Figure 4. Tracing rhodopsin diffusion at acidic pH. A: Absorbance changes at two sides of a ROS after half-ROS bleach (configuration a in Figure 1) converge demonstrating diffusion exchange by rhodopsin. ROS diameter is 12.9 μ. Here and onward, thin black line shows absorbance changes at nominally unbleached side, while the black bold trace is recorded from the bleached side. Arrow marks the bleaching flash. The two traces were adjusted vertically to bring absorbances at 750 nm to zero (stretch starting from 450 s). B: Absorbance difference between unbleached and bleached side, shown by bold blue line, dissipated along an exponent with the time constant of 133 s (smooth black line). This yielded the apparent diffusion constant, D_a=1.4×10⁻⁹ cm²s⁻¹.