Friday, August 27, 2004

Defining photosynthesis

Photosynthesis is a pretty miraculous thing. Organisms (most people think of plants) capture radiant energy from the great nuclear reactor in the sky and turn it into something they can use to do biological work. It's a fundamental part of life as we know it that's been studied for centuries, and thus you'd think most dictionaries and other places would have the definition of photosynthesis down pat.

Let's look at a few, starting with
  • "The process in green plants and certain other organisms by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a byproduct." (from the The American Heritage® Dictionary of the English Language, Fourth Edition, 2000)
  • "The process in green plants and certain other organisms by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source." (from the The American Heritage® Stedman's Medical Dictionary, 2002)
  • "The process of constructive metabolism by which carbohydrates are formed from water vapor and the carbon dioxide of the air in the chlorophyll-containing tissues of plants exposed to the action of light." (Webster's Revised Unabridged Dictionary, 1998)
  • Encarta's page on photosynthesis starts with, "Photosynthesis, process by which green plants and certain other organisms use the energy of light to convert carbon dioxide and water into the simple sugar glucose." (the article is quite long, and continues on to talk about other variations; this is just the first sentence)
  • A top 10 google-ranked page for "photosynthesis definition" (link; it was the first thorough non-dictionary link) provides this definition, "Photosynthesis is the conversion of carbon dioxide and water into a sugar called glucose using sunlight energy. Oxygen is produced as a waste product." (the rest of the text on the page discusses solely plants)
While these definitions share many features, one outshines the rest: they're all wrong.

The first glaring mistake in one of these definitions is the restriction to plants, since various species of Bacteria, Archaea, and Protista can also photosynthesize.

The primary problem with the definitions is that much of the biochemical information they specify is incomplete. All of the definitions include that the organisms take in carbon dioxide, yet there are numerous photosynthesizing organisms (primarily Bacteria) that do not use carbon dioxide as a carbon source, but still use light as an energy source. The prime example of this style of growth (termed photoheterotrophy) are purple nonsulfur bacteria, a family of bacteria that can use fatty acids, organic acids, amino acids, sugars, alcohols, and aromatic compounds (e.g. benzoate) as carbon sources in place of carbon dioxide. More commonly known is photoautotrophy, wherein organisms (e.g. plants) get their energy from light and their carbon from carbon dioxide.

Two of the definitions above specify that photosynthesizing organisms produce oxygen (and the others imply it by specifying water as a part of photosynthesis), yet oxygen production is not a common characteristic of all photosynthesizing organisms. This isn't even a terribly new observation, as the theory that photosynthesizing organisms always produce oxygen was first brought into doubt in 1883 by Theodor Englemann when he reported that purple sulfur bacteria appeared to be living solely on energy provided by the sun, yet did not produce oxygen.

Oxygen is produced by many photosynthetic organisms because they use water (H2O) as an electron donor. Photosynthetic organisms that don't produce oxygen (anoxygenic photosynthesizers) use some compound other than water as an electron donor. Hydrogen sulfide (H2S), thiosulfate (S2O32-), elemental sulfur (S0), and ferrous iron (Fe2+) can all be used by purple sulfur bacteria as electron donors, each potentially producing unique products. As an example, bacteria in the genus Ectothiorhodospira use hydrogen sulfide as an electron donor, and produce elemental sulfur as a product instead of oxygen (they typically live in salty aquatic environments).

So, if photosynthesis is not limited to plants, and doesn't necessarily include carbon dioxide, water, and oxygen, how is it defined? Here's one good definition:
"Photosynthesis is a series of processes in which electromagnetic energy is converted to chemical energy used for biosynthesis of organic cell materials; a photosynthetic organism is one in which a major fraction of the energy required for cellular syntheses is supplied by light." (Gest 2002)
By not focusing on the reagents used in one style of photosynthesis, this definition allows the full host of photosynthetic organisms, from bacteria to plants, to be included.

To be fair to the web, not all the definitions of photosynthesis out there are wrong. Here's a sampling of definitions I found that are pretty good (again starting with
  • "synthesis of chemical compounds with the aid of light sometimes including the near infrared or near ultraviolet; especially : the formation of carbohydrates from carbon dioxide and a source of hydrogen (as water) in chlorophyll-containing cells (as of green plants) exposed to light involving a photochemical release of oxygen through the decomposition of water followed by various enzymatic synthetic reactions that usually do not require the presence of light" (from the Merriam-Webster Medical Dictionary, 2002)
  • "synthesis of compounds with the aid of radiant energy (especially in plants)" (Wordnet 2.0, 2003)
  • "Photosynthesis is a biochemical process by which the energy of light is converted into chemical energy in plants, algae, and certain bacteria." (Wikipedia)
  • The top google rank page for "photosynthesis" (link, link to page with definition) describes it as, "Photosynthesis is carried out by many different organisms, ranging from plants to bacteria. The best known form of photosynthesis is the one carried out by higher plants and algae, as well as by cyanobacteria and their relatives, which are responsible for a major part of photosynthesis in oceans. All these organisms convert CO2 (carbon dioxide) to organic material by reducing this gas to carbohydrates in a rather complex set of reactions." (the article continues on, going into extreme depth)
Gest, H. 2002. History of the word photosynthesis and evolution of its definition. Photosynthesis Research 73: 7–10. (pdf link)

Madigan, M., Martinko, J., and Parker, J. 2003. Brock Biology of Microorganisms. 10th edition. Prentice Hall, NJ.

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