Shulou(Shulou.com)11/24 Report--

When you breathe, odor molecules are inhaled into the nose, captured by a layer of mucus, and finally taken to the top of the nasal cavity, an area called olfactory epithelium. This area contains special receptor sites that detect molecules and send signals to the brain.

We know that our sense of smell is captured by the odor molecules at the receptor site, causing these neurons to discharge. But because we cannot observe the receptors directly, we have to infer from the circumstantial evidence how they work. There are now two main theories about how odor receptors work: shape and vibration.

According to shape theory, odor receptors have a specific shape suitable for odor molecules, just like a key inserted into a lock. However, this theory is not entirely valid, because we only have about 300 different receptors, but we can detect about 10000 different odors, so there must be some more complex principles behind it.

The latest theory is that each receptor matches only one part of the molecule, so any molecule with that part has a similar smell, such as any molecule with hydrogen sulfide bonds that smells like rotten eggs. However, this theory does not explain everything. Some molecules are made up of exactly the same groups, but they are arranged in different ways, but their odors are very different. For example, vanillin and isovanillin have the same molecular groups, but in different order, but the former smells like vanilla, while the latter has a very foul and disgusting smell.

So there is another theory of olfactory receptors: vibration theory. In the vibration model of odors, odor receptors can distinguish different molecules according to their vibration patterns. Each chemical bond has a certain resonance frequency, which vibrates naturally. It's kind of like guitar strings always resonate at the same frequency, so they always give you the same pitch.

Different molecules have different vibrational frequency feature sets, depending on what atoms they are made of and how they are connected. In the past, scientists used this property to use light to calculate the chemical composition of molecules in Raman spectra. When we illuminate a beam of molecules with a laser, the light is absorbed to make the molecular bonds vibrate and then emit light at different frequencies. We can look at the frequency of this new light and figure out what these molecules are made of.

This is a good way to detect different kinds of molecules, and maybe our odor receptors are doing something similar. But over the years, the vibration theory of odors has not been very popular because our noses cannot perform Raman spectra and our noses are in the dark.

But there is another way to detect the vibration of molecules, which uses the quantum tunneling effect of electrons. Quantum tunneling is a phenomenon in which quantum particles such as electrons can travel beyond the reach of ordinary "classical" particles, and they can jump over barriers by disappearing from one side and immediately appearing on the other. This is one of the counterintuitive behaviors of quantum particles, and it is the result that quantum particles behave as waves.

In certain cases, electron tunneling can actually be used to find the resonant frequency of the molecule. If we take two metals and separate them with a small barrier, then apply a voltage so that the electrons are pushed to one side. Usually in classical physics, electrons cannot pass through this barrier, but if the gap is very small, it can be quantum tunneled to the other side. But there is an additional condition that electrons in the metal have a certain amount of energy, and only when there is a hole with the same energy can it tunnel to the other side. But if the energy of the hole on the other side is low, the electron cannot tunnel because there is no extra energy to get there.

But if we introduce a molecule into the gap, something interesting will happen. If the energy difference between the electron and the hole is the same as the energy required for one of the vibrating molecules to resonate, the electron is allowed to pass through the tunnel and uses its extra energy to vibrate the molecule when it tunnels.

Scientists have built machines that use this property to detect molecules, a technique known as inelastic electron tunneling spectroscopy. We can put in different molecules and change the energy difference between the electron and the hole to see if the electron will tunnel, which will tell you the resonance of the molecule and tell us what it is made of.

Maybe our noses are doing the same thing, maybe our odor receptors behave like metals and gaps, waiting for odor molecules to enter, which causes electrons to pass through the receptors and trigger nerves. In order to find out the answer, scientists have done some very clever experiments. According to this theory, if you can change the resonance frequency of a molecule, you will also change its smell. They used the same molecule, some of which replaced all hydrogen atoms with heavier deuterium, changing the resonance frequency. Next, different subjects were asked to sniff them to see if they could tell the difference between them, and the final conclusion was that they did smell different.

Although the vibration model is successful, it doesn't explain everything. We have heard of chiral molecules, which are made up of all the same substances but arranged into mirror images of each other. Because they are made up of the same atoms and bonds, they have the same vibrations, so they look the same as vibrational tests such as Raman spectra and inelastic electron tunneling spectra. So according to the vibration theory, they should smell the same, but this is not the case.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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