Publisher Summary This chapter discusses membrane proteins. These proteins are almost as simple as fibrous proteins. Membranes are films of lipids (fat) and protein molecules. They envelop cells and closed volumes within them (the so-called compartments). The peculiar role of membrane proteins (they amount to half of the membrane weight) is to provide transmembrane transport of various molecules and signals. The membrane is a kind of "insulator" while its proteins act as "conductors". These conductors are specific, each ensuring transmembrane transport of molecules of a particular kind or signals from particular molecules. The chapter describes the photosynthetic reaction center. Its function is to ensure the transport of light-released electrons from one side of the membrane to the other, thereby creating the transmembrane potential that underlies photosynthesis. The photosynthetic reaction center performs electron transport from the upper to the lower compartment that is against the apparent difference in electric potential among the compartments.