The document describes the endoplasmic reticulum (ER), a network of membranes in both plant and animal cells, divided into rough and smooth ER based on the presence of ribosomes. Rough ER is involved in protein synthesis and membrane manufacturing, while smooth ER is associated with lipid synthesis, carbohydrate metabolism, and calcium regulation. Malfunctions of ER can lead to various diseases, highlighting its crucial role in cellular function.