Figure 8-9The carotenoids /3-carotene (a) and xantho-phyll (b).

Figure 8-10 The light-absorbing structure of a phycocyanin.

net effect of the entire complement of photosynthetic pigments is to greatly broaden the spectrum of wavelengths usable as energy sources for photosynthesis.

Reactions of Photosynthesis 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 (CH₂O) is a unit of carbohydrate. In algae and higher plants, D is oxygen, and H₂D is H₂O. The product 2D on the right-hand side of the equation in higher plants is O₂. 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.

Van Niel's general equation for photosynthesis was confirmed when isotopes of the elements became available. Using isotopes of carbon and oxygen allowed the atoms of CO₂ and H₂O to be labeled and followed through photosynthesis. If an organism carrying out photosynthesis was supplied with water containing the heavy isotope ¹⁸O, the label showed up in the oxygen given off as a by-product. If CO₂ containing the ¹⁸O isotope was supplied, the label showed up in the carbohy­drates produced in photosynthesis and in the water molecules assembled as a by-product of the reactions (see Reaction 8-2). These results established that the oxygen evolved in photosyn­thesis is derived from the H₂O molecules entering the process, as Van Niel proposed.

When the acceptor is NO₃, which is reduced to ammonia in photosynthesis, the expression reads:

Since Van Niel's time, it has become clear that plants and bacteria can also use electron acceptors besides CO₂. For example, higher plants can reduce nitrate (NO₃~) as well as CO₂. This has led to an even more generalized expression for photosynthesis, using A to indicate any electron acceptor:

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