Scientists in the nineteenth century were able to come to establish that there are four primary tastes that can be classified by chemical makeups. As can be predicted, sugars are sweet, salts or ionic molecules are salty, alkaloids are bitter and acids are typically sour. Generally, sweet and salty tasting chemicals dissociate or ionize in the saliva, whereas bitter and and sour tasting things do not. Furthermore, recent studies in Japan have been able to classify a fifth taste, known as umami. Umami is the "savoury" taste obtained from amino acids, primarily glutamic acid and monosodium glutamate.
Salty Taste
The taste of salt, to a certain extent, is interpreted to be a positive taste. This is due to the fact that salt is needed by our bodies as a regulatory mechanism in homeostasis. On the other hand, when the body is not in need of any excess salt, the taste becomes negatively interpreted so that the individual no longer desires to consume it. The taste of salt generally comes from the sodium chloride molecule. The receptors specifically designed to interpret the taste of salt are comprised of amiloride-sensitive sodium channels and are called ENaC (Epithelial Sodium channel) channels. These are classified as one of the most selective channels found in the body. In other words, the epithelial membrane of the nerve channel is very permeable to sodium ions, and a molecule known as amiloride is used as a channel blocker to inhibit the permeability of the channel to sodium. Amiloride has four different amino functional groups. However, it is considered a member of the amide organic family because of its amino group in the middle of the molecule that is attached to a carbon atom with a carbonyl group on it. As a result of its unsymmetrical shape and polar covalent bond, amiloride it considered to be a polar molecule, making it soluble in water. Additionally, the hydrogen bonds between the N-H atoms and the double bonds between carbons and oxygens require lots of energy to be broken, resulting in high boiling and melting points. See the figure below for the structure of amiloride.
So as the positively charged sodium ions enter into the channel, they are diffusing from an area of high concentration outside of the channel, to an area of low concentration inside the channel. Rather than the outside being positive and the inside of the membrane being negative, a charge reversal takes place and a depolarization occurs. In this case, after the depolarization has occurred, channel on the inside of the sodium channel called voltage-sensitive calcium ion channels are able to detect the change of charge in the sodium channel and as a result, release positively charged calcium ions to increase the action potential in the nerve/channel. Therefore once the channel senses that the sodium ions are attempting to stimulate the nerve, it releases more positive ions to help enhance its affect. Furthermore, once the depolarization has been created, the positively charged sodium ions on the inside on the membrane are attracted to the rest of the negatively charged resting membrane. Consequently, the action potential moves along the channel and the message begins transmission through the Glossopharyngeal nerve to the brain with the stimulus from all other taste receptors. The larger the concentration of salt on the tongue, the greater the depolarization.
What defines a food that will taste salty? Well, the taste is best defined by the presence of cations. As for sodium might be the most well known or popular salt, other positively charged cations can be classified as salty tasting too. For example, calcium, potassium, lithium, rubidium and cesium ions all have the ability to pass through the cation channel designated to perceive salts. However, not all cations taste as salty as sodium. The closer that the ion resembles sodium, the more salty it will taste. Hence the reason why food containing molecules with cesium or rubidium might taste barely salty at all, whereas lithium and potassium more closely resemble the taste of sodium.
What defines a food that will taste salty? Well, the taste is best defined by the presence of cations. As for sodium might be the most well known or popular salt, other positively charged cations can be classified as salty tasting too. For example, calcium, potassium, lithium, rubidium and cesium ions all have the ability to pass through the cation channel designated to perceive salts. However, not all cations taste as salty as sodium. The closer that the ion resembles sodium, the more salty it will taste. Hence the reason why food containing molecules with cesium or rubidium might taste barely salty at all, whereas lithium and potassium more closely resemble the taste of sodium.
