Endoplasmic Reticulum: Structure, Function, and Composition

Endoplasmic Reticulum

In all eukaryotes (animals and plants), a complex system of sacs and tanks exists in the cytoplasm, separated by a membrane. This system, the endoplasmic reticulum (ER), covers the entire cell and delimits two non-interconnected spaces: the inside of the ducts and the hyaloplasm. It is part of the cytoplasmic vacuolar system, which also includes the nuclear envelope and the Golgi apparatus.

The ER consists of a series of tubular cavities or tanks, isolated from the cytosol by a membrane. This membrane may or may not be covered with ribosomes, which are responsible for protein and phospholipid biosynthesis. The ER is composed of flattened, stacked vesicles. The rough endoplasmic reticulum (RER) has ribosomes attached to its surface, specifically to the large subunit. The RER tanks are continuous with the nuclear envelope, which also has ribosomes attached.

The RER produces proteins for the cell or for export (secretion). The smooth endoplasmic reticulum (SER) consists of thin, interconnected tubes, appearing as tubular sections in cross-sections. It lacks ribosomes and is typically located in glycogen-rich areas. The SER synthesizes steroids and lipids.

The ER membrane is thinner than the plasma membrane (50-60 Å) and has two faces: the luminal face (towards the lumen) and the hyaloplasmic face (towards the hyaloplasm).

The ER cavities can be virtual; in active cells, they are dilated, while in resting or non-protein-synthesizing cells, they may appear collapsed. When the cavities are empty, the membranes can stick together. All cells have both RER and SER, but the abundance of each type varies with cell type and function.

• Plasma cells producing immunoglobulins have abundant RER in a concentric arrangement.
• Pancreatic cells have more RER.
• Hepatocytes have abundant SER in an irregular arrangement, forming Berg bodies.
• Muscle fibers have abundant SER, wrapping around myofibers, called the sarcoplasmic reticulum.
• Nerve cells have more SER, forming Nissl bodies.

The ER membrane contains approximately 30% lipids, 70% proteins, and a negligible amount of carbohydrates. Phospholipids and phosphatidylcholine are abundant, while glycolipids, cholesterol, and sphingomyelin are present in smaller amounts.

ER membrane proteins are categorized into three groups based on function:

1. Group 1: Approximately 30 different protein chains that have not been identified.
2. Group 2: Numerous enzymes.
3. Group 3: Two electron transport chains.