A Mechanistic Model for Photosynthesis

To begin our discussion of a mechanistic model for photosynthesis, let us return to Eq. (12.1b) for the overall process of photosynthesis in green plants  

It should be recalled that for purple sulfur bacteria the equation is [Eq. (12.2)] 

To make the two systems directly analogous. Van Niel suggested that the reaction for photosynthesis in green plants should be written as 

This of course is a grossly oversimplified picture, as we shall see later, but it serves as a starting point for our discussion. In actuality, four reducing equivalents and four oxidizing equivalents are required to convert one molecule of CO2 to carbohydrate and to liberate one molecule of O2 from water. 

Recent experimental results indicate that photosynthesis probably does indeed result from a scheme similar to the one presented above, although a large number of the individual participants in the scheme remain unknown to date. Let us now briefly review these experimental findings. 

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Finally, cell-quota theory treats the entire cellular content of a nutrient as being the pool controlling growth rate (under limiting conditions), and deals with multiple nutrient interactions empirically. At the third level of description, MECHANISTIC models aim to embody realistic accounts of the main biochemical processes and pools within cells. A recent example (Flynn Hipkin, 1999 Flynn, 2001) deals with nitrogen, phosphorus, silicon and iron as well as the carbon content of cells, photosynthesis, and the uptake competition between ammonium and nitrate. However, the model embodies many parameters and there is currently insufficient information to use it to distinguish between groups or species of phytoplankters. Its characteristic nutrient quota parameters are included in Table 9.3 except for those for silicon, which are cell-based. 

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