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.