The heart contracts because of an electrical impulse. Where in the heart does this impulse start?

A. A chemical messenger that is triggered by the binding of a ligand on the cell surface and causes a change in DNA transcription:This describes the mechanism of some hormones or intracellular signaling molecules, particularly steroid hormones, but it is not a complete definition of a hormone itself.

B. Junctions between cells interconnecting their cytoplasm:This describes gap junctions, which allow direct cytoplasmic communication between adjacent cells. It is a structural feature, not a hormone.

C. A chemical messenger transported by the bloodstream that stimulates target cells in another organ often a good distance away:This accurately defines a hormone. Hormones are secreted by endocrine glands into the blood and travel to distant target cells, eliciting specific physiological responses.

D. Chemical messengers that travel across a synapse to stimulate another cell:This describes neurotransmitters, which act locally at synapses rather than being transported through the bloodstream.

E. Chemical messengers that diffuse from their point of origin locally to affect other cells physiology:This describes paracrine signaling, which is local and short-range. Hormones, in contrast, typically act on distant cells via the circulatory system.

The liver plays a central role in iron metabolism by storing excess iron in the protein ferritin. Ferritin safely binds iron, preventing it from catalyzing harmful free radical reactions, and releases it when the body needs it for processes such as hemoglobin synthesis. This storage mechanism helps maintain iron homeostasis and prevents iron deficiency or overload.