Chlorophyll a
Encyclopedia
Chlorophyll a is a specific form of chlorophyll
used in oxygen
ic photosynthesis
. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment
is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary electron donor in the electron transport chain
. Chlorophyll a also transfers resonance energy in the antenna complex
, ending in the reaction center where specific chlorophyll’s P680
and P700
are located.
but is not the only pigment that can be used for photosynthesis. All oxygenic photosynthetic organisms use chlorophyll a, but differ in accessory pigments like chlorophylls b
and c
. Chlorophyll a can also be found in very small quantities in the green sulfur bacteria
, an anaerobic photoautotroph
. These organisms use bacteriochlorophyll
and some chlorophyll a but do not produce oxygen. Anoxygenic photosynthesis
is the term applied to this process, unlike oxygenic photosynthesis
where oxygen is produced during the light reactions of photosynthesis
.
.
ion
encased in a 4-ion nitrogen
ring known as a chlorin ring
. The chlorin ring is a heterocyclic compound
derived from a pyrrole
that encases a metal. The Mg within the center uniquely defines the structure as a chlorophyll molecule.
s as side chains. Chlorophyll b
replaces a methyl group at the C-3 position on the ring (Green Box in Figure) with an aldehyde
group. The porphyrin ring of bacteriochlorophyll
is saturated, and lacking alternation of double and single bonds causing variation in absorption of light.
.
. Genes code for the enzymes on the Mg-tetrapyrroles of both bacteriochlorophyll a and chlorophyll a. It begins with glutamic acid
, which is transformed into a 5-aminolevulinic acid (ALA). Two molecules of ALA are then reduced to porphobilinogen
(PBG), and four molecules of PBG are then coupled, forming protoporphyrin IX. When forming protoporphyrin, Mg-chelatase
acts as a catalyst for the insertion of Mg into the chlorophyll a structure. The pathway then uses either a light-dependent process
, driven by the enzyme protochlorophyllide
, which is a precursor to the production of a chlorophyll a molecule, or a light-independent process driven by other enzymes, to form a cyclic ring, and the reduction of another ring in the structure. Attachment of the phytol
tail completes the process of chlorophyll biosynthesis.
Chlorophyll a absorbs light within the violet
, blue
and red
wavelengths while mainly reflecting green
. This reflectance gives chlorophyll its green appearance. Accessory photosynthetic pigments broaden the spectrum of light absorbed, increasing the range of wavelengths that can be used in photosynthesis. The addition of chlorophyll b next to chlorophyll a extends the absorption spectrum
. In low light conditions, plants produce a greater ratio of chlorophyll b to chlorophyll a molecules, increasing photosynthetic yield.
Light Gathering
Absorption of light by photosynthetic pigments converts photons into chemical energy. Light energy
radiating onto the chloroplast
strikes the pigments in the thylakoid
membrane and excites their electrons. Since the chlorophyll a molecules only capture certain wavelengths, organisms may use accessory pigments to capture a wider range of light energy shown as the yellow circles. It then transfers captured light from one pigment to the next as resonance energy, passing energy one pigment to the other until reaching the special chlorophyll a molecules in the reaction center. These special chlorophyll a molecules are located in both photosystem II
and photosystem I
. They are known as P680
for Photosystem II and P700
for Photosystem I. P680 and P700 are the primary electron donor
s to the electron transport chain.
for the process of photosynthesis to proceed. Within the reaction centers
of both photosystems there are a pair of chlorophyll a molecules that pass electrons on to the transport chain
through redox
reactions.
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
used in oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
ic photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment
Photosynthetic pigment
A photosynthetic pigment is a pigment that is present in chloroplasts or photosynthetic bacteria and captures the light energy necessary for photosynthesis.- Plants :...
is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary electron donor in the electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
. Chlorophyll a also transfers resonance energy in the antenna complex
Antenna complex
The light-harvesting complex of plants is an array of protein and chlorophyll molecules embedded in the thylakoid membrane which transfer light energy to one chlorophyll a molecule at the reaction center of a photosystem....
, ending in the reaction center where specific chlorophyll’s P680
P680
P680, or Photosystem II primary donor, refers to any of the 2 special chlorophyll dimers , PD1 or PD2. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited as if they were a single molecule...
and P700
P700
P700, or Photosystem I primary donor, is the reaction-center chlorophyll a molecule in association with photosystem I. Its absorption spectrum peaks at 700 nm. When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll...
are located.
