[3] Bacteriorhodopsin is a light-driven H+ ion transporter found in some haloarchaea, most notably Halobacterium salinarum (formerly known as syn.

The repeating element of the hexagonal lattice is composed of three identical protein chains, each rotated by 120 degrees relative to the others.

[9][10] The modifications are: Bacteriorhodopsin molecule is purple and is most efficient at absorbing green light (in the wavelength range 500-650 nm).

[17] The conformational changes alter the pKa values of conserved amino acids in the core of the protein, including Asp85, Asp96 and the Schiff base N atom (Lys216).

The bacteriorhodopsin photocycle consists of nine distinct stages, starting from the ground or resting state, which is denoted 'bR'.

Its homologs include the archaerhodopsins,[21] the light-driven chloride pump halorhodopsin (for which the crystal structure is also known), and some directly light-activated channels such as channelrhodopsin.

It has been excessively studied on both mica[23][24] and glass substrates using Atomic force microscopy and Femtosecond crystallography.

These also produce a proton gradient, but in a quite different and more indirect way involving an electron transfer chain consisting of several other proteins.

Furthermore, chlorophylls are aided in capturing light energy by other pigments known as "antennas"; these are not present in bacteriorhodopsin-based systems.