The beta-barrel is an intergral transmembrane protein which functions as a membrane channel for molecules. The beta-barrel consists of a varying number of beta strands, from as few as 8 to as many as 22. This form of transmembrane protein is abundant in the outer membrane of mitochondria, choroplasts, and many bacteria in contrast to alpha-helix cell membrane protein which is frequently found in eukaryotic cell membranes.
Molecular Biology of the Cell, Alberts et al, 5th Edition (2007) Garland Science, New York.
Chapter 10 (pages 634-635).