Our brains are complex structures in which many
processes happen -- more or less simultaneously, more or less
independently. We perceive ourselves as some sort of intelligence
localised a few cm behind the eyeballs, looking out through them as
through a computer screen, yet the brain functions that contribute
to our self image are scattered all through our skulls. There are
automatic functions that never reach our consciousness at
all. We don’t know, don’t care, and pretty much
cannot influence the details of our digestive systems, our blood
system, and so forth.
Let’s call the “I” portion
of the brain our ‘ego’. Ego would like to think
that it’s in charge (just as some husbands think that they
run their families). But there is evidence – somewhat
controversial – that says more primitive parts of the brain
“decide” on an action first, after which ego lumbers
along pretending that it wanted that action all the time.
Steven Pinker (http:
//www.time.com/time/printout/0,8816,1580394,00.html) quoted the
cognitive psychologist Bernard Baars that consciousness is “a
global blackboard on which brain processes post their results and
monitor the results of the others.”
Most textbooks show the ego dominating the
“lower” portions of our brain.Yet to be brutally honest, we must admit that we often
have an immediate response to a situation or person, and
then our intellect gets into the action by rationalizing
reasons for or against. It’s self-deception, and the
“reasons” are fabrications! Try arguing with a
person vehemently opposed to something, be it GM food, homosexual
marriage, abortion, stricter drug laws, or looser drug laws.
He (or she) will have numerous reasons to support his opinion, yet
no matter how many of these reasons are negated during your
argument, it will not change his mind. Obviously, the
“reasons” that purportedly led to his original opinion
were merely retroactive window-dressing.
This interaction between instinctive
pre-wired responses and emotion/logic is especially obvious when
food is involved. Choosing the right food is critical to our
survival. Do we spit out what’s in our mouth, or do we
swallow it? To answer this question, our taste buds analyse each
bite for both good and bad chemicals. Taste bud results go to
primitive parts of our brain, which determine what action to
take. Much later, our higher
bitsput words to these taste-bud
sensations. Although this esthetic analysis can be enjoyable,
it’s not essential. Fruit flies reject liquids
containing caffeine without having to think, “I dislike
coffee”. The low-level mechanisms for tasting are, to
me, more interesting and more important than any cookbook analysis
of “flavours”. As new research elucidates the details,
we see the complexity of living systems, especially the way that
components are re-used in all sorts of novel
combinations.
Each taste bud on our tonguehas about 50-100 sensory cells, and each cell
specialisesin just one component. The
sensing mechanism is located at the outer end of the cell, while
the inner end connects to the corresponding nervefibre: sweetness sensor
cells connect to sweet-reporting nerves, and so forth. The idea
that we taste sweetness in one part of tongue, saltiness elsewhere,
is quite false.
Conventional wisdom acknowledges five taste
components (“taste qualities”) detected by the taste
buds of our tongues. (For some reason, the official list omits
attributes like chili “heat” (pedas in
Malaysian) of chilies and viscosity.)
· Saltiness
· Sourness
· Sweetness
· Umami
· Bitterness
There are two different mechanisms for
sensing: Both saltiness and sourness (acidity) involve
structures in the cell membrane that allow penetration by sodium
chloride or protons, and a single family of related proteins is
involved.
On the other hand, sweetness, bitterness, and umami
detectors involve another family of proteins. Moreover, these
receptors don’t allow the target chemicals to penetrate.
Instead, the cell membrane “fishes” with dangling
chains of proteins that match (lock and key) with the target
chemicals dissolved in saliva. A match with a target chemical
leads to a shape change, which then triggers a series of reactions
within the cell, ultimately leading to a nerve being fired
off.
Life is parsimonious. Any useful tool,
once evolved, gets re-used again and again. The same
selective biochemistry found in our taste buds is duplicated inside
our bodies, where selective detectors monitor what’s
happening internally. This makes sense; if you have a
manufacturing process you don’t merely analyse what’s
going into your reactor, but you also measure what’s
occurring inside. These internal status reports do not
normally elicit any “taste” sensations in our
consciousness. Admittedly, if gut bitterness detectors in the
gut report a toxic chemical that got past the tongue’s
screening system, we’ll be uncomfortably aware of the urgent
need to eliminate that toxin. Even then, we don’t think
of our upset digestion as having “bitter taste”.
