Chlorophyll (KLOR-uh-fill) is the pigment that gives
plants, algae, and cyanobacteria their green color. The name
comes from a combination of two Greek words, chloros,
meaning ‘‘green’’ and phyllon, meaning ‘‘leaf.’’ Chlorophyll is
the substance that enables plants to create their own food
through photosynthesis.
At least five forms of chlorophyll exist. They are:
• chlorophyll a (also known as a-chlorophyll), with a formula
of C55H72O5N4Mg
• chlorophyll b (also known as b-chlorophyll), with a formula
of C55H70O6N4Mg
• Chlorophyll c1, with a formula of C35H30O5N4Mg
• Chlorophyll c2, with a formula of C35H28O5N4Mg
• Chlorophyll d, with a formula of C54H70O6N4Mg
Chlorophyll a occurs in all types of plants and in algae.
Chlorophyll b is found primarily in land plants. Chlorophyll
c1 and chlorophyll c2 are present in various types of algae.
Chlorophyll d is found in red algae.
All forms of chlorophyll have a similar chemical structure.
They have a complex system of rings made of carbon
and nitrogen known as a chlorin ring. The five forms of
chlorophyll differ in the chemical groups attached to the
chlorin ring. These differences result in slightly different
colors of the five chlorophylls.
French chemists Pierre-Joseph Pelletier (1788–1842) and
Joseph-Bienaime´ Caventou (1795–1877) first isolated chlorophyll
in 1817. In 1865, German botanist Julius von Sachs
(1832–1897) demonstrated that chlorophyll is responsible
for photosynthetic reactions that take place within the cells
of leaves. In the early 1900s, Russian chemist Mikhail Tsvett
(1872–1920) developed a technique known as chromatography
to separate different forms of chlorophyll from each
other. In 1929, the German chemist Hans Fischer (1881–
1945) determined the complete molecular structure, making
possible the first synthesis of the molecule in 1960 by the
American chemist Robert Burns Woodward (1917–1979).
Plants make chlorophyll in their leaves using materials
they have absorbed through their roots and leaves. The
synthesis of chlorophyll requires several steps involving
complex organic compounds. First, the plant converts a common
amino acid, glutamic acid (COOH(CH2)2CH(NH2)COOH)
into an alternative form known as 5-aminolevulinic acid
(ALA). Two molecules of ALA are then joined to form a ring
compound called porphobilinogen. Next, four molecules of
porphobilinogen are joined to form an even larger ring structure
with side chains. Oxidation of the larger ring structure
introduces double bonds in the molecule, giving it the ability
to absorb line energy. Finally, a magnesium atom is introduced
into the center of the ring and side chains are added to
the ring to give it its final chlorophyll configuration.
Plants store chlorophyll in their chloroplasts, organelles
(small structures) that carry out the steps involved in photosynthesis.
Each chloroplast contains many clusters of several
hundred chlorophyll molecules called photosynthetic units.
When a photosynthetic unit absorbs light energy, chlorophyll
molecules move to a higher energy state, initiating
the process of photosynthesis. The overall equation for the
process of photosynthesis is 6CO2 + 6H2O ! C6H12O6 + 6O2.
That simple equation does not begin to suggest the complex
nature of what happens during photosynthesis. Botanists
divide that process into two major series of reactions: the light
reactions and the dark reactions. In the light reactions, plants
use the energy obtained from sunlight to make two compounds,
adenosine triphosphate (ATP) and nicotinamide adenine
dinucleotide phosphate (NADPH). ATP and NADPH are
not themselves components of carbohydrates, the final products
of photosynthesis. Instead, they store energy that is
used to make possible a series of thirteen different chemical
reactions that occur during the dark stage of photosynthesis
that result in the conversion of carbon dioxide and water
to the simple carbohydrate glucose (C6H12O6).
