5.8: The Endoplasmic Reticulum

The endoplasmic reticulum or ER makes up for more than half of the membranes in a cell and accounts for 10% of total cell volume. It is also the primary protein and lipid synthesis factory for most cell organelles, such as the Golgi apparatus, lysosomes, secretory vesicles, and the plasma membrane. Despite being the most extensive and functionally complex subcellular organelle, ER was the last to be discovered. After years of deliberation, Keith Porter and George Palade in the year 1954, produced the first high-resolution electron microscope images to affirm the presence of ER in the eukaryotic cell.

The ER membranes were identified as sites for protein synthesis and crucial for intracellular transport using radio-labeled and fluorescent-labeled amino acids. The isolation of ER is a tricky task as it forms an intricate mesh with other subcellular organelles. However, on cell homogenization, the disrupted ER membranes reseal into small closed vesicles called microsomes. These vesicles form a microsystem capable of sustaining all ER-related functions like protein and lipid synthesis, calcium signaling, and glycosylation. Subcellular fractionation is the best and often-used technique for the purification of these membranes. When separating using a sucrose gradient, the rough ER microsomes sediments at a higher density than smooth ER microsomes.

The ER network in a cell is dynamic. It is constantly shape-shifting along with the cytoskeleton to bolster mechanical support for the cell structure. Despite the distinct cisternae and tubule morphologies of the ER membrane, interconversion between the two is possible and is governed by the expression of membrane proteins. The ER network rearranges by tubule growth, retraction, and fusion of adjacent ER-ER membranes.

As mentioned above, the ER network is required for optimal overall cellular health. Disruption of ER morphology is linked to pathological conditions, including neurological disorders like Alzheimer's disease, hereditary spastic paraplegia, and viral infections like hepatitis C virus, and dengue virus.

The endoplasmic reticulum or ER is an interconnected membranous organelle that runs continuously with the outer nuclear membrane and stretches extensively into the eukaryotic cell's cytoplasm.

The ER has three structural morphologies - the nuclear envelope, peripheral cisternae, and an interconnected tubular network.

The nuclear envelope is made of stacked inner and outer nuclear membranes to form a cisternal circle that contains the nucleoplasm and the genome.

The outer nuclear membrane continues into the peripheral cisternae - a network of flattened vesicles with a large lumen enclosed by ER membranes.

When viewed microscopically, this part of the ER resembles a beaded string due to the presence of membrane-bound ribosomes and is commonly known as the rough ER.

The attached ribosomes release newly translated polypeptide chains into the lumen of the rough ER, where the chaperone proteins from the ER quality control systems assist the polypeptide in folding into appropriate tertiary structures.

After passing the quality control check, proteins are packaged into vesicles and released towards the Golgi apparatus.

Further away from the rough ER are the interconnected ER tubules.

This region is devoid of attached ribosomes and is called the smooth ER. It synthesizes carbohydrates and lipids, which are packaged into vesicles or released through transporters on the ER membrane, for delivery to other parts of the cell.