The non-taste uses of tasting mechanisms will be summarized after
the individual tastes are discussed.
We need about a gram of salt a day to
survive. The taste buds that specialize in salt detection
have, at one end, protein complexes with pores that are just the
right size to admit sodium chloride (salt). When that chemical
reaches the cell interior, it triggers a response on the
appropriate nerve, informing the brain that what’s in our
mouth is “salty”. The proteins that allow salt to
penetrate the cell membrane are called “PKD” proteins,
which stands for “Polycystic Kidney Disease”. Mice and
people with genetic defects affecting PKD are susceptible to kidney
and liver disease.
My opinions on the current anti-salt campaign are elsewhere on
this website: Is Salt Really that
Evil?
Sourness: Sourness, or acidity, is
related to pH, the concentration of hydrogen ions in a
solution. If what’s in our mouth is sour, it might be
from spoiled or unripe food, so these taste cells would vote
against swallowing that food. The sourness-detecting taste bud
cells have pores that permit hydrogen ions to pass directly into
the cell interior. Pore formation involves the
PKD2L1 member of the plycystic kidney disease proteins.
Thus a common family of related proteins detects both saltiness and
sourness.
Sweetness-tasting cells have receptor proteins on the external
surface of their cell membranes. These proteins have lengthy
peptide strands that project into the liquid surrounding the cells
(like the food-catching fronds of many marine animals). These
strands can interact with “sweet” chemicals like
sucrose, without allowing the target chemical to penetrate the
cell. Presumably the receptors change shape, thereby triggering
reactions that ultimately send a signal to a sweet- reporting
nerve.
The significance of detecting sweetness is, of
course, that the food in our mouths might be fruit. This
sense is only approximate, since different sugars with identical
caloric values, such as glucose, fructose, and mannitol, have
different sweetness intensities. It’s
probably not physiologically important for us to know exactly what
sugar we’re eating; just that what’s in our mouth
probably has useful energy. Non- carbohydrate chemicals like
saccharin and aspartame can also fool the sweetness detector, but
that seems to be an evolutionary accident.
Sweetness sensitivity is regulated by a group of
similar genes collectively known as the TR1 family. There are
three T1 receptor proteins, with the inspired names of T1R1, T1R2,
and T1R3. When both T1R2 and T1R3 are present in a taste
cells, it is sensitive to low concentrations of sugars. On
the other hand, uncombined T1R2 or T1R3 can only respond to very
high concentrations of sugar.
Carnivorous felines like cats and tigers
don’t have sweetness sensors, because of a natural deletion
of the T1R2 gene. Sugar levels are obviously not important to
feline predators. But why, then, are chickens also
unable to detect sugars?
Our taste buds evolved before saccharin and
aspartame were discovered, and our sensitivity to these additional
sweeteners seems to be accidental. Evidence for this is that
mice can taste sugars but not aspartame nor monellin (a naturally
sweet protein from the serendipity berry, Dioscoreophyllum
cumminsii). It seems that the elongated detection frond
of human T1R2 protein has one domain that fits natural sugars, and
one or more domains that happen to fit other sweetening agents.
Across the animal kingdom about twenty forms of T1R genes have been
found, but it's not clear how they affect response to different
sugars.
"Umami" is a hard-to-define sensation of
“richness”, “satisfaction”, “mouth-
filling”, “meatiness”. Umami-tasting cells
have surface proteins with receptor sites just right forglutamicacid to fit
in. (Aspartic acid, a four-carbon analog of glutamate, may
also have weak umami activity.) Nutritionally, a food that is high
in glutamate is usually high in protein. The umami and the
sweetness receptors are genetically very similar: a combination of
T1R1 and T1R3 gives rise to an umami receptor. (T1R3 is common to
both sweetness and umami sensing.)
