Fig. 3. The relationship between the molecular activity of the sodium pump (Na⁺/K⁺-ATPase) and the surface pressure of (A) membrane lipids and (B) phospholipids from kidney and brain microsomes of rats (Rattus norvegicus) and cane toads (Bufo marinus). Surface pressures were calculated at an average molecular area (per lipid molecule) of 0.30 nm² and 0.45 nm², respectively, for membrane lipids and phospholipids. Each symbol represents the mean ± S.E.M. of six determinations. Each inset graph show the relationship for the 24 individual data points.