Final answer:
The variation in target cell responses to chemical messengers is due to different cell types, receptor sensitivities, and signal integration, influencing the magnitude of the response.
Step-by-step explanation:
Responses to chemical messengers produced by target cells vary in magnitude according to different cell types, protein expression, and signal integration. This variation in response is because different cell types produce different receptors that bind to the same ligand, leading to the activation of different cellular responses. Additionally, receptors can exhibit varying levels of sensitivity to hormones, causing cellular activity to be either up-regulated or down-regulated, resulting in different magnitudes of response.
Signal integration plays a crucial role where signals from two or more different cell-surface receptors merge, activating the same response in the cell, thereby ensuring that a cell commits to a specific response only after multiple external requirements are met. Moreover, factors like downregulation, where a cell reduces its receptor numbers in response to excessive hormone levels, and upregulation, where receptor numbers are increased in response to low hormone levels, also affect the magnitude of response to chemical messengers.