Professor Dave here, I'm going to tell you about receptors. Your body is made of trillions of cells, and they communicate with one another just like we do. But how can one cell talk and how can another cell listen? We want to understand the different kinds of chemical signaling that occur between cells. We now have a pretty good picture of what the plasma membrane looks like, and we mentioned that swimming around in the sea of phospholipids there are many surface proteins. Some of these proteins are called receptors, and these are the proteins that receive information from outside the cell, just like humans might use their eyes, ears, or noses to receive information about their surroundings. There are almost a thousand different types of receptors, and just like enzymes, receptors have an active site that is highly specific to one molecule or small collection of molecules, which we can refer to as ligands. These could be something like a hormone that was secreted by a glandular cell far away from this one, that will receive the message sent by the other cell, and this message may or may not be specific to this particular type of cell. Once this molecule reaches the receptor it will bind to the active site, where it will cause a conformational change in the receptor that will trigger some kind of response which can fall into one of a few different categories. One type of signaling response is called signal transduction, when the receptor binds to its substrate and causes a conformational change in the receptor which is a transmembrane protein that will span the plasma membrane. This change in shape will cause it to either release something inside the cell or bind to something else inside the cell. Once the receptor is activated it might bind to something called a G-protein, which is attached to the inner layer of the plasma membrane, and once the G-protein is activated, it might bind to an enzyme, causing another conformational change which will release another different signaling molecule like cyclic AMP, that will then diffuse within the cell and elicit a cellular response. These are called second messengers, because they are different molecules from the ligand that first activated the receptor, but they carry the message into the cell and amplify it, since they will be more numerous than the one molecule that initially made it to the receptor. Another pathway that can occur is when a signaling molecule binds to a receptor and the conformational change in the receptor immediately releases a relay molecule on the other side of the membrane. This relay molecule may then interact with various intracellular proteins in a phosphorylation cascade, where certain molecules exchange phosphate groups in a chain reaction. This will eventually reach its destination and elicit a cellular response. Some other receptors allow for the absorption of bulk quantities of a particular substance through a process called receptor-mediated endocytosis. This happens when several molecules of a particular substrate bind to multiple receptors, and the receptors cluster into a pit which can pinch off from the plasma membrane to form a vesicle, thus bringing the molecules bound to the receptors inside the cell, as well as any other solute that may have been inside the pit. This material will then be liberated from the vesicle somewhere inside the cell and the receptors will return to the plasma membrane. Certain receptors are not found within the plasma membrane, but are instead inside the cell, either in the cytoplasm or in the nucleus. In these cases, the signaling molecule must be able to pass through the plasma membrane on its own in order to reach the receptor, which it can do if it is relatively small and nonpolar, like nitric oxide or certain hormones. Once an intracellular receptor is activated by its ligand it might enter the nucleus and turn a gene on by acting as a transcription factor. More on these later. So what are these cellular responses that are triggered by messenger molecules? As we just said, the signal might result in a gene getting turned on or off. This is often to stimulate or halt the production of a particular protein. Other times a signal may cause a change in metabolism by activating the enzyme that breaks down stored glycogen. Or the signal might regulate cell division, which is why malfunctions in these pathways can cause tumor growth, if the signal to stop cell division is not being received due to a faulty receptor. We will learn more about the specific molecules that trigger these cellular responses later, so for now we just need to understand that receptors are proteins that sit either in the plasma membrane or somewhere within the cell, and certain signaling molecules like hormones or neurotransmitters fit into the active site of these receptors. Once they bind to the receptor they induce a conformational change in its structure which will set off some kind of reaction pathway, whether through other proteins or small second messenger molecules that then deliver the message to wherever it needs to go in the cell so that the appropriate cellular response takes place. And that's how ourselves talk to one another, by using molecules. Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always, feel free to email me: