Chlorophyll

Microscopic view of chloroplasts, which contain chlorophyll, present in a plant leaf.

The chlorophylls (from the Greek χλωρος, chloros, "green", and φύλλον, fýlon, "leaf") are a family of green pigments found in cyanobacteria and all organisms that contain chloroplasts or thylakoid membranes in their cells, including plants and various algae. Chlorophyll is a highly indispensable biomolecule, critical in photosynthesis, which is a process that allows plants and algae to store energy from sunlight.

History

Chlorophyll was discovered in 1817 by French chemists Pierre Pelletier (1788-1842) and Joseph Caventou (1795-1877), who succeeded in isolating it from plant leaves. Pelletier introduced the methods, based on the use of mild solvents, which made it possible for the first time to isolate not only chlorophyll, but substances of great pharmacological importance such as caffeine, colchicine or quininapates.

Description

Chlorophylls are a group of pigments found in those eukaryotic organisms that have chloroplasts (plants, algae) and in some prokaryotes: bacteria that do not have chloroplasts (cyanobacteria, green and purple bacteria), and whose pigments are found in internal membrane systems: vesicles, lamellae and chromatophores, belonging to the Eubacteria and Eucarya domains.

Chemical structure of the chlorophyll molecule

Type A chlorophyll.

The structure of chlorophyll molecules has two parts: a ring of porphyrin that contains magnesium and whose function is to absorb light, and a hydrophobic chain of phytol whose function is to keep the chlorophyll integrated in the photosynthetic membrane.

Localization in cells

Chlorophylls are found in the membranes of the thylakoids, which in cyanobacteria are invaginations of the plasmatic membrane, and in the plastids of eukaryotic cells they are vesicles distributed throughout their interior. Chlorophylls appear inserted in the membrane, to which they are anchored by the side chain made up of a phytol residue, associated with proteins and other pigments, with which they form the photosystems.

Each photosystem contains around 200 chlorophyll molecules, in addition to auxiliary pigments, with which it constitutes the so-called antenna. The antenna is made up of ordered assemblies of chlorophyll molecules, other pigments, and proteins, which are called light-collecting complexes. Only one chlorophyll a molecule in each photosystem properly converts radiant energy (light) into chemical energy, when it receives a photon with sufficient energy from the antenna molecules, which pass it by.

Absorption spectrum and color

chlorophyll absorption a and b to different wavelengths. It can be seen that they absorb the colors of the ends of the rainbow (made the blue and the red), but not the green, from which its color proceeds.

Chlorophylls typically have two types of absorption in the visible spectrum, one in the blue light environment (400-500 nm of wavelength), and another in the red zone of the spectrum (600-700 nm); however, they reflect the middle part of the spectrum, the most nourished and corresponding to the green color (500-600 nm). This is the reason why chlorophylls have a green color and give it to organisms, or to those tissues that have active chloroplasts in their cells, as well as to the landscapes they form.

Diversity and taxonomic distribution

Different forms of chlorophyll are unevenly distributed in the diversity of oxygenic photosynthesizers. The following table presents the different forms of chlorophyll and summarizes its systematic distribution.

1. Chlorophyll a is in all cases linked to the active center of the molecular complexes, called photosystems, that absorb light during photosynthesis, and differs from chlorophyll b in which the radical of position 3 of the tetrapirrholic group is -CH₃ (methyl) instead of - Right. (functional group of aldehydes).
2. Chlorophyll b characterizes the plates of the green algae and its descendants, the terrestrial plants (Viridiplantae). These plates, and the organisms that carry them, are green. There are also green dishes in some groups of protists who have assimilated unicellular green algae endosimbionts thus acquiring side dishes. You can cite euglenas, chlorraracnophytes and some dinoflagellates, like Gymnodinium viride. It is also found in some cyanobacteria (the Chloroxibacteria), which is why they are green plant instead of blue; some time ago they were attributed by this trait the character of ancestors of the green plates, but then it has been proved that it is an acquired character independently in several separate lines.
3. The chlorophylls c₁ and c₂ are characteristics of an extensive and diverse line of protists that matches the Chromist superfilo and which includes groups as important as the pardas algae, diatoms, xantophiceas, haptopphytes and cryptophytes.
4. Chlorophyll d It has only been known for decades by isolated observation and not repeated in a red algae. Then he found himself in a cyanobacteria (Acaryochloris marina), which seems especially suitable to explode red light when it grows under certain ascidias. It has recently been discovered that this chlorophyll is not proper to the red algae, but it comes from the cyanobacteria that lives epiphyte on these algae.
5. Chlorophyll f has been found in Australian stromatolit cyanobacteria.

Chlorophylls are also found in animals that harbor unicellular algae (zoochlorellae and zooxanthellae) within their cells or between them. Thanks to this symbiosis, photosynthesis contributes significantly to the nutrition of corals, tridacnas, nudibranchs and other marine animals.

Not all photosynthetic organisms have chlorophylls. Bacteria that are not cyanobacteria have very distinct pigments called bacteriochlorophylls.

Measurement of chlorophyll content

Pure chlorophyll has an intense green color.

The measurement of light absorption is complex due to the solvent used to extract the chlorophyll from the plant, since it affects the values obtained.

• In ethyl ether, chlorophyll a has a maximum absorbance approximately between 430 nm and 662 nm, while chlorophyll b has a maximum absorption between 453 nm and 642 nm.
• The maximum absorption value of chlorophyll is between 465 nm and 665 nm. Chlorophyll fluoride at 673 nm (maximum) and 726 nm. The maximum value of chlorophyll absortivity exceeds 10⁵M⁻¹cm⁻¹, which is among the highest for organic compounds of small molecule.
• In a concentration of 90% acetona-agua, the wavelength of the maximum absorption of chlorophyll is 430 nm and 664 nm; the maximums of chlorophyll b are 460 nm and 647 nm; the maximums of chlorophyll c1 are 442 nm and 630 nm; the maximums for chlorophyll c2 are 444 nm

By measuring the absorption of light in the red and far-red regions, it is possible to estimate the concentration of chlorophyll contained in a leaf.

The fluorescence coefficient can be used to measure chlorophyll content. When chlorophyll is excited, it fluoresces at a lower wavelength; the fluorescence emission ratio at 705 ± 10 nm and 735 ± 10 nm can provide a linear relationship of chlorophyll content comparable to chemical tests. The relationship between F₇₃₅/F₇₀₀ provides a correlation coefficient r² of 0.96 compared to chemical tests, in the range of 41 mg m⁻² to 675 mg m⁻². Gitelson developed a formula for direct reading of chlorophyll content in mg m⁻². The formula provided a reliable method for measuring chlorophyll content from 41 mg m⁻² to 675 mg m⁻² with a correlation value r² out of 0.95.

Ecology

Chlorophyll can be easily detected thanks to its behavior towards light. Optically measuring the chlorophyll concentration in a water sample is simple and allows a sufficient estimation of the concentration of phytoplankton (microscopic algae) and, indirectly, of biological activity; Thus, the measurement of chlorophyll is an important instrument for monitoring eutrophication processes.

The presence of chlorophyll is also measured by remote sensing systems, which provide information on the distribution of primary production, including seasonal oscillations and interannual fluctuations. In this way, the measurement of chlorophyll helps research on climate and ecological change on a global scale.