Chlorophyll ''a'' is a specific form of chlorophyll
used in oxygen
. It absorbs most energy from wavelengths
of violet-blue and orange-red light, and it is a poor absorber of green and near-green portions of the spectrum. Chlorophyll does not reflect light but chlorophyll-containing tissues appear green because green light, diffusively reflected by structures like cell walls, becomes enriched in the reflected light. This photosynthetic pigment
is essential for photosynthesis in eukaryote
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 chlorophylls P680
Distribution of chlorophyll ''a''
Chlorophyll ''a'' is essential for most photosynthetic organism
s to release chemical energy
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 chlorophyll ''b''
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.
is the term applied to this process, unlike oxygenic photosynthesis
where oxygen is produced during the light reactions of photosynthesis
The molecular structure of chlorophyll ''a'' consists of a chlorin
ring, whose four nitrogen atoms surround a central magnesium
atom, and has several other attached side chains
and a hydrocarbon tail
Chlorophyll ''a'' contains a magnesium ion
encased in a large ring structure known as a chlorin
. The chlorin ring is a heterocyclic compound
derived from pyrrole
. Four nitrogen atoms from the chlorin surround and bind the magnesium atom. The magnesium center uniquely defines the structure as a chlorophyll molecule.
The porphyrin ring of bacteriochlorophyll
is saturated, and lacking alternation of double and single bonds causing variation in absorption of light.
are attached to the chlorin ring of the various chlorophyll molecules. Different side chains characterize each type of chlorophyll molecule, and alters the absorption spectrum of light.
For instance, the only difference between chlorophyll ''a'' and chlorophyll ''b''
is that chlorophyll ''b'' has an aldehyde
instead of a methyl group at the C-7 position.
Chlorophyll ''a'' has a long hydrophobic
tail, which anchors the molecule to other hydrophobic proteins in the thylakoid membrane
of the chloroplast
Once detached from the porphyrin ring, this long hydrocarbon tail becomes the precursor of two biomarker
, which are important in the study of geochemistry
and the determination of petroleum sources.
The Chlorophyll ''a'' biosynthetic pathway
utilizes a variety of enzymes
In most plants, chlorophyll is derived from glutamate
and is synthesised along a branched pathway that is shared with heme
The initial steps incorporate glutamic acid into 5-aminolevulinic acid
(ALA); two molecules of ALA are then reduced
(PBG), and four molecules of PBG are coupled, forming protoporphyrin
is the enzyme that completes the biosynthesis of chlorophyll ''a''
by catalysing the reaction
:chlorophyllide ''a'' + phytyl diphosphate
chlorophyll ''a'' + diphosphate
This forms an ester of the carboxylic acid group in chlorophyllide ''a''
with the 20-carbon diterpene
Reactions of photosynthesis
Absorbance of light
Chlorophyll ''a'' absorbs light within the violet
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.
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. These two systems are different in their redox potentials for one-electron oxidation. The Em
for P700 is approximately 500mV, while the Em
for P680 is approximately 1,100-1,200 mV.
Primary electron donation
Chlorophyll ''a'' is very important in the energy phase of photosynthesis. Two electrons
need to be passed to an electron acceptor
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
*Photosystem II light harvesting protein
, another related chemical
, an accessory pigment of chlorophyll