Photosynthesis Essay, Research Paper

PHOTOSYNTHSIS INFORMATION: The Nature of Light

Light behaves both as a wave phenomenon and as discrete particles of energy called photons. If we look at light as a wave phenomenon, we can assign it a wavelength (the distance from one peak of the wave to the next) and an amplitude (the distance the wave oscillates from its centerline). Different wavelengths of light have different characteristic energies and properties. Light can also travel at various speeds in different media, producing a frequency at which the wave travels. The energy contained in a wave of light is related to its frequency. Where E is energy, h is Planck’s constant Energy = (6.626196 * 10^-34 Joule-seconds), and c is the speed of light. Short wavelengths have high energies and long wavelengths have lower energies.

Pigments

How is light captured by living things? Molecules, when struck by a wave or photon of light, reflect some of its energy back out, or it can absorb the energy, and thus enter into a higher energy or excited state. Each molecule absorbs or reflects its own characteristic wavelengths of light. Molecules that have evolved to absorb wavelengths in the visible region of the spectrum very well are called pigments.

Absorption and Action Spectra

An absorption spectrum for a particular pigment describes the wavelengths at which it can absorb light and enter into an excited state. The following diagram represents the absorption spectrum of pure chlorophylls in solution:

An action spectrum, on the other hand, describes the efficiency of a particular molecule at acheiving its purpose in absorbing light; this measurement shows what wavelengths of light the molecule can trap to conduct photosynthesis. And action spectrum closely follows an absorption spectrum for a particular pigment because the molecule has to be able to absorb light to enter into its excited state and pass the energy along.

Chlorophylls and the Accessory Pigments

Chlorophyll is a generic name for green pigments in plant cells….a substance that absorbs visible light primarily in the red, violet and blue regions of the light spectrum. There are several kinds of chlorophyll with chlorophyll a being the most important for light dependent reactions in the complex photosynthesis processes. Chlorophyll a and b exist in plastids in cells of higher plants while chlorophyll c,d and e are present only in algae. Photosynthesis is the process of converting light energy into chemical energy which can only be performed by plants. All life on earth depends upon the ability of plants to photosynthesize simple sugars which are the basic source of food from which all other forms of food originate.

A chlorophyll molecule is made up of carbon and nitrogen atoms joined in a complex ring with an atom of magnesium located in the center of the ring. The molecule has a long chain of 20 carbon atoms making up an alcohol “tail” attached to the ring. Each kind of chlorophyll may vary somewhat in its molecular structure giving it slightly different chemical and physical properties. Chlorophyll appears to have three functions:

It serves as antennae to absorb light energy. In this process it becomes “excited” (it produces electrons that exist beyond their normal “ground” state and are in a “charged”condition, so to speak, ready to move elsewhere as a source of electrical energy.

Chlorophyll transfers H+ electrons by a process known as resonance transfer across thylakoid membranes to P700 and P680 type chlorophyll a molecules.

Chlorophyll, with the aid of enzymes, converts light energy into chemical energy by a complex series of processes of oxidation involving loss of electrons. In these processes carbon dioxide and water are converted to glucose and oxygen.

323