*C. Vitamin C plus undamaged substrate*

Concentrations of vitamin C measured in cell pellets after 6 h or 24 h incubation are shown in Figure 7. The concentration within the cells is estimated on the basis of a volume of 5 μL per 106 cells. It is clear that vitamin C is taken up by HeLa cells and accumulates to a concentration far higher than that provided in the medium.

**Figure 7.** Uptake of vitamin C by HeLa cells. Cells were incubated for 6 h in medium containing vitamin C at 50 μM (light shading) or 200 μM (dark shading). Intracellular vitamin C was measured by HPLC at the end of this incubation or after a further 24 h incubation.
