through the bloodstream to target organs or cells, where it stimulates various physiological responses. This definition highlights the role of hormones in communication within the body, functioning as messengers that can influence a wide variety of processes, including metabolism, growth, development, and mood.

Other options describe different biological substances and their functions, but they do not encompass the full definition of a hormone. Neurotransmitters, for example, are chemical messengers that primarily transmit signals between neurons in the nervous system, rather than being broadly involved in systemic regulation. Similarly, proteins that act as catalysts pertain to enzymes that speed up biochemical reactions but do not generally serve as regulatory substances like hormones. Lastly, while some hormones can influence muscle contractions, the description of regulating solely muscle contractions does not capture the wider regulatory roles hormones play in multiple systems throughout the body. Hence, the broad functionality of hormones aligns perfectly with the definition provided in the correct answer.