Final answer:
Receptors are proteins that respond to signals and can be categorized into internal and cell-surface types, as well as by structure, location, and function. Internal receptors typically bind to hormones.
Step-by-step explanation:
Receptors are protein molecules inside or on the surface of a target cell that bind ligands (signals). There are two main categories: internal receptors and cell-surface receptors. Internal receptors are located inside the cell and often bind to hormones or other molecules that can cross the cell membrane. Cell-surface receptors are embedded in the cell membrane and respond to various substances, like neurotransmitters, that cannot penetrate the cell on their own. These receptors can also be classified by their structure as having either free nerve endings, encapsulated endings, or as specialized cells like photoreceptors. Further classification includes their location relative to stimuli, encompassing exteroceptors for external environmental stimuli, interoceptors for internal bodily stimuli, and proprioceptors that sense the position or movement within the body.
Functional classification depends on how receptors transduce stimuli into membrane potential changes. This can be through biochemicals that change membrane potentials, physical changes like pressure or temperature, or electromagnetic like visible light. For example, humans perceive light through rod and cone cells in the eyes, while other organisms have unique receptors, such as heat sensors in snakes or ultraviolet sensors in bees.