The main function of the photosynthetic leaf is to convert light energy into chemical energy and to assimilate CO 2 . Coordination of these functions as related to the heterogeneous leaf structure and the distribution of metabolism within the leaf has been investigated in many studies [1‐5, etc.]. Presumably, after hundreds of millions of years of selection, the typical mesophytic leaf structure and the distribution of metabolic activity within it are relatively optimal. The stimuli that result in the formation of specific structural or functional features of the leaf may emerge through generalization of the experimental data using the unified theory of ecological utility. To describe the functioning of biological systems on the basis of the rule of maximal ecological utility (MEU) [6], the function of partial ecological utility is entered for each defined parameter of the system. It is possible to use any limited monomial function [7]. At optimal functioning of a system, the general utility as a product of partial utilities (in the case of their independence from each other) should be maximized. Some authors suppose that photosynthesis follows the light absorption gradient [8]. However, light absorption decreases from the top, illuminated surface to the bottom of the leaf [5], while the maximal CO 2 fixation occurs in the middle of palisade mesophyll [9]. The results imply that a significant amount of absorbed light energy may not be utilized directly for CO 2 fixation at the top of the leaf. It is possible that low photosynthesis in the upper layer of the palisade is due to excess illumination, which induces the formation of oxygen radicals (O 2 uptake under saturated light might reach 50% of its maximal evolution [10]). If the presumption is correct, the optimization of CO 2 fixation within the leaf should take place when the total utility of the two functions, (1) light absorption for photosynthesis and (2) protection from light intensities that can cause photodestruction, is maximized. The goal of the present work was to use the MEU rule to describe an optimal model of photosynthesis in a bifacial, mesophytic leaf of a C 3 plant. Let us consider this on a simplified model, where the leaf is a flat plate, and its surface is illuminated by light ( I 0 ; ν in the PAR range). The light is absorbed by chlorophyll ( a / b = 3/1 [9]) in a layer of phototrophic cells in the depth of the plate; the plate itself has the same absorption spectrum. In such a leaf, the function r ( z ) (partial utility of protection from photodestruction) and the function s ( z ) (partial utility of light absorption for photosynthesis) depend on the depth of the photosynthetic layer within the relative thickness ( h ) of the leaf (the variable h is incorporated by variable z , where