How Do We Smell a Flower?

Q: How do we smell a flower?
Susan K. and her father, Ron, Albuquerque, New Mexico

A: You smell the sweet scent of a gardenia because molecules escape the flower's surface, and float into the air where your nose can detect them. A smell travels from gardenia to nose because molecules in the air bop around, banging into other molecules like zigzagging, tiny pool balls. Gardenia molecules eventually reach your nose. A gardenia smells stronger when it's warmer because warm molecules move faster. Faster molecules have more energy to escape the surface and so more do.

Smelling is a strange sense: not like seeing or hearing. You can see a far-away object because you receive its light waves. Likewise you hear something from afar by receiving sound waves. Smelling--perceived in the most primitive part of our brain--is an ancient sense, chemical in nature.

Let's return to the gardenia. You breathe in the gardenia molecules that meandered to your nose. A paltry five percent (by volume) of your nasal cavity receives the gardenia molecules and smells them. The smellers are postage-stamp sized, thin patches of yellowish gray moist tissues. One on each side of the nose, they are located behind and barely beneath the bridge of your nose. They line the upper part of the nasal cavity directly below the brain. These are the smell targets. Only about two to ten percent of the inhaled gardenia molecules reach the target. There they dissolve in the wet mucus that coats the target lining so they can reach the smell receptors.

Tiny hair-like threads--the smell receptors--cover the lining, dangle into mucus, and reach out like antennae for matching molecules. The hairs, made of protein and separated only by the thin goo layer from the outside world, are extensions of your brain. And they do get damaged in that exposed location. These unusual nerve cells last only about four to six weeks before new ones replace them. Most nerve cells never regenerate.

Some searching hairs--which respond chemically--detect, connect, and react to the gardenia molecules. If a receptor nerve cell reacts, it sends an ON signal. Otherwise its message is OFF. This begins a cascade of reactions that, in a fraction of a second, creates a perception of gardenia odor in the brain.

The olfactory circuitry can send signals corresponding to a theoretical limit of sixteen million different smells. In 1991 neuroscientists at Harvard and Columbia Universities discovered nearly 1,000 distinct odor receptors in humans. Moreover, our nerve cells can recognize and respond to some 10,000 chemical odors--an amazing repertoire.

(Answered Nov. 15, 2002; updated Sep. 27, 2007)