Magnificent molecules
In this issue: chlorophyll
Did you
know?
Duncan McMillan appreciates the green glow
of nature and all that it provides
Absinthe was banned in
the European Union up
until 1988 over fears that it
caused hallucinations, fits
and delirium.
Ethylene glycol
(1,2-ethanediol)
causes electrons to be excited out of the chlorophyll
molecules and into a chain of reactions.
Chlorophyll B
Despite everything we know about
it, photosynthesis is a little magical. This
process turns photons into carbohydrates, ie
sunlight into food. This compound has effectively
built the largest, and longest-living, things from nothing
more than air, water, and light. The molecule that
makes it happen is chlorophyll.
Greens and yellows
Nature's little power-converter is a porphyrin ring
with a long side-chain. The porphyrin ring (shown
in green), the key constituent of the haem unit in
haemoglobin, contains a single magnesium ion.
Porphyrin's arrangement of alternating (conjugated)
single and double carbon-carbon bonds produces a
stable structure that absorbs visible light. When it is
combined with a metal ion it is often brightly coloured.
Find out more
ar tif icial
Shining light on
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photosyn
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ar tifphoto (pdf
http://bit.ly/TM
d β-carotene
Carotenoids an
0611 (pdf, p4)
http://bit.ly/IC
Chlorophyll's side chain 'tunes' the absorption
spectrum of the molecule. There are two main types
of chlorophyll (a and b). The composition of their
side-chains are different so can maximise a plant's
light-capturing potential – a is standard green and b is
a bit more yellow.
So, chlorophyll pigments give leaves a fresh green
and yellow tint. In autumn, deciduous plants stop
producing chlorophyll as they prepare to shed their
leaves. This reveals other pigments such as carotenoids
and xanthophylls, whose rich oranges and browns are
the colours of the season.
In this clever cycle, electrons pass through these
reactions and ultimately produce chemical energy
products which allow carbohydrates to be synthesised
from carbon dioxide and water (from the soil). The
chlorophyll is regenerated by electrons from the oxidised
water, so the leaves give oxygen gas as a by-product.
Artificial photosynthesis
Now scientists are working to hijack
photosynthesis for industry. The
sun dumps more solar energy onto the
earth than we might ever have use for; but even with
increasingly efficient solar cells the biggest problem to
overcome is storage. This is not a problem for plants
because they use the energy from the sun to build
complex carbohydrates such as sugars and starches to
store chemical energy, rather than electric potential.
Researchers are working on an artificial photosynthesis
that could improve on nature. By reducing the complex
process to simpler reactions, they hope to achieve a
higher energy conversion than in the humble green leaf.
On a more human note, chlorophyll is used in
food colouring. It is also present in the green spirit
absinthe, from the herbs it contains. Becoming so
fashionable at one point in France, particularly among
the artistic community, it is said to have inspired the
likes of Degas, Wilde, van Gogh, and Hemingway.
So chlorophyll is not only the source of life for virtually
every living thing, but was a source of inspiration in the
lives of some of our greatest artists.
Capturing photons
4 | The Mole | July 2012
0412MOLE - Pages 4-11.indd 4
SCIENCE PHOTO LIBRARY
In plants, chlorophylls are found in cell components
called chloroplasts. They capture the energy from
incoming photons. The chlorophylls are precisely
arranged within the cell proteins so that the energy can
be collected, channelled and focused. This eventually
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