April 02, 2005

Last Lecture

Yesterday I gave the last lecture in my course on the physics of molecules. I got this nutty idea that a really fun device would be to explain all of our senses from a chemical/physical point of view and at the same time review the main ideas of the course. And the lecture turned out really well, probably one of my best, although you'd have to ask the students if it really went over the way I thought it did. (It seems to me when I look out during a lecture that they all look stunned and I'm not sure what that means. At least they are not sleeping.)

Aristotle wrote that we have five senses, hearing, touch, sight, taste, smell. But today we know that there are many more. Depending on how exactly you define them: Color, brightness, sound, taste, smell, touch, temperature, pain, balance, body location, hunger and thirst. And some people are even rumored to have common sense. There are animals that have senses that we don't have-- the ability to sense electric and magnetic fields.

Sight is a truly amazing sense, and describing how it works allowed me to review the nature of light (Einstein's photoelectron effect) and a little bit of photochemistry. Just three pigments are involved in color vision. Each one has a different response curve. These curves overlap-- and the amazing thing is that we can sense a wide variety of colors using only these three types of sensors. For example a yellow daffodil may excite pigment one a lot, pigment two some and pigment three just a little, while a red tulip would have a different pattern of excitations. The eye generates the signals and the brain does the rest. There is a fourth receptor involved with sensing the intensity of light, especially low light levels. Some animals have pigments that we don't have. There are insects and birds that have a pigment that senses ultraviolet light, and many flowers have special markings that only insects and birds can see. And, there are snakes (pit vipers and boas) that can see infrared light. They use this ability to sense the body heat of their prey, even in the dark. (The wise among you will guess that this is the part in the lecture when I was able to review blackbody radiation.)

The senses of smell and taste are the most 'chemical' in nature. You know they went to all that work to map the human genome, and what did they find? Well, one thing they found is that there are no less than 347 different kinds of scent receptors coded in human DNA. But, we are able to sense thousands of different smells, so something similar to color vision must be occurring: different receptors have different affinities for different scent molecules, and your brain interprets this pattern of signals as a certain smell. Some people are missing one or more of these 347 receptors-- for example to me, roses smell waxy (likely isoprene), and some people cannot smell 'banana ester.

Taste is similar to smell. You may have heard that there are four tastes, but there are really five (in addition to smelling your food): salt, sour, sweet, bitter and umami. The senses of salt and sour are similar-- ion channels in the cell membrane admit sodium or solvated protons (acidity) into the cell in the taste bud, resulting eventually in the sensation in the brain (allowing me to review diffusion/random walk/the Einstein-Smoluchowski equation). The senses of sweet and bitter are similar to the smell. There is a receptor on the outside of cells on your tongue that has arms like an octopus that wrap themselves around the molecule being sensed. If the molecule fits, you have the sensation of sweet, or for a different receptor, bitter. DIfferent molecules have varying affinities for this receptor, and there are some molecules that are quite a bit better at being sweet than sugar. For example, saccharine is about 300 times sweeter than sucrose-- it fits the receptor better (and allowed me to review hydrogen bonds and ionic interactions). There is one class of molecule, derivatives of guanidine, that can be over a million times sweeter than sugar. Just a spoonfull of lugdanum in a typical water tower would make a whole town's water taste sweet.

Umami is a really interesting taste and if you want to know more, please go look it up at one of the world's truly wonderful resources, www.wikipedia.org.

2 Comments:

At April 03, 2005 7:37 AM , Blogger Tim said...

This is strange because I was just speculating in a comment
href= "http://www.phpsolvent.com/wordpress/?p=345#comments" > over here
why some things are transparent and I was kind of hoping you would set us straight...

 
At April 04, 2005 9:37 AM , Blogger Matt_J said...

Tim, looks just fine what you've written. Its the electrons that are important, and nuclei are only important because the electrons are attracted to them and the nuclei shape their orbits. In order to define the optical properties of a substance you need to find out about two things-- absorption and scattering. Absorption works how you've described it. Photons of light have specific energies (corresponding to their wavelength or color) and materials have discreet energy levels. If the energy of the photon matches the energy spacing between the ground and an excited energy level, then absorption is possible. For visible light these energy levels involve different configurations of the valence electrons. So this is one condition for absorption, that the energy level spacing between these configurations matches the wavelength of the light. The other condition is that the transition has some kind of intensity. Remember that light is a wave, a wave of electromagnetism-- oscillating electrical and magnetic fields. The electron cloud in the molecule will start swinging from the light wave. This oscillation by the electrons in a substance gives rise to absorption if there is an excited state for the molecule to wind up in. If there is no upper state of the right energy, the only thing these electrons do is increase the refractive index of the material-- they slow down the light wave. The more the speed of light is decreased, the higher the refractive index of the material. You can observe the refractive index of water by putting a stick partway below the surface-- it appears to bend at the surface. This is part of scattering-- the direction of light changes at the interface between air (low refractive index) and water (higher refractive index). Another scattering phenomenon is reflection. Metals are really good at this because they have loosly bound electrons. A metal is essentially a gas of electrons flying around a macroscopic collection of positive nuclei-- so metals are almost perfect at oscillating in response to incoming electromagnetic radiation. So good in fact that they generate a perfect image of the incoming photon's electric field vector, and this image generates a new outgoing photon at the reflection angle. Some materials are only so-so at reflection. Whenever there is an interfact between materials of different refractive index reflection is possible-- for example a piece of glass or a water surface. Usually here there is partial reflection and partial transmission.

Sounds cool to make optical instruments of ice! It would be hard to get a homogeneous sample-- the best would be if you could get a single crystal of ice which would involve freezing it very slowly.

 

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