With exactly what do receptors interact?
Receptors bind with a substance (ligand) for which they are structurally shape specific. The ligand may be circulating and soluble in an animal's bloodstream, floating around in the water or soil, or floating around in the air. The ligand may be a small and relatively simply-structured organic molecule such as histamine or nitric oxide; a more complex organic molecule such as the "essence of rose" (we'd smell that one if we had a receptor for that particular essence); a region of a large protein molecule such as the hormone, insulin; part of a DNA or RNA molecule; the sugar molecule, glucose; or, an amino acid of some sort. In addition, and amazingly for us, there are receptors for certain types of electromagnetic radiation as well. We have receptors in our eyes for the particles of light known as photons. Through the interaction of photoreceptors in our retina with visible light, the subsequent resultant nerve impulses which occur, and the chemical and electrical interpretation of these impulses by a specific nerve system within our brain (the visual cortex), we are able to "see". As you can see, there is a receptor for just about everything we can imagine. Without these receptors, all cells would be non-responsive to all things which under normal conditions allows the cell to "know" what to do every single second.
Where can receptors be found?
Receptors can be found all over the place - inside a cell, and especially embedded within and an integral part of all the membranes that a given cell may have. When the life form of the organism is unicellular - such as a bacterium of some sort (like E. coli for example), receptors are present primarily within the membrane surrounding the contents of the cell, and every single member of the species will have the same array and number of different receptors unique to that species. And, even though a given receptor may have a similar function among all bacterial species such as transporting a potassium ion (K+) into the cell, the fine-detailed structure of the receptor may well be slightly different among different species.
When the life form of the organism is multicellular - such as a human, and there are organ systems present, a given receptor may be found associated only with a particular type of cell which comprises a particular type of tissue, which makes up a particular organ. However, this receptor among the same tissue among different humans may be exactly the same. The human receptor for insulin for example, will be identical among humans except for minor changes that may have occurred due to genetic mutations, etc. Similarly, the ligand for this receptor, the human insulin molecule, is identical among humans - we are members of the same species of course, known as Homo sapiens. In all cases however, no matter the organism, and no matter the species of the organism, the location of receptors will be in association with the membranes which make up the cell's outer structure which has direct access to the outside conditions (the plasma membrane) or which comprise internal structures such as mitochondria, ribosomes, etc.
What is it that receptors actually do?
Receptors are critical to the life of all cells, whether or not the cell represents an animal, a plant, a fungus, or a bacterium. Every function, response, interaction, pathway, process, and any other term you might think of that concerns the moment-to-moment existence of a cell, is controlled by various receptor/ligand-induced systems.
Human cells need a source of energy in order to maintain life. One such source is the sugar molecule, glucose. All of the materials necessary to utilize glucose are inside the cell. And, in order to obtain glucose we must absorb it in some way from our intestines. There are special receptors, called carrier proteins, within the membrane of cells which line the intestinal "wall". These carrier proteins are specific for glucose and energy is used to pump the glucose into the cells that line the intestines. Through interaction with other kinds of glucose carrier proteins, glucose eventually is deposited into the bloodstream. There, the glucose molecules reach cells throughout the body. Glucose cannot, however, simply diffuse across a cell's membrane and enter the cell. The movement must be facilitated by a cell's response to the hormone, insulin. Insulin receptors on cells, bind to a specific region of the insulin molecule, and this interaction triggers a molecular "wiggling" of the insulin receptor. The result of this slight movement causes a chemical reaction to occur (part of the insulin receptor is actually a kind of special enzyme which transfers phosphate groups). This reaction triggers further specialized reactions within the cell along a very precise activation pathway which ultimately result in the uptake by the cell of glucose.
Another kind of receptor, the acetylcholine receptor, specifically interacts with a chemical substance known as acetylcholine (asseh-teel-ko-leen) and this interaction allows nerve impulses to travel to a muscle and trigger movement of an entire muscle group. Other kinds of receptors in our nasal tissue (olfactory tissue) are connected by nerves to our brain. When certain chemicals react with receptors specific for them, we may experience an automatic withdrawal/avoidance reaction as one result. Our brain interpreted the nerve impulses in such a way as to lead to a "warning". The ability to respond to such a warning signal depends upon our genetic makeup - the genes which lead to all of these kinds of responses. Alternatively, if we possess receptors in our nasal tissues which can bind to molecules emitted by the flower of a lilac bush, we may react pleasurably to this "smell". I know I do.
Therefore, receptors are necessary for many, many things. Without receptors, we'd lead a pretty dull existence. And, although we might not experience pain, either physical or emotional, we'd also be unable to experience any of the joy of life.