The Overall Reaction
The basic elements of the generalized photosynthetic reaction synthesizing glucose in eukaryotes were worked out in the early nineteenth century:
Subsequent research, extending into this century, established that some bacteria carry out photosynthesis using electron and H+ donors other than H2O. Many substrates, such as H2S, thiosulfate, alcohols, and hydrogen itself, can be used. This led C. B. Van Niel in 1935 to propose a more basic expression for the reactions of photosynthesis in which the substance D is an electron donor and (CH20) is a unit of
carbohydrate. In algae and higher plants, D is oxygen, and H2D is H20. The product 2D on the right-hand side of the equation in higher plants is 02. In some bacteria D is sulfur, and H2D is H2S. In these bacteria, sulfur or 2S is liberated as the 2D product of Reaction 8-2.
Since Van Niel's time, it has become clear that plants and bacteria can also use electron acceptors besides C02. For example, higher plants can reduce nitrate (NO:l") as well as C02. This has led to an even more generalized expression for photosynthesis, using A to indicate any electron acceptor: When the acceptor is NO:!"~, which is reduced to ammonia in photosynthesis, the expression reads:
In this chapter, we will be concerned primarily with the photo-synthetic mechanism in which A is C02 and H2D is H20:
In the higher plants, six of the (CELO) units combine to produce a six-carbon sugar such as glucose.
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