A simulation of the cell under normal conditions: extracellular pH=7.6. The graphical axes follow the simulated cell; the square grid of black markers is fixed relative to the substrate. The lamellipodium is shaded according to pH value, as indicated by the color bar at the bottom of the figure. The pH values between the yellow and blue lines denote the contraction zone (i.e. the pH range over which the bundling stress and adhesion in the model cytoskeleton vary sharply). Note that cytoskeletal nodes in the middle of the lamellipodium remain almost fixed relative to the substrate (retrograde flow). Note also that there is a virtual circumferential flow of the boundary cytoskeletal nodes (consistent with the graded radial extension model). The steady gliding of the virtual cell exhibits the features of stability and persistence (see also http://www.cnr.berkeley.edu/~bottino/research/wormsperm/movies/movie_normal_long.qt).

A simulation of the cell with body friction increased fourfold in comparison to the ‘normal’ run. Note that the cell body moves much more slowly than in the normal run, but the lamellipodial shape is stably maintained and retrograde flow is slightly enhanced (see also http://www.cnr.berkeley.edu/~bottino/research/wormsperm/movies/movie_hi_cb_friction.qt).

A simulation of the cell under lower pH conditions (see Fig. 5, middle). Extracellular pH parameter pH_e=6.75 (‘normal’ is 7.6). The lower extracellular pH value results in a more acidic lamellipodial interior; this in turn inhibits the protrusive velocity of the leading edge. However, cell body traction continues (see also http://www.cnr.berkeley.edu/~bottino/research/wormsperm/movies/movie_pH_low.qt).

A simulation of the cell under the lowest pH conditions (see Fig. 5, top). Extracellular pH parameter pH_e=6.35 (‘normal’ is 7.6). In this simulation, the lower extracellular pH value results in an acidic lamellipodial interior, weakening both adhesion and gel assembly. This in turn causes the lamellipod to collapse and diminish cell body translocation (see also http://www.cnr.berkeley.edu/~bottino/research/wormsperm/movies/movie_pH_lowest.qt).