Chlorophyll in plants is a pigment molecule that imparts a green colour to the leaf and stems by absorbing a red and blue spectrum of light. The term chlorophyll has originated from the Greek terms “Khloros” which means green and “Phyllon” which means leaf. It plays a fundamental role in the process of photosynthesis by allowing a plant to absorb light energy and convert it into chemical energy. In addition to plants, chlorophyll pigment also exists in the mesosomes of cyanobacteria and chloroplast of algae.
There are two kinds of chlorophyll pigments, type a and b that predominates in the plant. Both chl-a and chl-b are the light-absorbing pigments that absorb a specific wavelength of white light. Chl-a, absorbs dark blue and red light, whereas chl-b absorbs light blue and red-orange light within the visible light spectrum.
Content: Chlorophyll in Plants
Meaning of Chlorophyll
Chlorophyll can define as the light-absorbing pigment molecule that reflects a green colour to the chloroplast containing tissue by absorbing light of longer wavelength (red) and light of shorter wavelength (blue) within the electromagnetic spectrum. There are two significant kinds of chlorophyll pigment, namely chlorophyll-a, and b. These two pigment differs by having different side-chain composition and the distinct absorption tendency.
Chlorophyll-a: It consists of a methyl group (CH3) in the side chain and tends to absorb more red light of visible spectrum.
Chlorophyll-b: It consists of the aldehyde group (CHO) in the side chain and tends to absorb more violet-blue light of visible spectrum.
|Year of discovery||Discoverer||Discovery|
|1817||Joseph Bienaime Caventou and Pierre Joseph Pelletier||Isolated and termed “Chlorophyll”|
|1864||Stokes||Through spectroscopy, demonstrated that chlorophyll is a mixture of two components (Chl-a and b)|
|1906||-||The presence of magnesium in chlorophyll has been detected|
|1906-1915||Richard Willstatter||Introduced general structure of chlorophyll|
|1940||Hans Fischer||Introduced the structure of chlorophyll-a|
|1960||Robert Burns Woodward||Introduced synthesis of chlorophyll-a|
|1967||Lan Fleming||Studied the remaining stereochemical elucidation|
|1990||Woodward and Co-authors||Published an updated synthesis of chlorophyll|
|2010||-||Presence of chlorophyll-f has been detected in cyanobacteria|
Why is Chlorophyll green?
Chlorophyll is a green pigment which absorbs the red and blue spectrum of visible light and transmits green light. Due to the reflection of green light, all the chlorophyll-containing tissues or organelles appear green coloured. The green colour of leaf and stem is also due to this chlorophyll pigment.
Structure of Chlorophyll
A typical composition of chlorophyll comprises of a porphyrin head and a long phytol tail. Chlorophyll is a chelating ligand, which includes a central metal ion attached to the complex organic compound containing a mixture of carbon, nitrogen and hydrogen elements.
The structure of chlorophyll is characterized by:
- The presence of magnesium (Mg2+) as a central metal ion.
- A varying side chain.
- And by the presence of an extra fifth ring or isocyclic ring, fixed to the porphyrin head.
A porphyrin head of chlorophyll typically includes four pyrrole rings fixed to the coordinated central metal, and thus called as “Tetrapyrroles”. The first pyrrole ring is substituted with the side chain that differs in both the chlorophyll pigments. Both the chlorophyll pigments a, and b have different side chain, CH3 and CHO respectively.
The porphyrin ring has a square planar arrangement, where the four nitrogen atoms join the four pyrrole rings to the central magnesium ion. Apart from plants, the porphyrin ring also exists in haemoglobin and vitamin-B12 by having a different central atom, like iron and cobalt respectively.
To the base of the porphyrin ring, an extra isocyclic ring is present. Porphyrin ring is a stable ring, around which an electron can migrate freely, due to which it has a high tendency to gain or lose electrons.
A phytol tail of chlorophyll binds to the porphyrin head via ester. It also refers as an unsaturated hydrocarbon chain, containing 39 H-atoms and 20 C-atoms with two C-C double bonds.
A phytol chain is composed of four isoprene units with a chemical name (2-methyl-1, 3-butadiene). One isoprene unit has a molecular formula C5H8 as it consists of five carbon atoms and eight hydrogen atoms.
Types of Chlorophyll in Plants
Chlorophyll-a and Chlorophyll-b are the two pigments that are commonly present in the plants.
Chl-a is the primary light-absorbing pigment and chl-b works as an accessory pigment of a plant. Both these pigments absorb light of certain wavelength from the incoming white light emitted by the sun.
White light includes seven different colours like violet, indigo, blue, green, yellow, orange and red and refers as “VIBGYOR”. Violet, blue, red and orange light are generally absorbed from the visible white light.
Chl-a shows great absorbencies towards the light of the red and orange spectrum, while chl-b shows great absorbencies towards the light of the violet and blue spectrum. Chlorophyll-a is a universal pigment that is present in all oxygenic photosynthetic organisms, while chlorophyll-b is ubiquitous in higher plants and some algae. As both are the plant pigments, these are embedded in the sac-like thylakoid membrane.
A thylakoid membrane involves many light-absorbing and accessory pigments that collectively form a Photosystem. A photosystem consists of an antenna or light-harvesting complex and an active reaction centre. Chl-a is a primary pigment that absorbs the light energy (photons) from the sun (carries a bundle of photons) and passes it to the other pigment molecules till it outreaches a reaction centre.
In a photosystem, a reaction centre functions as an “Electron donor” that transfer the photon to the electron acceptor molecule for the further cellular activities. Chlorophyll-b functions as an accessory pigment that will expand the light-absorbing capacity of light-absorbing particles.
Facts about Chlorophyll
There are some interesting facts about the chlorophyll that we must explore.
During plant senescence and fruit ripening
Plants get degreen during plant senescence and at the time of fruit ripening because during that period the chlorophyll pigment converts into colourless tetrapyrroles refer as NCC’s (Non-fluorescent chlorophyll catabolites).
Among the plant pigments, chlorophyll has the highest absorbing capacity due to which it dominates or masks the leaf by a green colour. But, when a chlorophyll starts to decompose, the colour of leaf changes to red, yellow, orange etc. For a plant to appear green, it must continuously replenish the chlorophyll.
Chemistry of chlorophyll
A crystalline form of chlorophyll will obtain if dried leaves are pulverized and treated with ethanol. An amorphous form of chlorophyll will form if dried leaves are pulverized and subjected to the treatment with ether or acetone. As chlorophyll is a mixture of two components, which includes a ratio of 3:1 of chl-a, and chl-b.
- Solubility: Chlorophyll is a hydrophobic organic compound that readily dissolves in lipids and is fat-soluble.
- Acid treatment of chlorophyll causes a replacement of the magnesium with the two H-atoms and results in the formation of derivative “Phaeophytin” (olive-brown colour solid). Further hydrolysis of pheophytin, cause splitting of phytol and results in the production of “Phaeophorbide”.
- Base treatment of chlorophyll results in the formation of a series of phyllins and magnesium porphyrin compounds.
- Denaturation: Prolonged cooking and steaming denature the conformation of chlorophyll.
Food sources of chlorophyll
Asparagus, bell peppers, broccoli, green cabbage, celery, kale, green olives, spinach, alfalfa etc.
Chlorophyll extracts from a plant are commercially used as additives in processed soaps, toothpaste, cosmetics, food products etc.
Chlorophyll has a wide range of medicinal uses. It can use as a natural body cleanser. Its regular uptake can reduce the faecal and urinary odour. Chlorophyll increases the bone, nail and teeth strength. It also provides immune support by increasing the RBCs count and can reduce colon and liver cancer by interfering with the procarcinogens. Chlorophyll also detoxifies the blood by eliminating impurities from our body.