Thanks to chlorophyll, plants supply energy to all living things and provide us with oxygen. But there is still much left to discover about it.

Chlorophyll is a group of green pigments with a similar chemical structure, which are found in plants, algae, and “blue-green algae” (cyanobacteria), both underwater and on land. These pigments play a vital role in the photosynthesis process, which uses light energy to produce sugars from water and carbon dioxide, while emitting oxygen that is so vital to life on Earth.

The name chlorophyll is derived from two Greek words: Chloros (green) and phyllon (leaf). It was first isolated in 1817 by French chemists Joseph Bienaimé Caventou and Pierre-Joseph Pelletier. But it was only in the 20th century, more than 100 years later, when researchers discovered that there are several types of chlorophyll and determined their structure. Today, six types of chlorophyll molecules are known, each designated by a Latin letter. The last of them, chlorophyll f, was found only in 2010.

Chlorophyll’s basic structure is a porphyrin ring with a magnesium atom at its center. This structure is very similar to that of the heme molecule, which is found in the hemoglobin in our red blood cells, with an iron atom at its center. The various chlorophyll types differ by the chemical groups that are bound to the porphyrin ring.

Because of its structure, chlorophyll absorbs light in the blue and red parts of the visible spectrum, and reflects the green light (with wavelengths of 500–600 nanometers) back to our eyes. Therefore, plants and algae appear green. Different types of chlorophyll absorb light of slightly different wavelengths, affecting the plant’s absorbance and use of light energy.

In the marine environment, the chlorophyll affects the absorption and reflection of light by the water. In water, chlorophyll is found in the single-celled algae called phytoplankton and in “blue-green algae,” which are actually species of cyanobacteria. Satellite photographs now enable researchers to map areas that are rich in these organisms and monitor changes, for example, due to increase in the ocean temperature.

The chlorophyll molecule | Illustration: Science Photo Library
The ring structure brings to mind the hemoglobin molecule.The chlorophyll molecule | Illustration: Science Photo Library

From Light Energy to Chemical Energy

Chlorophyll molecules are organized in and around designated photosystems that perform the photosynthesis. In plants and algae, the photosystems are found inside cellular organelles called chloroplasts. “Blue-green algae,” however, like other bacteria, do not have cellular organelles, and their photosystems are located inside the cells’ membranes.

The majority of the chlorophyll molecules in the photosystems are responsible for absorbing the light energy and transferring it to the complexes that convert it into chemical energy. These complexes, located at the center of the photosynthesis system, are called “reaction centers” and they, too, contain chlorophyll molecules. When light energy reaches the chlorophyll molecules in the complex, it excites one of their electrons, causing it to move to another molecule. The chemical energy harvested in this charge-separation process is utilized later to produce sugar from carbon dioxide. Simultaneously, in order to replace the chlorophyll’s lost electron, a water molecule undergoes an oxidation process that yields an oxygen molecule.

The main type of chlorophyll found in the reaction centers is chlorophyll a, which is shared by most of the photosynthetic organisms. The other types of chlorophyll can vary from one organism to another. Some photosynthetic bacteria, however, have other pigments, called bacteriochlorophyll, which consist of a group of pigments with structures similar to that of chlorophyll, but absorbing light at different wavelengths. Rather than releasing oxygen as a photosynthesis byproduct, these bacteria produce another molecule, for example: sulfur.

Chlorophyll is also found in fruit, which do not perform photosynthesis. It is responsible, for instance, for the characteristic green color of the olive fruits and the oil produced from them. It is widely used in the food industry, as the green colorant E140. Like many other natural pigments, chlorophyll has also found its way into the cosmetics industry and can thus be found in hair and skin care products.

Two hundred years after it was first isolated, the world of science still takes an interest in chlorophyll. Researchers are looking for new and rare family members of chlorophyll, investigating its mechanism of action and developing ways to mimic it. They are carefully monitoring the climate changes and their effects on photosynthesis - the vital process that supplies us with food and oxygen.