Adapted from Irvine & Schell (2001). This is a simplified version, designed to illustrate only specific points discussed in the text. Thus, no phosphatase reactions (other than the conversion of Ins(1,3,4,5)P4 to Ins(1,3,4)P3) are shown, and no pathways from higher plants are illustrated. Thick black arrows indicate reactions universal (as far as we know) in eukaryotes, and thinner coloured arrows illustrate reactions confined to specific groups of organisms (again, as far as we know, and within the limits of the discussion in the text): purple, slime moulds; blue, animals; red, yeast. For fuller versions of these pathways, see Irvine & Schell, 2001; Shears, 2001). Note that the isomeric configuration of (PP)2InsP4 is not yet known for animal cells (Irvine & Schell, 2001).