Some prokaryotes express proton pumps called bacteriorhodopsins , proteorhodopsins , and xanthorhodopsins to carry out phototrophy .  Like animal visual pigments, these contain a retinal chromophore (although it is an all- trans , rather than 11- cis form) and have seven transmembrane alpha helices ; however, they are not coupled to a G protein. Prokaryotic halorhodopsins are light-activated chloride pumps.  Unicellular flagellate algae contain channelrhodopsins that act as light-gated cation channels when expressed in heterologous systems. Many other pro- and eukaryotic organisms (in particular, fungi such as Neurospora ) express rhodopsin ion pumps or sensory rhodopsins of yet-unknown function. Very recently, a microbial rhodopsin with guanylyl cyclase activity has been discovered.   While all microbial rhodopsins have significant sequence homology to one another, they have no detectable sequence homology to the G-protein-coupled receptor (GPCR) family to which animal visual rhodopsins belong. Nevertheless, microbial rhodopsins and GPCRs are possibly evolutionarily related, based on the similarity of their three-dimensional structures. Therefore, they have been assigned to the same superfamily in Structural Classification of Proteins (SCOP). 
When Walker published the novel in 1982, one of the most highly praised features of the book was its use of language. Mel Watkins of The New York Book Review commented that the novel “assumes a lyrical cadence of its own...The cumulative effect is a novel that is convincing because of the authenticity of its folk voice.” The language was particularly important to Walker. She later explained what happened after she sent her finished novel to a leading black women’s magazine which, she believed, probably would recognize its merits quicker than anybody else. The magazine turned the novel down, however, on the ground that “black people don’t talk like that” (Garrett & McCue, 1990, p. 229). The subsequent success of the novel exposes such totalizing statements as untrue, for it is Celie’s powerfully idiomatic voice that captures her specific situation and that of so many African Americans of her time.