Part 1 of Essay
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 (cisternae), which form a connecting channel throughout the cytoplasm ....

Part 2 of Essay
.... or incorporated into the cell 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 ....