Distribution of Chlorophyll a
Chlorophyll a is essential for most photosynthetic organisms to release chemical energyChemical energy
Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction or, to transform other chemical substances...
but is not the only pigment that can be used for photosynthesis. All oxygenic photosynthetic organisms use chlorophyll a, but differ in accessory pigments like chlorophylls b
Chlorophyll b
Chlorophyll b is a form of chlorophyll. Chlorophyll b helps in photosynthesis by absorbing light energy. It is more soluble than chlorophyll a in polar solvents because of its carbonyl group. Its color is yellow, and it primarily absorbs blue light....
and c
Chlorophyll c
Chlorophyll c can refer to:* Chlorophyll c1* Chlorophyll c2* Chlorophyll c3...
. Chlorophyll a can also be found in very small quantities in the green sulfur bacteria
Green sulfur bacteria
The green sulfur bacteria are a family of obligately anaerobic photoautotrophic bacteria. Most closely related to the distant Bacteroidetes, they are accordingly assigned their own phylum....
, an anaerobic photoautotroph
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. These organisms use bacteriochlorophyll
Bacteriochlorophyll
Bacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by Von Neil in 1932 . They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Groups that contain bacteriochlorophyll conduct...
and some chlorophyll a but do not produce oxygen. Anoxygenic photosynthesis
Anoxygenic photosynthesis
Phototrophy is the process by which organisms trap light energy and store it as chemical energy in the form of ATP. There are three major types of phototrophy: Oxygenic photosynthesis, Anoxygenic photosynthesis, and Rhodopsin-based phototrophy...
is the term applied to this process, unlike oxygenic photosynthesis
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
where oxygen is produced during the light reactions of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
.
Molecular Structure
The molecular structure of chlorophyll a consists of a N-ring with a Mg center, side chains, and a hydrocarbon tailHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
.
Chlorin Ring
Chlorophyll a contains a central magnesiumMagnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
encased in a 4-ion nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
ring known as a chlorin ring
Chlorin
In organic chemistry, a chlorin is a large heterocyclic aromatic ring consisting, at the core, of three pyrroles and one pyrroline coupled through four methine linkages...
. The chlorin ring is a heterocyclic compound
Heterocyclic compound
A heterocyclic compound is a cyclic compound which has atoms of at least two different elements as members of its ring. The counterparts of heterocyclic compounds are homocyclic compounds, the rings of which are made of a single element....
derived from a pyrrole
Pyrrole
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...
that encases a metal. The Mg within the center uniquely defines the structure as a chlorophyll molecule.
Side Chains
Side chains are attached to the porphyrin ring of chlorophyll a. Different side chains characterize each type of chlorophyll molecules, and alters the absorption spectrum of light. Chlorophyll a contains only methyl groupMethyl group
Methyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...
s as side chains. Chlorophyll b
Chlorophyll b
Chlorophyll b is a form of chlorophyll. Chlorophyll b helps in photosynthesis by absorbing light energy. It is more soluble than chlorophyll a in polar solvents because of its carbonyl group. Its color is yellow, and it primarily absorbs blue light....
replaces a methyl group at the C-3 position on the ring (Green Box in Figure) with an aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
group. The porphyrin ring of bacteriochlorophyll
Bacteriochlorophyll
Bacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by Von Neil in 1932 . They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Groups that contain bacteriochlorophyll conduct...
is saturated, and lacking alternation of double and single bonds causing variation in absorption of light.
Hydrocarbon Tail
A tail attached to the porphyrin ring (of the chlorophyll a molecule) is a long hydrocarbon tail. This long hydrophobic extension anchors the chlorophyll a molecule to other hydrophobic proteins in the thylakoid membrane of the chloroplastChloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
.
Biosynthesis
Chlorophyll a biosynthetic pathway utilizes a variety of enzymesOne gene-one enzyme hypothesis
The one gene-one enzyme hypothesis is the idea that genes act through the production of enzymes, with each gene responsible for producing a single enzyme that in turn affects a single step in a metabolic pathway...
. Genes code for the enzymes on the Mg-tetrapyrroles of both bacteriochlorophyll a and chlorophyll a. It begins with glutamic acid
Glutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...
, which is transformed into a 5-aminolevulinic acid (ALA). Two molecules of ALA are then reduced to porphobilinogen
Porphobilinogen
Porphobilinogen is a pyrrole involved in porphyrin metabolism.It is generated by aminolevulinate and the enzyme ALA dehydratase. PBG is then converted into hydroxymethyl bilane by the enzyme porphobilinogen deaminase, also known as hydroxymethylbilane synthase.Acute intermittent porphyria causes...
(PBG), and four molecules of PBG are then coupled, forming protoporphyrin IX. When forming protoporphyrin, Mg-chelatase
Magnesium chelatase
Magnesium-chelatase is a three-component enzyme that catalyses the insertion of Mg2+ into protoporphyrin IX. This is the first unique step in the synthesis of bacteriochlorophyll...
acts as a catalyst for the insertion of Mg into the chlorophyll a structure. The pathway then uses either a light-dependent process
Light-dependent reactions
The 'light-dependent reactions', or light reactions, are the first stage of photosynthesis, the process by which plants capture and store energy from sunlight. In this process, light energy is converted into chemical energy, in the form of the energy-carrying molecules ATP and NADPH...
