Today on ScienceSketch the chemical weapons in your food.
This is not a nefarious plot by chemical companies to kill you, rather they're chemical compounds
made by plants to defend themselves.
Let's face it.
Life as a plant is not easy.
You're fixed in one spot, and if an animal tries to eat you, you can't scare it away.
You can't run away.
There's no physical way to fight back.
Plants use a different strategy.
They use a chemical strategy.
Over millions of years of natural selection, plants have developed chemical defenses to
protect themselves from pathogens, microbes, hungry insects, and vertebrates.
In their fruit, leaves, and roots, plants produce a complex suite of molecules, called
phytochemicals.
These include many essential nutrients and have been the source of life-saving medicines.
But often these chemicals are toxic, bitter, or noxious, and made to keep herbivores at
bay.
In their quest for food, animals learn to keep their distance from dangerous plants,
and those that don't learn the lesson, may get sicker, weaker and eventually perish.
As we shall see, humans, perversely, find many of these noxious chemicals appealing
and use them to make food more interesting.
Activating the heat receptors in the mouth is the fastest and most direct way to tell
an herbivore, "Don't eat me!"
Plants in the chili pepper family produce a chemical called capsaicin which activates
the heat receptor called TRPV1.
Mammals have the TRPV1 receptor, but birds and reptiles do not.
Perhaps dinosaurs did not have them either.
But who knows?
Another chemical defender found in food is allyl isothiocyanate.
This activates the TRPA1 pain receptor in the nasal passages.
It is found in mustard seeds, radishes, horseradish and wasabi.
And lesser amounts in vegetables in the Brassica family (cabbage, brussel sprouts, broccoli
and kale).
If you've ever eaten a dallop of wasabi or a Dijon mustard, you're familiar with the
burn from allyl isothiocyanate.
This compound is highly reactive owing to the isothiocyanate group.
It is so reactive that plants store it as a precursor compound, called glucosinolate,
and sequester it in a compartment.
When the root or leaf is crushed, the glucosinolate is released which reacts with an enzyme, myrosinase,
cleaving the glycoside bond, giving the intermediate which rearranges to allyl isothiocyanate.
In this way, the plant only makes the chemical weapon when it is attacked by an animal or
a wasabi maker.
Another strong irritant in the chemical weapon arsenal is found in onions.
Propanethial S-oxide is a powerful lachrymator.
As onions are crushed or sliced, isoalliin is converted by an enzyme, alliinase, to make
an intermediate which is further acted upon by lachrymatory factor synthase to make the
offending agent.
This chemical readily diffuses in the gas phase creating a cloud of tear gas.
Everyone knows that cyanide is poisonous, but few realize it is present in many foods
that we eat.
Apple seeds, apricot and peach pits, and bitter almonds deliver cyanide, a small molecule
composed of a carbon triple bonded to a nitrogen.
Plants do not store free cyanide which could be dangerous to the plant cells, but instead
store a benign substance that releases cyanide when acted upon by an enzyme.
These seeds contain amygdalin, a cyanogenic glycoside.
After being chewed up, beta glucosidase cleaves the glycoside bond releasing an intermediate
which is further acted upon by hydroxynitrile lyase producing cyanide and benzaldehyde (which
has a pleasant fruity-nutty flavor).
Cyanide poisonings from food are rare, but they do happen occasionally.
Bitter almonds, which are banned for sale in the US contain about 6.2 milligrams cyanide
per almond, making an intake of 60 bitter almonds a fatal intake for an adult.
Cyanide binds to heme iron in cytochromes and hemoglobin, shutting down oxygen transport
and the electron transport chain in mitochondria.
Solanine is a natural insecticide found in plants of the nightshade family including
tomatoes, eggplant, and potatoes.
Solanine is a neurotoxin inhibiting acetyl cholinesterase and disrupting ion channels.
Fatal poisonings are rare, high exposure to solanine can cause nausea, vomiting, and headaches.
But in more severe cases, victims can experience hallucinations, loss of sensation, and paralysis.
The FDA regulates the amount of solanine in potatoes to 20 mg per 100 g, and solanine
levels are usually low, but exposure to sunlight will turn potatoes green and increase solanine
levels significantly.
Potatoes should be stored in the dark, and the green ones should not be eaten, even if
cooked.
Almost all plants have some form of chemical defense, and this video has only touched on
a few examples from the wild world of phytochemicals.
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