Sour Taste
Sour tasting foods generally have a negative connotation, meaning that compared to other tastes, it is commonly despised or less easily consumed in foods. This is because sour tasting foods are normally resultants of acids, and the consumption of acids tends to affect the delicate pH balance of our bodies. So, by limiting the pleasure that is associated with this taste, our body is protecting its pH balance. The sour taste originally comes from acidic foods, in which their molecules dissociate into proton/positive hydrogen ions. Recently a channel given the name PKD2L1 has been discovered as an acid-sensing channel. This channel belongs to the TRP (transient receptor potential) ion channel family and therefore a non-selectively permeable cation channel. Therefore, dissociated acids on the tongue flow through a proton channel into presynaptic taste cells where they furthermore cause excitation and release a transmitter to the brain.
Common chemical molecules that dissociate into hydrogen ions to characterize a sour or tangy taste are carboxylic acids which have a distinctive carboxyl functional group, -COOH. The carboxyl group makes the acids very polar, and since likes dissolve in likes, they readily soluble in water. As well, the double bonded oxygen and the hydroxyl group require a greater amount of energy to be broken so the acids typically have high melting and boiling points. These organic compounds and generally weak acids can be found in citrus fruits and can also depict the taste of wine vinegars, sour mild and yogurt, and the gamey taste of meat. Primarily, these tastes come from the carboxylic acid, lactic acid, which is produced by a bacteria culture. Some other common carboxylic acids that are found in various food we eat include oxalic acid which is found in spinach and rhubarb leaves, citric and ascorbic acids found in many fruits and vegetables and acetylsalicylic acid which helps to compose Aspirin.
The moment that these acids enter the mouth and become subjected to saliva, their immediate breakdown is able to be recognized by the taste buds. For example, if food or drink is consumed that contains a carboxylic acid at the same time as food or drink that contains molecules with as alcohol group, a reaction known as esterification may occur in which the two molecules react to produce an ester and water. Compared to the alcohol and the acid, the ester may stimulate a completely different taste receptor, and therefore the combination of the two things consumed at once would result in a completely different taste. However, it is most common that when acids or foods containing molecules like the carboxylic acid are consumed, they will dissociate in water/saliva. Strong acids, such as HCl, HBr, and H2SO4 will undergo complete dissociation in water to result in the hydronium ion, H3O, in which one proton is donated to the water. Carboxylic acids on the other hand are considered to be weak acids due to the fact that they partially dissociate in water. The pi bonding between the carbon and the electronegative oxygen, and the sigma bond between the same carbon atom and a second electronegative oxygen, followed by the hydrogen, allow for the hydrogen to be pulled from the molecule and join the water molecule. Thus consequently, the dissociated hydrogen ions from both strong an week acids then travel through the proton channel to stimulate an action potential, then transmitting a signal to the brain which then interprets the taste as sour. The lower the pH and greater acidity of the saliva mixture, the more sour the consumed food or drink will be interpreted as.
The moment that these acids enter the mouth and become subjected to saliva, their immediate breakdown is able to be recognized by the taste buds. For example, if food or drink is consumed that contains a carboxylic acid at the same time as food or drink that contains molecules with as alcohol group, a reaction known as esterification may occur in which the two molecules react to produce an ester and water. Compared to the alcohol and the acid, the ester may stimulate a completely different taste receptor, and therefore the combination of the two things consumed at once would result in a completely different taste. However, it is most common that when acids or foods containing molecules like the carboxylic acid are consumed, they will dissociate in water/saliva. Strong acids, such as HCl, HBr, and H2SO4 will undergo complete dissociation in water to result in the hydronium ion, H3O, in which one proton is donated to the water. Carboxylic acids on the other hand are considered to be weak acids due to the fact that they partially dissociate in water. The pi bonding between the carbon and the electronegative oxygen, and the sigma bond between the same carbon atom and a second electronegative oxygen, followed by the hydrogen, allow for the hydrogen to be pulled from the molecule and join the water molecule. Thus consequently, the dissociated hydrogen ions from both strong an week acids then travel through the proton channel to stimulate an action potential, then transmitting a signal to the brain which then interprets the taste as sour. The lower the pH and greater acidity of the saliva mixture, the more sour the consumed food or drink will be interpreted as.
