Figure 1. Mechanisms underlying rod-shaped cell-wall growth. (a)
Top: in rod-shaped bacteria such as E. coli, new cell wall is inserted along
the cylindrical midcell (shaded green region) and not at the poles (cyan).
Recent evidence suggests that insertion occurs in bursts (green patches)
and is coordinated by the bacterial actin homolog MreB. Bottom: the
circumferential motion of cytoplasmic MreB polymers (purple) is dependent
on cell-wall synthesis, suggesting that MreB tracks represent new glycan
strands (green) that have been added into the old wall with peptide
crosslinks (red). (b) In tip-growing organisms such as S. pombe, new cell
wall is added and remodeled at the growing cell tip(s), and turgor pressure
provides force for elongation. Cell-wall synthases and new membrane are
targeted to the cell tip by membrane trafficking directed by actin cables
that emanate from the cell tips. (c) Stresses in a spherical shell are the
same in every direction, while for a thin cylindrical shell the circumferential
stress is twice as large as the longitudinal stress (equation 2).