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A theoretical description of the simultaneous processes of photosynthesis and photorespiration in a single leaf is developed, based on the hypothesis that carbon dioxide and oxygen compete for the active site of ribulose diphosphate carboxylase. Michaelis-Menten kinetics and competitive inhibition at the end of a diffusion path provide the basic structure of the model. Data of Ludwig (1972) from sunflower are analyzed according to the formulation. This description is part of a more general physiological-ecological model of photosynthesis presented previously (Tenhunen et al., 1976a, b) and continues to elaborate sub-processes in terms of physiologically meaningful parameters. The description is considered a working hypothesis. Data on photorespiration from the literature are reviewed as they relate to this working hypothesis. Several lines of investigation are thereby suggested that will help clarify the role of photorespiration in whole leaf photosynthesis and determine the over-all utility of this modeling approach.

A sequential extraction scheme is utilized to determine the geochemical fractionation of Fe, Mn, Cr, Cu and Zn in different grain size classes for sediment samples collected from within and around Little Traverse Bay, Lake Michigan. The partitioning of Fe, Cr and Cu in both sediment and presentiy-eroding coastal bluff samples are similar. Also, the concentrations of these metals in fine-grained particles are not proportional to the increased surface area of these grains compared with larger grain sizes. Both of these results indicate that these metals have undergone little authigenic change since entering the lacustrine environment. However, sediment Zn concentrations are significantly higher than in coastal bluff samples, and also have a different geochemical fractionation. This result is attributed to the precipitation of Zn in authigenic phases. In general, Mn shows relatively little mobility in the bay's oxidizing environment, except in areas where Mn nodules are forming.

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