|Smooth and rough endoplasic reticulum; Credit: Wikimedia|
The endoplasmic reticulum is a network of flattened sacs and tubules that perform various functions in our cells. This network is divided into two regions that differ in both, function and structure. These regions are called, the rough endoplasmic reticulum and the smooth endoplasmic reticulum.
Smooth Endoplasmic ReticulumThe smooth endoplasmic reticulum is a network of vesicles, sacs, and tubules which is continuous with the rough endoplasmic reticulum, but lacks membrane bound ribosomes.The smooth ER contains enzymes involved in biosynthesis of triglycerides, sterols, and phospholipids.The smooth endoplasmic reticulum has various functions, including:
Steroid synthesis: Smooth endoplasmic reticulum synthesizes and metabolizes steroids. Steroids are organic compounds that are essential for synthesis of various compounds, such as; sex hormones, cholesterol, and anabolic steroids.
Glycogen degradation and gluconeogenesis: The enzyme, glucose-6-phosphatase removes phosphate from glucose-6-phosphate. This controls the formation of glucose from glycogen via gluconeogenesis.
Reactions in lipid metabolism: The smooth ER is site of assembly of lipoproteins. Lipolysis also begins in the smooth ER with the release of fatty acids from triglyceride.
Storage and release of calcium ions: in striated muscle the smooth ER is known as sarcoplasmic recticulum and is responsible for the sequestration and release of calcium ion. Calcium ions are essential for muscle contraction and relaxation.
Detoxification reactions: These reactions make toxic compounds water-soluble so that they can be excreted. These reactions include:
Hydroxylation reactions: These reactions utilize hydroxylase complexes.
Conjugation reactions: in these reactions the polar groups are transferred from an active carrier to the toxic water-insoluble molecule.
Rough Endoplasmic ReticulumThe rough endoplasmic reticulum is an extensive organelle composed of convoluted flattened sacs, which is continuous with the nuclear membrane. The outer surface is studded with ribosomes, giving it the name of ‘rough’ ER. These ribosomes are called membrane bound ribosomes and are attached to the cytosolic side of the endoplasmic reticulum.
Ribosomes on the cytosolic side of the endoplasmic reticulum are responsible for the assembly of various proteins. The process of protein assembly is called translation. These ribosomes synthesize proteins that are destined for the plasma membrane, lysosomes, Golgi apparatus, and secretion. The functions of rough endoplasmic reticulum also include:
Protein folding: Proteins synthesized by the membrane studded ribosomes fold into 3-D shapes in the lumen of the rough endoplasmic reticulum. The shape of the protein is essential for its optimal function.
Secretory glycoproteins: Most secretory proteins are covalently bonded with carbohydrates that is they are glycoproteins. The carbohydrate unit is covalently bonded to the proteins by specialized molecules in the endoplasmic reticulum. These secretory proteins are packaged into vesicles before being transported out of the rough endoplasmic reticulum.
Membrane factory: The rough endoplasmic reticulum grows by adding phospholipids and membrane proteins to its own network. It assembles phospholipids with precursors in the cytosol.