The ability to sense glutamate seems to be a late
arrival in evolution. Rats are brilliant at detecting toxins
(“bitterness”), in part because they cannot vomit, so
it's doubly important for them to avoid eating dangerous levels of
natural toxins. They cannot, however, distinguish between sweetness
and umami. Perhaps distinguishing between protein-rich and
carbohydrate-rich food is not too important for these rodents.
Our umami-sensing cells also react to
ribonucleotide monophosphates, such as inosine mono phosphate
(IMP), perhaps by making the T1R1/T1R3 combination more
sensitive. Since ribonucleotides are usually found in muscles
from dead animals, this may have evolved as an added incentive for
omnivores to eat animal protein. Similar ribonucleotides occur in
some dried mushrooms; is this a mushroom’s way of encouraging
foraging animals to spread mushroom spores?
Grazing animals such as sheep and cattle eat more
straw if it’s been sprayed with glutamate and/or salt, but no
one has ever tested their response to ribonucleotides. My
prediction is that these obligate vegetarians will not pay any
attention to the ribonucleotide monophosphates.
(Wouldn’t this be a good project for a Young Farmers’
group?)
Glutamate-rich foods include tomatoes, dried
mushrooms, Italian cheese, peas, asparagus, and anchovies, as a
partial list. There are glutamate-rich cooking ingredients like
fish sauce, soya sauce, fermented shrimp, anchovies, Marmite, and
Vegemite. And, of course, monosodium glutamate (MSG), the
favourite hate of the chemically illiterate. Personally, I
rarely use MSGbecause I employ other
umami-rich (glutamate-rich) ingredients with more complex
flavours. On the other hand, MSG can be safer in Italian
cooking than strongly flavoured anchovies.
The sensation we call “bitterness” is an advance
warning that the food in our mouth might be poisonous. Each
bitter-sensing cell has at least twenty different surface proteins
that interact with different classes of toxins. There are numerous
dangerous natural chemicals, each group apparently needing a
different biochemical method for detection. Regardless of
which particular sensing protein is activated, the signal to our
brain goes via one type of bitter-reporting nerve, so all bitter
chemicals taste the same: -- caffeine and quinine, for
instance. We don’t need to know the detailed chemistry
of toxins in order to spit the contaminated food out.
Animals vary in how extensive a repertoire of
toxins can be detected. Across a number of animals (fish,
amphibian, bird, and mammals) there are 167 versions of T2R genes,
the family of bitterness/toxin sensors. No animal has a
complete repertoire. In fact we don’t know what toxins most
of these genes code for.
Frogs have 49 different genes for toxin detection,
mice have 33, but chickens have only three! Why doesn’t
the chicken, an animal with a wide range of feeds in the wild, have
a better apparatus for detecting toxins? Perhaps the speedy
chicken digestive system (less than two hours from input to exit)
doesn’t allow much time for many toxins to act.
Chickens (like cats) cannot detect sweetness either. Any
karma-burdened gourmand who is reincarnated as a chicken will be
amply punished for his sins! (Shi and Zhang, 2005)
Fruit flies have the exact same caffeine-detecting
sensor that humans have! Hungry wild fruit flies, even when
thirsty, won’t drink sugar solutions to which caffeine has
been added. However, mutant flies with this gene missing,
will sip from caffeine-laden liquid. That’s an
incredibly deep evolutionary link, when fruit flies and humans
share exactly the same detection mechanism. The missing
caffeine-gene doesn’t stop the fruit flies from avoiding
other bitter chemicals.
There are bitterness-detecting sensors in our nose
and in our intestines. Some toxins might be chemically masked
by, say, an attached sugar. When the sugar is stripped off
during digestion, the dangerous toxin would be revealed. It’s
not too late: the potential poison can be eliminated by either
vomiting or diarrhoea.
The sourness mechanism is also involved in
maintaining the pH of the cerebrospinal fluid within close limits.
