Chlorophyll



Chlorophyll is a green [|pigment] found in almost all [|plants], [|algae], and [|cyanobacteria]. Its name is derived from the [|Greek] words χλωρος, //chloros// ("green") and φύλλον, //phyllon// ("leaf"). Chlorophyll is an extremely important biomolecule, critical in [|photosynthesis], which allows plants to absorb [|energy] from light. Chlorophyll absorbs light most strongly in the blue portion of the [|electromagnetic spectrum], followed by the red portion. However, it is a poor absorber of green and near-green portions of the spectrum, hence the green color of chlorophyll-containing tissues. Chlorophyll was first isolated by [|Joseph Bienaimé Caventou] and [|Pierre Joseph Pelletier] in 1817

Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light. Chlorophyll molecules are specifically arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts. In these complexes, chlorophyll serves two primary functions. The function of the vast majority of chlorophyll (up to several hundred molecules per photosystem) is to absorb light and transfer that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems. The two currently accepted photosystem units are Photosystem II and Photosystem I, which have their own distinct reaction center chlorophylls, named P680 and P700, respectively. These pigments are named after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the types of chlorophyll in each photosystem are distinct and determined by each other and the protein structure surrounding them. Once extracted from the protein into a solvent (such as acetone or methanol), these chlorophyll pigments can be separated in a simple paper chromatography experiment and, based on the number of polar groups between chlorophyll a and chlorophyll b, will chemically separate out on the paper. The function of the reaction center chlorophyll is to use the energy absorbed by and transferred to it from the other chlorophyll pigments in the photosystems to undergo a charge separation, a specific redox reaction in which the chlorophyll donates an electron into a series of molecular intermediates called an electron transport chain. The charged reaction center chlorophyll (P680+) is then reduced back to its ground state by accepting an electron. In Photosystem II, the electron that reduces P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms such as plants produce O2 gas, and is the source for practically all the O2 in Earth's atmosphere. Photosystem I typically works in series with Photosystem II; thus the P700+ of Photosystem I is usually reduced, via many intermediates in the thylakoid membrane, by electrons ultimately from Photosystem II. Electron transfer reactions in the thylakoid membranes are complex, however, and the source of electrons used to reduce P700+ can vary.
 * Chlorophyll and photosynthesis **

The history of chlorophyll is directly related to the history of the discovery of photosynthesis as shown below.
A famous chemist named Joseph Priestley (left) who was English discovered during 1771 that if he put a burning candle under a glass jar that the flame would extinguish after a little while; but when he put a mint plant in the jar with the candle he found that the candle would continue to burn. He also noticed that if he put a mouse in the jar with the mint plant the mouse continued to live as opposed to putting the mouse under the glass jar without a mint plant. A Dutchman, Jan Ingenhousz, made the next contribution to photosynthesis. He learned of Priestley's experiments, and spent a summer near London doing over 500 experiments, and he discovered that light or sunlight to be specific plays an important role in the occurrence of photosynthesis. He said..."//I observed that plants not only have the faculty to correct bad air in six to ten days, by growing in it...but that they perform this important office in a complete manner in a few hours; that this wonderful operation is by no means owing to the vegetation of the plant, but to the influence of light of the sun upon the plant//". Soon after Jean Senebier (right) working in Geneva noticed that "fixed air" or CO2. Theodore de Saussure also working in Geneva discovered that water also played a major role in the occurrence of photosynthesis. The last piece of the puzzle of photosynthesis water put in by a German surgeon, Julius Robert Mayer (right), who discovered that all plants convert solar energy received through the leaves into chemical energy that the plant needs to survive. He said..."//Nature has put itself the problem of how to catch in flight light streaming to the Earth and to store the most elusive of all powers in rigid form. The plants take in one form of power, light; and produce another power, chemical difference.//" The prefix chloro, of chlorophyll does not indicate the presence of chlorine in the molecule; rather chloro comes from the Greek word //chloros//, which means yellowish green. Therefore chlorophyll was named after the Greek word because it gave plants their green color.

Chlorophyll is the molecule that traps sunlight and is called a photoreceptor. It is found in the chloroplasts of green plants, and is what makes green plants green. The basic structure of this molecule is a porphyrin ring, co-ordinated to a central atom. This is very similar in structure to the heme group found in hemoglobin.