Which of the following best describes a hormone?
A chemical messenger that is triggered by the binding of a ligand on the cell surface and causes a change in DNA transcription
A chemical messenger transported by the bloodstream that stimulates target cells in another organ often a good distance away
Chemical messengers that travel across a synapse to stimulate another cell.
Chemical messengers that diffuse from their point of origin locally to affect other cells physiology
The Correct Answer is B
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 how some hormones act inside a cell, but it is not the definition of a hormone.
B. A chemical messenger transported by the bloodstream that stimulates target cells in another organ often a good distance away: This is the textbook definition of a hormone: an endocrine signal transported in the blood to distant target organs.
C. Chemical messengers that travel across a synapse to stimulate another cell: This describes neurotransmitters, not hormones.
D. Chemical messengers that diffuse from their point of origin locally to affect other cells physiology: This describes paracrine or autocrine signals, not hormones.
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Related Questions
Correct Answer is D
Explanation
A. initiate blood clotting: Platelets and coagulation factors initiate clotting; erythrocytes do not initiate clot formation.
B. transport nutrients: Plasma carries most nutrients (glucose, amino acids, lipids), not erythrocytes.
C. defend the body against pathogens: White blood cells (leukocytes) are the main defenders; erythrocytes do not have immune functions.
D. transport some carbon dioxide: Erythrocytes carry a portion of CO₂ (dissolved as bicarbonate via carbonic anhydrase inside RBCs and also some carbaminohemoglobin).
E. regulate erythropoiesis: Erythropoiesis is regulated mainly by erythropoietin (from kidneys) and iron/nutritional signals, not by erythrocytes themselves.
Correct Answer is D
Explanation
A. CP: Creatine phosphate (phosphocreatine) is an energy-reserve molecule in muscle that donates phosphate to ADP to regenerate ATP; it is not a second messenger in hormone signal transduction and would not appear as the intracellular signaling oval shown.
B. DPG: 2,3-diphosphoglycerate (2,3-DPG) is an RBC metabolite that modulates hemoglobin’s O₂ affinity; it is not produced as a cytoplasmic second messenger following receptor activation.
C. PEP: Phosphoenolpyruvate (PEP) is a glycolytic intermediate involved in metabolism, not a hormone-activated intracellular signaling molecule.
D. CAMP: Cyclic AMP (cAMP) is a common second messenger produced by adenylate cyclase after many peptide hormones bind cell-surface receptors; the diagram’s small oval (C) inside the cytoplasm leading to downstream effects (D) fits cAMP.
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