Final answer:
Only certain cells are affected by hormones because these cells have specific receptor proteins that allow the hormone to bind and initiate a response. These receptors determine the cell's sensitivity to the hormone and can vary through up-regulation or down-regulation, adapting to hormone levels.
Step-by-step explanation:
The reason only certain cells are affected by hormones is because only some cells have receptor proteins that can bind to a given hormone. These receptors are specific to a particular hormone and allow it to have an effect when it binds to them.
The hormone will travel throughout the bloodstream until it finds a cell with the correct receptor and binds to it, which initiates various responses depending on the type of hormone, such as a steroid hormone or a non-steroid hormone.
Hormones, acting as messengers, are crucial in regulating physiological processes. They play a significant role in human reproduction, development, metabolism, and maintaining homeostasis, among other functions.
The presence and quantity of hormone receptors determine which cells will respond to a given hormone, thus determining the sensitivity of the cell to the hormone. Cell sensitivity to hormones can change through processes like up-regulation and down-regulation, which adapt the number of receptors available in response to hormonal levels.