Double Membrane Surrounded Compartments

An extensive array of internal membranes in eukaryotes creates compartments within a cell for distinct biochemical functions. For instance, a double membrane surrounds the nucleus, the location of most of the cell s genetic material, and the mitochondria, the location of most ATP synthesis. A single membrane defines the other internal compartments, such as the endoplasmic reticulum. Some compartments can exchange material by the process of membrane budding and fusion. As with all membranes, the proteins associated with these membranes determine the specific biochemical function. Specific amino acid sequences in the proteins direct these molecules to the appropriate compartment. 

In the remainder of this chapter, we examine how proteins synthesized on cytosolic ribosomes are sorted to mitochondria, chloroplasts, and peroxisomes (see Figure 16-1). Both mitochondria and chloroplasts are surrounded by a double membrane and have internal subcompartments, whereas peroxisomes are bounded by a single membrane and have a single luminal compartment known as the matrix. Because of these and other differences, we consider peroxisomes separately in the last section. 

In bacterial cells there is no membrane surrounding the DNA nucleoid, and both DNA transcription and RNA translation proceed within the one-cell compartment. A ribosome can begin translation of mRNA before transcription is complete. In contrast, in eukaryotes the nucleus is bounded by a double membrane. Transcription occurs within the nucleus, and the completed mRNA must pass into the cytoplasm to be translated. Frequently, the immediate polypeptide product of translation is subsequently modified, sometimes in a process that enables it to be transported aroruid or out of the cell. 

Three types of compartments are of significance in secondary metabolism (a) microcompartments, (b) organelles surrounded by a single membrane, and (c) organelles surrounded by double membranes. 

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