Fig. 3. Rapid mobility of DsRed1 within the mitochondrial matrix. A long mitochondrion in the periphery of a HeLa cell was located (A, arrowheads). A small area at one end of the mitochondrion (B, white square) was photo-bleached. (C) The fluorescence recovery was monitored for 90 s at 2 Hz. Fluorescence from a region at the photo-bleached end of the mitochondria (blue circle in 3B and corresponding blue line in D) was compared with the fluorescence from a region that was also photo-bleached but 5 µm closer to the unbleached end of the mitochondria (purple circle in 3B and corresponding purple line in D). The half-maximum fluorescence recovery (0.5 F/F₀) took 5 s longer in the site 5 µm further from the unbleached end of the mitochondrion, giving an estimated linear diffusion rate for DsRed1 of approximately 1 µm s^-1 along the mitochondrion. Scale bar, 5 µm.