Rate of diffusive uptake by discrete absorbers. (A) The number of particles per unit time absorbed by the cell body, Icell(N), is plotted for bcell = 0.25 μm and Lcell = 1 μm, in units of the maximum rate of uptake by the cell body, Icellmax. The radius of an individual absorber is s = 1 nm, a typical size for transport proteins (26). By increasing the number of absorbers on the cell body from N = 10,000 to N = 20,000, the rate of uptake is increased by only 5% of the maximum rate. (B) The number of particles per unit time absorbed by the stalk, Istalk(N), is plotted for bstalk = 50 nm and Lstalk = 1 μm (red), 5 μm (green), and 10 μm (blue). Addition of N = 10,000 absorbers to stalks of length 1, 5, and 10 μm increases the rate of uptake by ≈50%, 140%, and 210% of Icellmax. The absorbing surface is represented in red in the figures (Left).