Membranes not only segregate heritable genetic molecules into the nucleus and mitochondria, but also separate various cellular functions into distinct areas of the cell. The compartmentalization of cellular functions, such as molecular synthesis, modification, and catabolism, increases the local concentration of reactive molecules, thus improving the cell's biochemical efficiency. This partitioning also protects inappropriate molecules from becoming substrates for these processes. One example of this segregation is the endoplasmic reticulum (ER), which consists of a complex of membranous compartments where proteins are synthesized. Glycoproteins are synthesized by ribosome-ER complexes known as rough ER (RER), while lipids are produced in the smooth ER. The Golgi apparatus consists of numerous membrane-bound sacs where molecules generated in the ER become modified for transportation out of the cell.
In addition, peroxisomes and lysosomes segregate digestive and reactive molecules from the remainder of the cellular contents to prevent damage to the cell's internal molecules and infrastructure.
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