A hormone is best described as a:

• A. enzyme that catalyzes metabolic reactions within the cytoplasm
• B. chemical messenger that is secreted into the bloodstream and acts on distant target cells
• C. lipid molecule that forms the main component of cell membranes
• D. structural protein that provides support to the cell membrane

Answer: (B)

Verified step by step guidance

1. Understand the definition of a hormone: A hormone is a chemical messenger produced by glands in the endocrine system. It is secreted into the bloodstream and travels to distant target cells to regulate physiological processes.
2. Eliminate incorrect options: Hormones are not enzymes, so the option 'enzyme that catalyzes metabolic reactions within the cytoplasm' can be ruled out.
3. Eliminate the option 'lipid molecule that forms the main component of cell membranes': This describes phospholipids, not hormones.
4. Eliminate the option 'structural protein that provides support to the cell membrane': Structural proteins like actin or spectrin provide support, but they are not hormones.
5. Conclude that the correct description of a hormone is 'chemical messenger that is secreted into the bloodstream and acts on distant target cells,' as this aligns with the biological role of hormones.