Which of the following statements about hormones is correct?
a. They tend to be large molecules.
b. They exert their effects only on the same cells that produce them.
c. They can exert strong effects only when they are present in high concentrations.
d. They trigger a response by binding to target-cell receptors.

Verified step by step guidance

1. Step 1: Understand what hormones are. Hormones are chemical messengers produced by glands in the endocrine system. They travel through the bloodstream to tissues and organs, affecting various physiological processes.
2. Step 2: Analyze option a. Hormones can be large molecules, such as proteins, but they can also be small molecules, like steroids. Therefore, this statement is not universally correct.
3. Step 3: Analyze option b. Hormones typically exert their effects on cells other than those that produce them. They travel through the bloodstream to reach target cells, so this statement is incorrect.
4. Step 4: Analyze option c. Hormones can exert strong effects even at low concentrations due to their high specificity and affinity for their receptors. Therefore, this statement is incorrect.
5. Step 5: Analyze option d. Hormones trigger responses by binding to specific receptors on target cells, initiating a cascade of cellular events. This statement is correct as it describes the fundamental mechanism of hormone action.

Key Concepts

Hormone Structure

Hormones can vary in size and structure, ranging from small molecules like amino acids and steroids to larger peptide and protein hormones. Their size and structure influence their solubility and the mechanism by which they travel through the bloodstream to reach target cells.

Hormone Target Specificity

Hormones exert their effects on specific target cells that possess receptors for them, rather than affecting only the cells that produce them. This specificity is crucial for the regulation of physiological processes, as hormones can travel through the bloodstream to reach distant target cells.

Hormone Concentration and Effect

Hormones can exert significant effects even at low concentrations due to the amplification of their signal within target cells. This is achieved through mechanisms such as signal transduction pathways, where binding to receptors triggers a cascade of intracellular events, leading to a physiological response.