The liver stores excess iron in ferritin.

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.

A. Nitrogen:Nitrogen makes up approximately 78% of the air we breathe, making it the most abundant gas in the atmosphere. It is largely inert and does not participate directly in human respiration.

B. Water vapor:Water vapor is present in variable amounts depending on humidity but generally constitutes a very small fraction of air, far less than nitrogen. Its concentration is not sufficient to be the highest component.

C. Carbon dioxide:Carbon dioxide is present at only about 0.04% in the atmosphere. Despite its critical role in respiration and pH regulation, it is not the major constituent of inhaled air.

D. Hydrogen:Hydrogen exists in trace amounts in the atmosphere and plays no significant role in the composition of breathable air. Its concentration is negligible compared to nitrogen or oxygen.

E. Oxygen:Oxygen makes up roughly 21% of atmospheric air, making it the second most abundant gas. While essential for cellular respiration, its concentration is much lower than that of nitrogen.