Why do tobacco plants make nicotine, which affects the human brain? What’s in it for them?

The short answer is that tobacco plants make nicotine to discourage insects from eating them (it is poisonous to them), so what one really has to do is make a connection between insects and people.

Given the huge variety of life, it is often hard to remember that it is all put together in very similar ways. The fundamental mechanism of heredity based on DNA is common across all life, as is most of the core biochemistry that goes on. Among other things, that means that the basic mechanisms by which nerves work are similar across animal species.

Nerves have receptors to which various chemicals can bind, changing how the nerves operate. Nicotine binds to receptors called nicotinic acetylcholine receptors, which means that in addition to being activated by the neurotransmitter acetylcholine, which nerves use to tell other nerves to fire, they are also activated by nicotine. Simplifying things a bit, nicotine often acts as a stimulant by making nerves fire more.

Now imagine what would happen if you ate an amount of tobacco the size of your head and scale that down to an insect eating an amount of tobacco about the size of its head. Even if you smoked that amount of tobacco, you would be a mess, but with smoking, only a small fraction of the nicotine survives the burning process. If you ate that much, you would certainly die. For an insect it’s even worse because they are more sensitive to nicotine.

When people use nicotine in smaller quantities they are basically taking small doses of what evolved to get an insect’s nervous system firing so much that it dies. Knowing this, you can make a pretty good, cheap, biodegradable insecticide by putting some tobacco into a sprayer filled with water and letting it sit for a while.

Nicotine in pure form is an oily liquid, which gets its name from the scientific name for the common tobacco plant, Nicotiana tabacum. That name came from Jean Nicot de Villemain, a French diplomat who sent tobacco from Brazil to Paris in 1560.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.

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