, driven by the enzyme protochlorophyllide
Protochlorophyllide
Protochlorophyllide, or monovinyl protochlorophyllide, is an immediate precursor of chlorophyll a that lacks the phytol side-chain of chlorophyll. Unlike chlorophyll, protochlorophyllide is highly fluorescent; mutants that accumulate it glow red if irradiated with blue light...
, which is a precursor to the production of a chlorophyll a molecule, or a light-independent process driven by other enzymes, to form a cyclic ring, and the reduction of another ring in the structure. Attachment of the phytol
Phytol
Phytol is an acyclic diterpene alcohol that can be used as a precursor for the manufacture of synthetic forms of vitamin E and vitamin K1. In ruminants, the gut fermentation of ingested plant materials liberates phytol, a constituent of chlorophyll, which is then converted to phytanic acid and...
tail completes the process of chlorophyll biosynthesis.
Absorbance of Light
Light SpectrumChlorophyll a absorbs light within the violet
Violet (color)
As the name of a color, violet is synonymous with a bluish purple, when the word "purple" is used in the common English language sense of any color between blue and red, not including either blue or red...
, blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
and red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...
wavelengths while mainly reflecting green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
. This reflectance gives chlorophyll its green appearance. Accessory photosynthetic pigments broaden the spectrum of light absorbed, increasing the range of wavelengths that can be used in photosynthesis. The addition of chlorophyll b next to chlorophyll a extends the absorption spectrum
Absorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
. In low light conditions, plants produce a greater ratio of chlorophyll b to chlorophyll a molecules, increasing photosynthetic yield.
Light Gathering
Absorption of light by photosynthetic pigments converts photons into chemical energy. Light energy
Radiant energy
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...
radiating onto the chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
strikes the pigments in the thylakoid
Thylakoid
A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as...
membrane and excites their electrons. Since the chlorophyll a molecules only capture certain wavelengths, organisms may use accessory pigments to capture a wider range of light energy shown as the yellow circles. It then transfers captured light from one pigment to the next as resonance energy, passing energy one pigment to the other until reaching the special chlorophyll a molecules in the reaction center. These special chlorophyll a molecules are located in both photosystem II
Photosystem II
Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
and photosystem I
Photosystem I
Photosystem I is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known...
. They are known as P680
P680
P680, or Photosystem II primary donor, refers to any of the 2 special chlorophyll dimers , PD1 or PD2. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited as if they were a single molecule...
for Photosystem II and P700
P700
P700, or Photosystem I primary donor, is the reaction-center chlorophyll a molecule in association with photosystem I. Its absorption spectrum peaks at 700 nm. When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll...
for Photosystem I. P680 and P700 are the primary electron donor
Electron donor
An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....
s to the electron transport chain.
Primary Electron Donation
Chlorophyll a is very important in the energy phase of photosynthesis. Two electrons need to be passed to an electron acceptorElectron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
for the process of photosynthesis to proceed. Within the reaction centers
Photosynthetic reaction centre
A photosynthetic reaction center is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis...
of both photosystems there are a pair of chlorophyll a molecules that pass electrons on to the transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
through redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
reactions.
See also
- Photosystem II light harvesting proteinPhotosystem II light harvesting proteinPhotosystem II light-harvesting proteins are the intrinsic transmembrane proteins CP43 and CP47 occurring in the reaction centre of photosystem II. These polypeptides bind to chlorophyll a and beta-carotene and pass the excitation energy on to the reaction centre...
- ChlorophyllChlorophyllChlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
, green pigment that contains sub essential and accessory pigments - Chlorophyll bChlorophyll bChlorophyll b is a form of chlorophyll. Chlorophyll b helps in photosynthesis by absorbing light energy. It is more soluble than chlorophyll a in polar solvents because of its carbonyl group. Its color is yellow, and it primarily absorbs blue light....
, an accessory pigment of chlorophyll, found in many land plants - Chlorophyll cChlorophyll cChlorophyll c can refer to:* Chlorophyll c1* Chlorophyll c2* Chlorophyll c3...
, an accessory pigment of chlorophyll - Methyl groupMethyl groupMethyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...
, hydrophobic functional group attached to a - Photosystem IPhotosystem IPhotosystem I is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known...
- Photosystem IIPhotosystem IIPhotosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
- P680P680P680, or Photosystem II primary donor, refers to any of the 2 special chlorophyll dimers , PD1 or PD2. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited as if they were a single molecule...
- P700P700P700, or Photosystem I primary donor, is the reaction-center chlorophyll a molecule in association with photosystem I. Its absorption spectrum peaks at 700 nm. When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll...