taste taste the gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor cells located on taste buds in the oral cavity, mostly on the tongue. taste, along with olfaction and trigeminal nerve stimulation (registering texture, pain, and temperature), determines flavors of food and other substances. humans have taste receptors on taste buds and other areas, including the upper surface of the tongue and the epiglottis. the gustatory cortex is responsible for the perception of taste. taste bud the tongue is covered with thousands of small bumps called papillae, which are visible to the naked eye. within each papilla are hundreds of taste buds. the exception to this is the filiform papillae that do not contain taste buds. there are between 2000 and 5000 taste buds that are located on the back and front of the tongue. others are located on the roof, sides and back of the mouth, and in the throat. each taste bud contains 50 to 100 taste receptor cells. taste receptors in the mouth sense the five taste modalities: sweetness, sourness, saltiness, bitterness, and savoriness (also known as savory or umami). scientific experiments have demonstrated that these five tastes exist and are distinct from one another. taste buds are able to distinguish between different tastes through detecting interaction with different molecules or ions. sweet, savoriness, and bitter tastes are triggered by the binding of molecules to g protein-coupled receptors on the cell membranes of taste buds. saltiness and sourness are perceived when alkali metal or hydrogen ions enter taste buds, respectively. the basic taste modalities contribute only partially to the sensation and flavor of food in the mouth—other factors include smell, detected by the olfactory epithelium of the nose; texture, detected through a variety of mechanoreceptors, muscle nerves, etc.; temperature, detected by thermoreceptors; and "coolness" (such as of menthol) and "hotness" (pungency), through chemesthesis. as the gustatory system senses both harmful and beneficial things, all basic taste modalities are classified as either aversive or appetitive, depending upon the effect the things they sense have on the body. sweetness helps to identify energy-rich foods, while bitterness serves as a warning sign of poisons. among humans, taste perception begins to fade at an older age because of loss of tongue papillae and a general decrease in saliva production. humans can also have distortion of tastes (dysgeusia). not all mammals share the same taste modalities: some rodents can taste starch (which humans cannot), cats cannot taste sweetness, and several other carnivores including hyenas, dolphins, and sea lions, have lost the ability to sense up to four of their ancestral five taste modalities. basic tastes the gustatory system allows animals to distinguish between safe and harmful food, and to gauge foods' nutritional value. digestive enzymes in saliva begin to dissolve food into base chemicals that are washed over the papillae and detected as tastes by the taste buds. the tongue is covered with thousands of small bumps called papillae, which are visible to the naked eye. within each papilla are hundreds of taste buds. the exception to this are the filiform papillae that do not contain taste buds. there are between 2000 and 5000 taste buds that are located on the back and front of the tongue. others are located on the roof, sides and back of the mouth, and in the throat. each taste bud contains 50 to 100 taste receptor cells. the five specific tastes received by taste receptors are saltiness, sweetness, bitterness, sourness, and savoriness, often known by its japanese name umami which translates to 'deliciousness'. as of the early 20th century, western physiologists and psychologists believed there were four basic tastes: sweetness, sourness, saltiness, and bitterness. the concept of a "savory" taste was not present in western science at that time, but was postulated in japanese research. by the end of the 20th century, the concept of umami was becoming familiar to western society. one study found that salt and sour taste mechanisms both detect, in different ways, the presence of sodium chloride (salt) in the mouth. acids are also detected and perceived as sour. the detection of salt is important to many organisms, but specifically mammals, as it serves a critical role in ion and water homeostasis in the body. it is specifically needed in the mammalian kidney as an osmotically active compound which facilitates passive re-uptake of water into the blood. because of this, salt elicits a pleasant taste in most humans. sour and salt tastes can be pleasant in small quantities, but in larger quantities become more and more unpleasant to taste. for sour taste this is presumably because the sour taste can signal under-ripe fruit, rotten meat, and other spoiled foods, which can be dangerous to the body because of bacteria which grow in such media. additionally, sour taste signals acids, which can cause serious tissue damage. sweet taste signals the presence of carbohydrates in solution . since carbohydrates have a very high calorie count (saccharides have many bonds, therefore much energy), they are desirable to the human body, which evolved to seek out the highest calorie intake foods . they are used as direct energy (sugars) and storage of energy (glycogen). many non-carbohydrate molecules trigger a sweet response, leading to the development of many artificial sweeteners, including saccharin, sucralose, and aspartame. it is still unclear how these substances activate the sweet receptors and what adaptational significance this has had. the savory taste (known in japanese as umami) was identified by japanese chemist kikunae ikeda, which signals the presence of the amino acid l-glutamate, triggers a pleasurable response and thus encourages the intake of peptides and proteins. the amino acids in proteins are used in the body to build muscles and organs, transport molecules (hemoglobin), antibodies, and the organic catalysts known as enzymes. these are all critical molecules, and as such it is important to have a steady supply of amino acids, hence the pleasurable response to their presence in the mouth. pungency (piquancy or hotness) had traditionally been considered a sixth basic taste. in 2015, researchers suggested a new basic taste of fatty acids called 'fat taste', although 'oleogustus' and 'pinguis' have both been proposed as alternate terms. sweetness the diagram above depicts the signal transduction pathway of the sweet taste. object a is a taste bud, object b is one taste cell of the taste bud, and object c is the neuron attached to the taste cell. i. part i shows the reception of a molecule. 1. sugar, the first messenger, binds to a protein receptor on the cell membrane. ii. part ii shows the transduction of the relay molecules. 2. g protein-coupled receptors, second messengers, are activated. 3. g proteins activate adenylate cyclase, an enzyme, which increases the camp concentration. depolarization occurs. 4. the energy, from step 3, is given to activate the k+, potassium, protein channels.iii. part iii shows the response of the taste cell. 5. ca+, calcium, protein channels is activated.6. the increased ca+ concentration activates neurotransmitter vesicles. 7. the neuron connected to the taste bud is stimulated by the neurotransmitters. sweetness, usually regarded as a pleasurable sensation, is produced by the presence of sugars and substances that mimic sugar. sweetness may be connected to aldehydes and ketones, which contain a carbonyl group. sweetness is detected by a variety of g protein coupled receptors (gpcr) coupled to the g protein gustducin found on the taste buds. at least two different variants of the "sweetness receptors" must be activated for the brain to register sweetness. compounds the brain senses as sweet are compounds that can bind with varying bond strength to two different sweetness receptors. these receptors are t1r2+3 (heterodimer) and t1r3 (homodimer), which account for all sweet sensing in humans and animals. taste detection thresholds for sweet substances are rated relative to sucrose, which has an index of 1. the average human detection threshold for sucrose is 10 millimoles per liter. for lactose it is 30 millimoles per liter, with a sweetness index of 0.3, and 5-nitro-2-propoxyaniline 0.002 millimoles per liter. "natural" sweeteners such as saccharides activate the gpcr, which releases gustducin. the gustducin then activates the molecule adenylate cyclase, which catalyzes the production of the molecule camp, or adenosine 3', 5'-cyclic monophosphate. this molecule closes potassium ion channels, leading to depolarization and neurotransmitter release. synthetic sweeteners such as saccharin activate different gpcrs and induce taste receptor cell depolarization by an alternate pathway. sourness the diagram depicts the signal transduction pathway of the sour or salty taste. object a is a taste bud, object b is a taste receptor cell within object a, and object c is the neuron attached to object b. i. part i is the reception of hydrogen ions or sodium ions. 1. if the taste is sour, h+ ions, from acidic substances, pass through h+ channels. depolarization takes place ii. part ii is the transduction pathway of the relay molecules. 2. cation, such as k+, channels are opened. iii. part iii is the response of the cell. 3. an influx of ca+ ions is activated. 4. the ca+ activates neurotransmitters. 5. a signal is sent to the neuron attached to the taste bud. sourness is the taste that detects acidity. the sourness of substances is rated relative to dilute hydrochloric acid, which has a sourness index of 1. by comparison, tartaric acid has a sourness index of 0.7, citric acid an index of 0.46, and carbonic acid an index of 0.06. sour taste is detected by a small subset of cells that are distributed across all taste buds called type iii taste receptor cells. h+ ions (protons) that are abundant in sour substances can directly enter the type iii taste cells through a proton channel. this channel was identified in 2018 as otopetrin 1 (otop1). the transfer of positive charge into the cell can itself trigger an electrical response. some weak acids such as acetic acid, can also penetrate taste cells; intracellular hydrogen ions inhibit potassium channels, which normally function to hyperpolarize the cell. by a combination of direct intake of hydrogen ions through otop1 ion channels (which itself depolarizes the cell) and the inhibition of the hyperpolarizing channel, sourness causes the taste cell to fire action potentials and release neurotransmitter. the most common foods with natural sourness are fruits, such as lemon, lime, grape, orange, tamarind, and bitter melon. fermented foods, such as wine, vinegar or yogurt, may have sour taste. children show a greater enjoyment of sour flavors than adults, and sour candy containing citric acid or malic acid is common. saltiness the simplest receptor found in the mouth is the sodium chloride (salt) receptor. saltiness is a taste produced primarily by the presence of sodium ions. other ions of the alkali metals group also taste salty, but the further from sodium, the less salty the sensation is. a sodium channel in the taste cell wall allows sodium cations to enter the cell. this on its own depolarizes the cell, and opens voltage-dependent calcium channels, flooding the cell with positive calcium ions and leading to neurotransmitter release. this sodium channel is known as an epithelial sodium channel (enac) and is composed of three subunits. an enac can be blocked by the drug amiloride in many mammals, especially rats. the sensitivity of the salt taste to amiloride in humans is much less pronounced, leading to conjecture that there may be additional receptor proteins besides enac to be discovered. the size of lithium and potassium ions most closely resemble those of sodium, and thus the saltiness is most similar. in contrast, rubidium and caesium ions are far larger, so their salty taste differs accordingly. the saltiness of substances is rated relative to sodium chloride (nacl), which has an index of 1. potassium, as potassium chloride (kcl), is the principal ingredient in salt substitutes and has a saltiness index of 0.6. other monovalent cations, e.g. ammonium (nh4+), and divalent cations of the alkali earth metal group of the periodic table, e.g. calcium (ca2+), ions generally elicit a bitter rather than a salty taste even though they, too, can pass directly through ion channels in the tongue, generating an action potential. but the chloride of