HeLa cells under normoxic (a–d) and hypoxic (e–h) conditions. Panels (a) and (e) represent time point 0, and (b) and (f) show cells after 72 h in normoxic or hypoxic environments, respectively. Panels (c) and (g) show the oxygen profile over a period of 72 h, where the pericellular oxygen concentration was measured with an automated microsensor. The calculated usage of oxygen per cell per hour is depicted in panels 2(d) and (h).