The Electrochemical Basis of Vision

Before we can understand the hows and whys of vision we need to review some of the basic physics and chemistry that make it possible in the first place.

And then there was light

We know that we need light to experience vision and light is a form of energy. All life on Earth runs on energy and almost all life is dependant on energy that comes directly from the sun. A few deepwater communites get their energy from heat derived from radioactive decay in the earths interior. But for all organisms alive, the key to life is the ability to trap and use some of this energy. Since our focus is ultimately on vision and since the sun is responsible for driving almost the entire biosphere we will concentrate on sunlight.

Living organisms can be divided into two major categories.

Those that can utlitze light energy to make the organic molecule of which all living things are composed - Primary prodcuers

Those that utilize those organisms in some way (primarily by eating them!) - Consumers

Light energy is carried in small units called photons or quanta which have properties of both waves and particles. Each photon has some energy content which depends on it's wavelength. The smaller the wavelength of the photon, the more energy it carries. So the work that a single photon can do is related to it's wavelength and is a known, and measureable, amount.

Light intensity, on the other hand, refers to the number of photons measured at a given time and has nothing to do with wavelength. You can shine shine a brighter light at an object are more photons available but the photons themselves still carry the same amount of energy. We are interested in the energy that can be used in a chemical reactions, specifically chemical reactions inside living cells.

Energy enters chemical reactions in one of two ways. As:

1. Energy of activiation

This is the energy required to "excite" the molecule into reacting. This energy can come from collisions with other molecules or, in a photochemical reaction, the energy is supplied by photons.

2. Heat of reaction

Energy released by a chemical reaction that is then available to activate further reactions through collisions.

In chemistry, a unit of measure commonly used is a mole, which represents a very large number (7.023 x 10²³). The energy in a mole of photons is measured in units called einsteins. This unit of measure is named after (who else) Albert Einstein who first postulated that all the energy in a single photon is transferred to a single electron. This one to one relationship is a key to understanding how light and matter can interact in photobiology¹ and is known an Einstein's Law of Photochemistry.

The energy one einstein of light carries can be easily determined if the wavelength is known. This energy can then be expressed in calories, which is what chemists use to express energies of activiation in chemical reactions. Thus, we can assess the various wavelengths in their effectiveness in activating chemical reactions. This makes it easier to relate light enery with chemical energy and from this we can make determinations of optimal wavelengths for a given chemical reaction. For example, the energy required to break a single covalent bond between atoms, a process which forms free radicals and can therefore be a potent means of chemical activation, is in the range of 40-90 kilocalories per mole which corresponds to light wavelengths of 710 to 320 millimicrons (which is light we can see).

The effects that photons have on matter depends on their wavelength and these effects vary greatly. So greatly that we have different names for different parts of this electromagnetic spectrum (See the electromagnetic spectrum graphic above). These are known by such familiar names as radio, X-ray, and microwave radiation. Ultraviolet, infra-red and gamma rays also refer to photons of varying wavelengths.Scientists came to discover, after looking at the entire spectrum of light, that if you are to make use of this light in biochemistry, we have a very small band available to us to use which we generally think of as "visble light."

Wavelengths smaller than this range have too much energy and tend to break chemical bonds, which has disasterous effects on our proteins and DNA. This is what makes ultraviolet radation from the sun, and radioactivity in general, so bad for us. The unexpected breaking of molecules into free radicals and the denaturing of proteins that we need can result in everything from premature aging to cancer.

The longer wavelengths tend to be absorbed by water (of which we are mostly composed). When water absorbs these photons it gets warmer. A microwave oven is a good example of this. Not only is this a hard way to make a living from light (we would cook) but these wavelengths tend to get filtered out by the atmosphere long before they reach the ground.

in summary then, visible light is the group of wavelengths that, to paraphrase Goldilocks, would be "just right." These photons carry the right amount of energy to meet the needs of most ordinary chemical reactions. the next step is to identify the molecules that use this light energy and the ways that they do it. The group of chemicals we are interested in fall into a group known as bioactive pigments. These pigments are responsible for:

Photosynthesis

Phototropism

Vision

The next section explores photosynthesis.