The outer membrane of a heterocyst-forming cyanobacterium is a permeability barrier for uptake of metabolites that are exchanged between cells.
Nicolaisen K, Mariscal V, Bredemeier R, Pernil R, Moslavac S, López-Igual R, Maldener I, Herrero A, Schleiff E, Flores E. Mol Microbiol. 2009 Oct; 74(1):58-70. Epub 2009 Aug 24.