Sweet Taste
Sweet tasting foods are generally interpreted very positively due to the fact that for evolutionary purposes, most of these foods must be consumed to obtain a sufficient amount of energy. The taste that we interpret as being sweet, compared to the stimulus of salty and sour tasting foods is activated by a complex as opposed to the depolarization of a channel. Substances, such as sucrose and other carbohydrates bind to G-protein-coupled receptors known as T1R2 and T1R3 receptors on the surface of the taste buds in the apical membrane. Once binded to the receptor, a chain reaction occurs as a G-protein known as Gusducin is activated, and in turn activates phospholipase C which carries on to generate IP3 and diacyl glycerol (see molecules below). As a result of the generation and activation of each of these three intracellular messengers, they work both directly and indirectly to depolarize the TRPM5 channel and create an action potential. TRPM5 stands for transient receptor potential cation channel, which is a common channel activated by and transmitting messages for the stimulation of sweet, bitter and umami taste receptors. As a result, similarly to the depolarization of the sodium and hydrogen ion channels, the messengers work to depolarize the channel by transforming the negative interior charge of the resting membrane to a positive charge. Activated messengers such as the IP3 and the diacyl glycerol cause a protein “kinase” to be activated which in turn closes the potassium ion channel into the TRPM5 channel, resulting in excess potassium ions that increase the positive charge inside the cell. This furthermore triggers positive calcium ions to enter the TRPM5 channel through depolarized, activated calcium ion channels to increase the rate of the depolarization and to create a greater charge reversal. Furthermore, once the depolarization has been created, the positively charged sodium ions on the inside on the membrane are attracted to the rest of the negatively charged resting membrane. Consequently, the action potential moves along the channel in which the cells are found to synapse upon the glossopharyngeal and chorda tympani nerves to the brain. Last but not least, a hormone know as Leptin is used to inhibit the stimulus of sweet cells by opening positive sodium ion channels which hyperpolarizes the cell by making it even more negatively charged, so that when the messengers attempt to depolarize the channel the charge does not surpass the threshold level to create an action potential.
What you are now wondering is, “what type of chemical molecules attach to the T1R2 and T1R3 sweet receptors?”. The answer is sugars, which may refer to any type of monosaccharide or disaccharide. Monosaccharides being simple carbohydrates and disaccharides being complex carbohydrates composed of two monosaccharides. In other words, simple sugars and complex sugars. Two common examples of these sugars are sucrose and fructose. Fructose being a basic monosaccharide, and sucrose which is a common table sugar being a disaccharide composed of glucose and sucrose combined. It is important to understand that all monosaccharides fall into the category as aldoses or ketoses which are also technically aldehydes and ketones. For example, glucose is an aldehyde sugar and fructose is a ketone. Therefore, glucose has a double bonded oxygen atom attached to a carbon that is attached to one carbon and one hydrogen. Whereas fructose has a double bonded oxygen attached to a carbon that is attached to two other carbons. As a result of the carbonyl group, these sugars have fairly high melting and boiling points since a lot of energy is needed to break their bonds. Some artificial sweetners are composed of some very different structural compounds, including Saccharin, cyclamate, aspartame, sucralose and plant proteins such as monellin and thaumatin. (see diagrams below)
Bitter Taste
The bitter taste is generally a reaction to the toxins and poisons found in our food. Thus, our gustatory systems are naturally built to recognize such molecules as being potentially hazardous in order to protect our bodies and digestive systems from dangerous substances. Unlike sweet and sour tasting foods, those that are bitter have something in common with the interpretation of sweet tasting foods, in which they too are comprised of compounds that break down from saliva and fit into a receptor complex like two puzzle pieces to stimulate the taste bud. Bitter substances/chemicals bind to receptors known as T2R receptors. Interestingly, these bitter receptors have also been found to exist on the cilia and smooth muscle cells of the bronchi and trachea. This quite possibly could be to protect the body from inhaled poisons that would most likely share similar traits to a bitter tasting molecule, so that a message would be sent to the brain to expel. Once they are in place, a chain of activations commences before any nerve is stimulated. Primarily, the substances that bind with this bitter receptor include quinine and phenylthiocarbamide, also known as PTC. PTC (see diagram of molecule below), is a molecule belonging to the organic amide family because its amino group extends from a carbon atom with a double bond to another molecule. With a combination of the amino group, the double bonded sulfur atom and the benzene functional group, PTC is polar in nature and therefore soluble in water because the N-H bonds also have the ability to form hydrogen bonds with water. As humans, we have a total of twenty five different T2R receptors, and each one only slightly differentiates in its preference from bitter molecule to molecule. Following the fit of the molecule to the receptor, alike the process involved in sweet tasting molecules, the G-protein known as Gusducin is activated which furthermore progresses to activate phospholipase C and in turn initiates the generation of diacyl glycerol and IP3. The production of these two messengers is from the hydrolysis of a chemical molecule known as phosphatidylinositol-4,5-bisphosphate (PIP2).
