Geology Reference
In-Depth Information
plants to carbon dioxide with a carbon-11 “label.” In this way, they could use the telltale
radioactivity to follow carbon dioxide as it was taken into plant tissues, though the fleeting
twenty-one-minute half-life of carbon-11 made these experiments exceedingly difficult.
Ruben and Kamen's 1940 discovery of a way to manufacture carbon-14, a much more
suitable tracer isotope with a leisurely half-life of 5,730 years, revolutionized biophysical
research andledtoarapidunderstanding ofhowplantstakeadvantage ofwater,carbondi-
oxide,andsunlight.Inbrief,aclever(andveryancient)proteincalledrubisco—achemical
found in the type of pioneering cyanobacteria thought to date back three billion years or
more—concentratescarbondioxideandwater,absorbstheSun'senergy,andassemblesthe
raw materials into essential bio-building blocks. In the photosynthesis reaction that yields
the oxygen we breathe, algae or plants consume six molecules of carbon dioxide plus six
molecules of water to make one molecule of the sugar glucose, with six molecules of oxy-
gen as a by-product. This chemical transformation is another example of our old friend,
the redox reaction (like rusting iron). In this case, the carbon atoms in carbon dioxide gain
electrons and are thus reduced, while water or some other electron donor is oxidized. In
photosynthesis, the Sun's rays provide the energetic boost to shift electrons.
As straightforward as the bare-bones chemical reaction might sound—carbon dioxide
plus water (or some other chemical that can contribute electrons) makes sugars and other
biomolecules—the details of photosynthesis are immensely complicated and are still being
worked out. For one thing, microbes have figured out quite a few different ways to har-
vest sunlight and other sources of energy. Most oxygen-producing plants and algae today
use the bright green pigment chlorophyll to absorb light in red and violet wavelengths. But
throughout Earth'shistory,avariety ofcells have employed other photosynthetic pathways
that produced no oxygen at all. Alternate light-absorbing pigments have evolved to decor-
ate red and brown algae, purple bacteria, and strikingly beautiful diatoms and lichens in a
wide range of colors. A few inventive microbes even power their photosynthetic reactions
with infrared radiation—wavelengths that are utterly invisible to our eyes but that our bare
skin senses as heat energy.
The complex origins of photosynthesis are the subject of the research of biochemist
RobertBlankenship,whoholdschairedpositionsinboththechemistryandbiologydepart-
ments at Washington University in St. Louis. Blankenship and his coworkers, including
former colleagues from the influential astrobiology team at Arizona State University,
search for signs of early life, both on Earth and on other worlds. Their strategy is to exam-
inethevariedphotosyntheticpathwaysofmanydifferentkindsoflivingmicrobes—purple,
brown,yellow,andgreen—scrutinizingtheirgenomesforsimilaritiesanddifferences.Data
come from multiple aspects of the intricate photosynthetic apparatus: the varied nature of
photosynthetic pigments, the exact molecular sequences of protein “reaction centers” that
shiftelectronsfromonemoleculetoanother,themanywaysthosetransferredelectronsare
