RNA is transported out of the nucleus through the nuclear pores. Proteins needed inside the nucleus are transported in through the nuclear pores. Ribosomes Ribosomes are the sites of protein synthesis , where RNA is translated into protein. ER bound to ribosomes is called rough ER because the ribosomes appear as black dots on the ER in electron microscope photos , giving the ER a rough texture. These organelles are quite small, made up of 50 proteins and several long RNAs intricately bound together.
Ribosomes have no membrane.
Ribosomes disassemble into two subunits when not actively synthesizing protein. Mitochondria have two membranes, an inner and an outer, clearly visible in this electron microscope photo of a mitochondrion . Note the reticulations, or many infoldings, of the inner membrane, This serves to increase the surface area of membrane on which membrane-bound reactions can take place. The existence of this double membrane has led many biologists to theorize that mitochondria are the descendants of some bacteria that was endocytosed by a larger cell billions of years ago, but not digested. This fascinating theory of symbiosis, which might lend an explanation to the development of eukaryotic cells, has additional supporting evidence.