Again, similar to the activation of the TRPM5 (transient receptor potential) ion channel for sweet foods, the messengers work to depolarize the channel by transforming the negative interior charge of the resting membrane to a positive charge. Activated messengers such as the iodine triphosphate and the diacyl glycerol cause a protein “kinase” to be activated which in turn closes the potassium ion channel into the TRPM5 channel, resulting in excess potassium ions that increase the positive charge inside the cell. This furthermore triggers positive calcium ions to enter the TRPM5 channel through depolarized activated calcium ion channels, to increase the rate of the depolarization and to create a greater charge reversal takes place and a depolarization occurs. Once the depolarization has been created, the positively charged sodium ions on the inside on the membrane are attracted to the rest of the negatively charged resting membrane. As a result, the abolishment or reduction if the G-protein, Gustucin, would be used to reduce the taste sensitivity to bitterness because without its activation, nothing else would happen to further the stimulation.
Apart from quinine and phenylthiocarbamide, a few other common known bitter molecules include cycloheximide, denatonium, 6-n-propyl-2-thiouracil and β-glucopyranoside.
Umami Taste
Out of the five tastes, umami was the fifth one to be discovered and it is still debated among many scientists whether or not it should have its own classification. Similar to sweet tasting foods, this taste is generally classified as being a good taste because for evolutionary reasons, foods containing "Umami" classified molecules are good sources of useful energy. Umami taste is defined as “savory,” and primarily originates from the binding of several specific amino acids and nucleotides (namely salts of glutamic acid) with the receptor complex. Similar to that of sweet and bitter tasting foods, the molecules bind of G-protein coupled receptors known as T1R1 and T1R3. Again, through activation of the G-protein, phospholipase C, IP3, and diacyl glycerol. Additionally, to increase the effects of the taste, guanosine 5'-monophosphate (GMP) and inosine 5'-monophosphate (IMP) commonly bind to a secondary site on the T1R1 receptor to enhance the effects of the molecule binding to the main site. After the receptors have activated the secondary messenger, the non-selective cation channel, TRPM5, opens to begin depolarizing the cell and allowing for the positive calcium ions to enter and speed up the process. See the process described under either sweet or bitter taste for a more in depth description of the action potential creation.
The umami taste comes primarily from meats such as chicken, steak and fish. It is most commonly associated with a molecule known as monosodium glutamate which is frequently added to foods to enhance their flavor, along with inosine monophosphate and guanosine monophosphate (CMP) which also bind to the receptor in a different place to enhance the effect. Monosodium glutamate is also the main ingredient in substances like soy sauce, and not only does it activate umami receptors, it has been discovered to also activate an internal receptor in the taste cell called the NMDA receptor which is an ion-channel complex that open up and allows the entrance of sodium and calcium ions to depolarize the receptor cell so that a smaller amount of other umami molecules are needed to complete the stimulation.
Glutamic acid (see carboxyl group) has two carboxyl groups and one amino group. Therefore it is a polar molecule with the ability to form hydrogen bonds with water between the oxygen and hydrogen molecules. As well, the double bonded carbonyl groups require more energy to be broke, resulting in a high melting point.
Monosodium glutamate, as can be seen, the molecule is practically identical to that of the glutamate acid, except for the hydrogen from one of its carboxyl groups has been removed. This can occur after the acid has been reacted with a base (NaOH) to form an acid and an alcohol, through saponification or hydrolysis.