Endoplasmic Reticulum (ER) - an Overview

The endoplasmic reticulum (ER) is a complex network of membranous tubes, sacs, and flattened channels known as cisternae, which forms a distinct compartment within the cytoplasm of the cell. Composed of a double layer of lipids with various enzymes attached to its surface, the ER is continuous with the plasma membrane, nuclear membrane, and Golgi complex.

There are two types of ER based on their appearance and functions: Rough ER (RER) and Smooth ER (SER).

Rough ER (RER) is so named because it has ribosomes attached to its surface facing the cytoplasm, giving it a rough appearance when viewed under an electron microscope.

Smooth ER (SER) is a network of interconnected tubules that is continuous with the RER, but lacks ribosomes on its surface. Most cells contain both types of ER, although the relative proportions may vary among different cell types.

The functions of Rough ER (RER) include providing mechanical support to the cell and playing a role in protein synthesis and modification. Proteins synthesized by ribosomes on the RER surface enter the cisternal space where they are modified through processes such as folding and addition of lipids and carbohydrates. Eventually, the ER membrane encloses these modified proteins, forming sacs that have different destinations. These sacs may merge with the Smooth ER (SER) or enter the Golgi membrane system for further processing.

Smooth ER (SER) houses different enzymes depending on the type of cell. In the liver, for example, the enzymes in the smooth ER are involved in detoxification processes. Smooth ER also plays a role in lipid synthesis, including the production of steroids. In muscle cells, a specialized form of smooth ER called sarcoplasmic reticulum is responsible for storing and releasing calcium ions, which are critical for muscle contraction.

Smooth ER and Rough ER