The relative ability of a nonphotosynthesizing organ or tissue—so-called sink sites (see mass flow)—to acquire and use the products of photosynthesis (i.e. photoassimilates) produced elsewhere in the plant. Sink strength is one factor determining the rate of flow of photoassimilate (chiefly sucrose) from leaves via phloem to sink organs such as grains, fruits, or tubers. The strength of these sinks is determined by their size and physiological ability to maintain an effective concentration gradient of sucrose between phloem and tissue cells. The latter depends on the efficiency of the sucrose transporter, a cellular protein that actively unloads sucrose from phloem, and of enzymes like invertase, which converts sucrose into glucose and fructose.