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Thousands of structures with membranes crowd the eukaryotic cytoplasm, the most numerous being called the endoplasmic reticulum (ER), a dense network of membranes running through the cytoplasm of every cell and enclosing a network of interconnected tubes (cisterna ....

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.... membrane. The membranes of the ER usually form a tightly packed and flattened sac-like structures, that form inter-connected compartments within the cytoplasm. The internal space formed by the membrane sheets is known as the ER lumen. In most cells the ER lumen forms a single internal compartment. The membranes of other organelles are not connected to the ER and appear to form separate distinct compartments within the cytoplasm. The ER membranes and lumen contain a large number and variety of enzymes which catalyse many different types of chemical reactions. In some cases the membranes serve as a framework for systems of enzymes that carry out sequen ....