The biological consequences of exposure to TiO2, UV light and their combined effects were studied on the cellular envelopes of Tetrahymena. For Tetrahymena cells treated with TiO2 or UV light alone, the cell membrane shrunk while still maintaining the original elliptoid shape. Cells treated by TiO2 under UV light irradiation experienced the most serious damage by peroxidation. A pear-shaped cell was formed due to serious shrinkage and cilia loss. An increase in the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene was observed, indicating a significant decrease in membrane fluidity. Quantum dot (QD) labeling revealed that damaged cells could not function properly or absorb extracellular materials selectively. QDs were able to enter the damaged cells or be absorbed on the cell surface. Attenuated total reflection Fourier transform infrared spectra revealed that amide groups and PO2− of the phospholipid phospho-diester, both in the hydrophobic end exposed to the outer layer, were the easiest to be oxidized. Other groups like CH2 and CH3 were also involved, but showed a resistance to photocatalytic peroxidation. The differences can be attributed to the bond energy as well as the ordering and position of the groups.