What do we have? Why do orange and lemon - they smell different? That's what we're going to look at. In fact, we need to remember, for all the molecules that we deal with, they have some sort of asymmetry. And in chemistry, we use the term to call chirality. It refers to molecules that they are mirror image of each other. So think about that. We have two hands; they look alike, but actually, they are mirror image of each other. The same thing when we're dealing with some molecules. We can have this particular molecule and this particular molecule. In fact, they have the same composition, but in terms of how they are form, they are simply mirror image of each other. So with that - in fact, do we have the ability to tell them apart? So that's exactly what it is about; orange and lemon. Now with all this molecules being mirror image of each other, it tells us that molecules have one form versus their mirror image form. And in the living world, in fact, a lot of molecule - they have such kind of unique property such as when we are working with sugar. We find that most of the sugar that we're dealing with, in fact, they are right-handed; we call the dextro sugar. And for most of the amino acid that makes up protein, these are left-handed; we call levo. All right? So if we are made of amino acid protein and with all this chemical, so that means we are also asymmetric in terms of our structure. So if they are asymmetric, will we be able to tell them apart, such as lemon and orange? So here it is. We say that proteins are made asymmetric because they're made of chiral amino acid. So here what it is. Molecules can be mirror image of each other. So we need to be able to tell them apart. When these molecules, they are odorant molecules, we need to use a receptor. And this receptor should be able to interact with this molecule in one way, but if I'm providing a mirror image of it, then it would not be able to recognize. How to make analogy of that? You all have gloves. Think about that. If you have glove which is for your left hand; call that glove is a odorant receptor. Now if you put your right hand into that glove, do you think you'll be able to fit? The answer is no, it won't fit. So the sensory - the detection of a particular odorant by the receptor - is just like your hand and the glove; they need to match when they are of the same format, whether it's right-handed or left-handed. What happen between lemon and orange? In fact, they emit the same kind of molecule. Due to the biochemical synthesis, they produce the same kind of chemical called limonene. But lemon, they produce one version and the orange produce the mirror image of the same version. So what happen is that certainly, you have that experience when you smelled lemon and orange. They are certainly different and you can tell them apart. Similarly, when we cook something, these kind of aroma molecule, they also have mirror image of different versions. Such in this case, it is very easy for you to tell that it's mint because of this particular molecule being produced. But if the same molecules is produce as a mirror image, it would smell like the caraway, right? So with that, we need to understand when we now have a particular receptor, it's sensing a molecule that comes in. They pile on top of each other. So what do they do? The same thing as we learn from the taste; they act through a receptor. So this is the receptor that here, they would be able to bind onto this aroma molecule. They go through a series of signal transduction process and at the end, what they would do is that they will elicit the release of calcium inside the cell. So we call that is excitation. And this calcium signal would be able to pass on in this neuron as impulse along the axon and dentrite and cell and would send to afar. So that's how we perceive. Now, think about that. When we perceive a particular odorant molecule, sometimes when we have a prolonged exposure to it, we'll find that well, we no longer detect the smell. What happen is that, in fact, it is called olfactory fatigue. It's simply defining this particular kind of response and this kind of process of odor perception. They would be able to be activated again only when this odor molecule has been removed and later on they come on and bind to it again and reactivate it. Now, having that, we say that when these olfactory cells or the receptor being exposed to your odor molecule for prolonged period of time, their binding site, they are - if they're not clear of this molecule, they cannot bind any new molecule. And without any new molecule, they cannot excite and they cannot send a signal and so therefore, you simply - you don't sense the same smell again. This, in fact, involve multiple mechanisms. Some of them involve that - the binding of this particular odorant molecule by the receptor, results in some of the modification of the receptor, so they change the shape so they cannot bind the same thing again. And some of - some of the time, in fact, these occupied receptor, they would be internalized so that they would not be able to be put on the surface of the cells to perceive the molecule again. Or sometimes, when they're excited, it would lead to the cell dying, such, as you remember, that at one point that we're talking about the chili - the chili pepper, the capsaicin acid, activating its receptor resulting in the cell being killed. Now - let's now come together and look at - well, what happen when you have this volatile compound as this odorant molecule when they're in contact with the odor receptors? What do you do? Basically, what you do is that if this is some of the food - let's say, well, this is a lemon in front of me. What it is is that it's going to generate some of this odorant molecule or the aroma that comes out of it. So what I do? I come in, I sniff. And then I'm testing - it has space around it, what kind of molecule is around? That works. Sometimes we can smell without using our nose. What we do is that we eat some food into our mouth. And what we do? When we are trying to swallow it, that creates something like a vacuum inside your oral cavity and then, in this vacuum, some of the food - if they have some odorant molecule - they come out of it. And then when we breathe, this molecule, they come out from the nasal cavity out. And so therefore, these aroma molecule will be in contact with the olfactory epithelium. So for that, we will be able to sniff, we can eat and masticate and result in the release of this molecule so that we will be able to smell them from the back of your oral cavity and then to the nasal cavity. Now very importantly, we say that well, in that case, how can we tell actually what kind of molecule it is and what kind of smell it is? We need to understand that for the world around us, in fact, it's not made of a single smell or a single molecule that tells us what it smell. A lot of the food, in fact, they are made of multiple component. You cook a dish with some beef, you have some vegetable, you have some chicken also and a sauce. Each of them is going to release some of these aroma molecule and we should be able to tell them apart. Very simply, look at a situation like a cola drink. In fact, don't think of it as a single compound. We can use a method, which is called gas chromatography; what it allows us to do is to pass all this aroma molecule into a column with resin that is going to retain some of this aroma molecule and allow them to pass it through with a different rate. And by the time that they come out, we try to monitor what kind of molecules they are. So here are two cola drinks. And based on this gas chromatography profile, you may find that, in fact, cola drink, they smell very much the same. But, in fact, when you look at what are the component that come out of it, clearly they are very sharp bends which are present in one drink, but absent in the other. And, in fact, it defines why we prefer one drink better than the other, is because of these kind of different profile of individual aroma molecules present in the drink.