Acid-sensitive PKD2L1-linked nerve cells exist along the whole
length of the spinal column (Huang et al, 2006). However, we
don’t consciously “taste” the sourness of our
spinal fluid; the control mechanism works out of sight.
Additional pH sensors maintain blood pH and, in the small
intestine, counteract the acid material coming from the
stomach. Whether the PKD genes are involved here is not yet
known.
The same sugar- detecting proteins found in our
tongues are also present in our guts (Dyer et al 2005).
Imagine you’ve just eaten something starchy. The
initial attack by salivary starch-degrading enzymes could activate
the sugar-receptors in our intestines, and that could trigger
release of additional starch-degrading enzymes.
Glutamate- receptor proteins are also found in our
intestinal lining. Since glutamate is a major component of most
proteins, even partial digestion of a protein-rich food would be
enough to signal for even more proteolytic enzymes.
Pancreatic fluid (about a liter a day for adults) is the major
source of protein-digesting enzymes in the gut. Rodent experiments
have confirmed that glutamate either in the mouth or in the
intestine, stimulates activity of nerves leading to the
pancreas.
Intestinal walls prefer glutamate to any other
food, so less than 5% of ingested glutamate ever reaches the rest
of the body. The presence of a glutamate-sensing protein in
the intestine walls could both switch on glutamate uptake and
signal the pancreas to release more proteolytic enzymes.
This is an oversimplified picture. There are more ancient, less
sensitive and less selective sensing mechanisms that can take over
when the main sensors are out of action. For instance, animals with
T1R1 and T1R3 genes knocked out can still detect umami (Bradbury,
2004). Sourness (acidity) sensing seems to involve both a
proton-selective channel (pH) and another channel(s) that picks up
weak organic acids like acetic acid (vinegar). Although a
sensitive sodium-chloride sensor accounts for three-fourths of our
salt sensing, there is an alternative mechanism.
Our responses to possible tastes vary enormously.
Some people have 100 times more taste receptor cells than others!
(Scott and Verhagen, 2000; get web reference) Some people who
have a double dose of the gene to taste the bitter chemical
propylthioruacil are more likely to dislike broccoli.
Taste components combine in unexpected ways. Umami
glutamate can intensify the saltiness of a low concentration of
salt, while suppressing the intensity of a sour acid. Tomato
varieties thought to be “not acid enough”, are made
palatable by adding extra sugar rather than extra acid. The
addition of vanilla flavouring to caffein-spiked milk unexpectedly
enhanced bitterness.
In living cells, simplicity and efficiency are not as important
as ruggedness. When a new-and-improved enzyme or sensor evolved,
the old enzyme or sensor is not discarded but continues to be
available. This is not the way advocated by efficiency experts. If
Life had been designed by a Cost Accountant, Earth would be a
lifeless planet, where the never-ending wind would stir up bits of
paper on which could still be read, “Plans for Simplifying
Cell Metabolism and Removing Duplicated Genes”.
(Only a limited selection; interested readers
can find many more on Google.)
Jane Bradbury. Taste perception: cracking the code.
2004. PLoS Biol. 2004 March; 2(3): e64.
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3681
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Jayaram Chandrashekar1, Mark A. Hoon2, Nicholas J. P. Ryba2 and
Charles S. Zuker1. The receptors and cells for mammalian taste.
2006. Nature 444, 288-294 (16 November 2006)
http://www.nature.com/nature/journal/v444/n7117/full/nature054
01.html
J. Dyer, K.S.H. Salmon, L. Zibrik and S.P. Shirazi-Beechey1
Expression of sweet taste receptors of the T1R family in the
intestinal tract and enteroendocrine cells. Biochem. Soc. Trans.
(2005) 33, (302–305)
http://www.biochemsoctrans.org/bst/033/bst0330302.htm
Scott TR, Verhagen JV. Taste as a factor in the management of
nutrition. Nutrition. 2000 Oct; 16(10):874-85
Sherry Seethaler. UCSD-led team discovers how we
detect sour taste. 2006. http://www-
biology.ucsd.edu/news/article_082306.html
