Biological membranes which serve as interfaces between different cellular regions to achieve compartmentation, vary widely in their chemical composition. Gas vacu-ole membranes of blue-green algae are almost entirely protein, whereas artificial membranes may be constructed entirely from phospholipids. Some membranes contain glycosylated proteins or lipids, while others do not. Striking variations in protein and lipid composition occur among species often to the extent that the compositions of different organelles within a species have greater similarities than the same organelle from different species. Plasma membranes, while serving the same essential functions in all cells, exhibit wide variations in composition according to cell type and, with regard to specific receptors, considerable variation in composition among species. Variation may be encountered even within a single cell type and with stage of development or in response to environmental or cellular effectors. Thus, membrane biogenesis emerges as a process of considerable complexity and diversity. The outcome is an interface or compartment that is important to the regulation or segregation of cellular metabolism. Even within a single morphologically identifiable cell compartment such as endoplasmic reticulum or Golgi apparatus, not all regions morphologically defining that cell compartment are alike.
Membrane constituents are first assembled from simpler precursors by well-established biochemical pathways (Fig. 5.1). Whether it is the assembly of amino acids into membrane proteins or incorporation of head groups into phos-pholipids, the final stages of biosynthesis frequently are catalyzed by enzymes closely associated with a membrane. Few, if any, membrane constituents or terminal biosynthetic enzymes of membrane constituents occur free in the cytoplasm in significant concentrations (Table 5.1).
KeywordsEndoplasmic Reticulum Sialic Acid Golgi Apparatus Rough Endoplasmic Reticulum Biosynthetic Enzyme
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