Final answer:
Blebbing involves detachment of the cell membrane from the actin cytoskeleton and is characterized by spherical protrusions, whereas lamellipodia formation is an actin-driven process with myosin II activity, resulting in flat, broad extensions of the membrane tightly associated with the actin network.
Step-by-step explanation:
The difference in membrane protrusion by blebbing and by lamellipodia is related to their cellular mechanisms and the actin cytoskeleton interactions. Blebbing is characterized by the detachment of the cell membrane from the underlying actin cytoskeleton, leading to the formation of spherical protrusions due to the hydrostatic pressure within the cell. Conversely, the formation of lamellipodia involves a complex interplay of actin filament polymerization, branching, and crosslinking, resulting in a flat, broad extension of the cell membrane closely associated with the actin network.
In lamellipodia, polymerization of actin filaments at the leading edge and their subsequent branching, possibly mediated by the Arp2/3 complex, create a dendritic network that pushes the plasma membrane forward. This is accompanied by the formation and maturation of focal adhesions, which provide traction against the substrate. Myosin II activity is also associated with lamellipodia and the contractile force it generates, particularly in the lamella region behind the lamellipodia.
In contrast, blebs form when the actin cortex is disrupted and does not support the membrane, leading to a local outward bulging. Thus, while lamellipodia require a tight interaction between the membrane and actin cytoskeleton, blebs arise from a loss of such interaction.