Chlorophyll is a complex organic molecule that enables plants, and some other organisms, to carry out photosynthesis — the conversion of carbon dioxide and water to glucose and oxygen using sunlight. The glucose is a source of energy that is used to power growth and development, and the oxygen created as a byproduct is essential to all animal life. As a result, this green pigment is one of the most important biological compounds. It also gives plants their distinctive green color.
It can be helpful to think of chlorophyll as fulfilling the same function in plants as the human digestive system. They both take raw materials and convert them into useable sources of energy in the form of glucose. In the case of the pigment, all the plant needs is water absorbed through its roots, air absorbed through its pores, and light from the Sun. Then the plant, whether it is a redwood tree, a clump of algae, or a rose, absorbs glucose into its cells to unfurl another leaf or bud another flower. Energy it does not immediately need it stores as starch for later use.
The process of photosynthesis can be summarized as the reaction
but is in reality much more complex, and involves a number of steps. The end products have more energy than the raw materials, which means that the reaction cannot take place without energy being supplied. Chlorophyll does this by capturing energy from sunlight. Since CO
is absorbed, and O2
produced, it can be said that plants “breathe in” carbon dioxide and “breathe out” oxygen.
The Structure and Function of Chlorophyll
The pigment contains a porphyrin ring, which is a circular arrangement of carbon atoms. Within this are four nitrogen atoms bonded to a central magnesium ion. This allows electrons to float freely within the structure, which means that the compound can supply electrons to other molecules, and also accept them.
The pigment also strongly absorbs light in the red and blue parts of the spectrum, which is why it is green in color — the remaining light is reflected back, and is mostly in the green part of the spectrum. The absorbed light is used to energize electrons, which are ultimately employed create glucose from carbon dioxide and water. The electrons are replaced by using sunlight to split water into hydrogen ions and oxygen, which frees two electrons for each water molecule. This is a remarkable achievement, considering that splitting water in a laboratory requires very high temperatures or an electric current.
Chlorophyll is contained within plant cells in structures known as chloroplasts. These occur mainly in the leaves, in the cells between the upper and lower layers. Within the chloroplasts are grana, which consist of tiny bodies known as thylakoids stacked on top of one another. The pigment is contained within the membranes of the thylakoids.
Why Plants are Green
Due to its green color, chlorophyll absorbs only part of the available light, so it might be asked why plants do not use a pigment that can use all the available energy: such a pigment would be black, as it would absorb all the light that strikes it. The answer is not clear. It may be that no black colored molecule exists that would have exactly the chemical properties required to do the job. Alternatively, a black pigment might absorb too much energy, possibly causing plants to overheat, or producing undesirable chemical reactions. Another theory is that some early photosynthetic organisms used retinol, a purple pigment that absorbs green light, and that chlorophyll use evolved in later life forms to exploit the parts of the Sun’s spectrum that were not being used by the retinol-based organisms.
In temperate regions, many perennial plants go dormant during the winter, losing their leaves. Prior to this, in the autumn, chlorophyll production ceases, and the pigment is broken down, causing the leaves lose their green color. When this happens, other pigments, which were masked by the strong green hue of this compound, become visible. Carotenoids are one example — they give the yellow and orange colors commonly seen in autumn.
Chlorophyll is used in the food industry as a natural colorant, and in Europe is given the E-number E140. A number of common food products are colored green by this compound, and it also give the green color to the alcoholic drink, absinthe. The pigment is often sold in a variety of liquid forms as a health supplement. Although it is harmless, there appears to be no scientific basis for the various claimed benefits.