15.1
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 prima…
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.
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Q1: What are the three main structural regions of the endoplasmic reticulum?
The ER has three distinct structural morphologies: the nuclear envelope, which contains the nucleoplasm and genome; peripheral cisternae, flattened vesicles with large lumens enclosed by ER membranes; and interconnected tubular networks. The peripheral cisternae appear beaded due to attached ribosomes, forming the rough ER, while the tubular region lacks ribosomes and is called the smooth ER.
Q2: How does the rough endoplasmic reticulum differ from the smooth endoplasmic reticulum in function?
The rough ER synthesizes proteins; attached ribosomes release newly translated polypeptides into the lumen where chaperone proteins assist folding. After quality control, proteins are packaged into vesicles for delivery to the Golgi apparatus. The smooth ER, devoid of ribosomes, synthesizes carbohydrates and lipids, which are packaged into vesicles or released through transporters for cellular distribution.
Q3: What percentage of total cell membranes and volume does the endoplasmic reticulum comprise?
The endoplasmic reticulum makes up more than half of all membranes in a cell and accounts for approximately 10 percent of total cell volume. Despite being the most extensive and functionally complex subcellular organelle, the ER was the last to be discovered, with high-resolution electron microscope images first produced in 1954 by Keith Porter and George Palade.
Q4: How do microsomes form and what functions do they retain?
When cells are homogenized, disrupted ER membranes reseal into small closed vesicles called microsomes. These vesicles form a microsystem capable of sustaining all ER-related functions including protein and lipid synthesis, calcium signaling, and glycosylation. Subcellular fractionation using sucrose gradients separates rough ER microsomes, which sediment at higher density, from smooth ER microsomes.
Q5: What role do chaperone proteins play in the rough endoplasmic reticulum?
Chaperone proteins from the ER quality control systems assist newly translated polypeptide chains in folding into appropriate tertiary structures within the rough ER lumen. After passing the quality control check, properly folded proteins are packaged into vesicles and released toward the Golgi apparatus for further processing and distribution.
Q6: How does the ER network maintain its dynamic structure within the cell?
The ER network is dynamic and constantly shape-shifts along with the cytoskeleton to provide mechanical support for cell structure. Interconversion between cisternae and tubule morphologies is governed by membrane protein expression. The ER network rearranges through tubule growth, retraction, and fusion of adjacent ER membranes, maintaining optimal cellular health.
Q7: What pathological conditions are linked to disruption of endoplasmic reticulum morphology?
Disruption of ER morphology is associated with several pathological conditions including neurological disorders such as Alzheimer's disease and hereditary spastic paraplegia, as well as viral infections like hepatitis C virus and dengue virus. These connections highlight the critical importance of maintaining proper ER structure for overall cellular health and function.