FIGURE 2. Chemotactic response and adaptation.

The system activity, A, of a model system (the reference system described in Methods) which was subject to a series of step-like changes in the attractant concentration, is plotted as a function of time. Attractant was repeatedly added to the system and removed after 20 min, with successive concentration steps of l of 1, 3, 5 and 7 M. Note the asymmetry to addition compared with removal of ligand, both in the response magnitude and the adaptation time. The chemotactic drift velocity of this system is presented in the inset. Inset: the different curves correspond to gradients l = 0, 0.01, 0.025 and 0.05 M/m. An average change in receptor occupancy of less than 1% per second is sufficient to induce a mean drift velocity of the order of microns per second.