Saturday, November 10, 2007

The Photosynthesis Song and a Pet Peeve

Photosynthesis is a process where light energy is captured and converted to chemical energy in the form of ATP. The basic process is similar to that in membrane-associated electron transport. In both cases an energy source (light or reducing equivalents respectively) is used to create a proton gradient across a membrane. The dissipation of this gradient as protons move back into the cell drives the synthesis of ATP.

In addition to ATP, most types of photosynthesis can also be coupled to synthesis of NADPH. The chemical energy molecules derived from photosynthesis (ATP and NADPH) are used in many different biochemical pathways such as DNA synthesis, protein synthesis, fatty acid synthesis and carbohydrate synthesis.

Most photosynthetic organisms can fix carbon dioxide and make carbohydrates using ATP and NADPH [The Calvin Cycle] [Fixing Carbon: the Rubisco Reaction]. Many nonphotosynthetic organisms can do this too. In photosynthetic bacteria and photosynthetic protists this pathway uses only a small part of the chemical energy created by photosynthesis. In large plants the synthesis of carbohydrates can use up a significant portion of the ATP and NADPH generated by photosynthesis.

The fixation of CO2 and the synthesis of carbohydrates used to be called the "dark reactions" of photosynthesis back in the days when all we knew about were big plants. We didn't know anything about the biochemistry of photosynthetic bacteria or other species. Because our attention was focused on big plants, it was thought that photosynthesis was always coupled to the carbohydrate synthesis pathways. Now we know that this isn't true, so the old-fashioned equation,

is just not a valid representation of photosynthesis. The real products of photosynthesis don't even appear in the equation and, furthermore, those products are used for many different purposes inside the cell (especially in bacteria). Not only that, in some photosynthetic bacteria the electron donor isn't water but some other inorganic molecule like H2S and S2 is produced instead of oxygen. The equation is very misleading on many levels and should be abandoned.

Greg Laden has posted the video shown below and commented on the fact that the word "miracle" is used [The Photosynthesis Song ... Bad Word Choice?]. I'm much more worried about the misleading science in the video than I am about one instance of the word "miracle."


  1. This is why science blogging is good.

    I've added a link to your post to my post. By the way, I also sent a link to the author of the video.

  2. I love Photosynthesis so much that I shot this picture as a tribute to it: