Endoplasmic reticulum is membrane system continuous from nuclear envelope and extends to cytoplasm. The word endoplasmic means “within the cytoplasm,” and reticulum is Latin for “little net”. In many eukaryotic cells, it accounts more than half of the total membrane. It has lumen inside. The space of the two layers nuclear envelope are connected to the lumen as the endoplasmic reticulum continuous with the nuclear envelope.
There are two distinct and connected to each other:
- Rough endoplasmic reticulum
- Smooth endoplasmic reticulum
Rough endoplasmic reticulum
Rough endoplasmic reticulum is abundant in cell and has ribosomes attached on the outer surface of the membrane. It is observed as stacks of flattened sacs with many ribosomes attached. Ribosomes synthesize polypeptide chains. Those newly formed chains which have the signal will enter rough endoplasmic reticulum and secrete secretory proteins. The secretory proteins are mostly glycoprotein, proteins that have carbohydrates covalently bonded to them. Secretory proteins are transported out from rough endoplasmic reticulum to Golgi bodies by wrapped insides the transport vesicles. For instance, pancreatic cells synthesize insulin protein in rough endoplasmic reticulum and secrete them into bloodstream to regulate the blood glucose level.
It is also cell membrane factory. It grows inside cell by adding membrane proteins and phospholipids to its own membrane.
Smooth endoplasmic reticulum
Smooth endoplasmic reticulum does not have ribosomes attached on the surface. It is looked like connecting pipe. The smooth ER functions in many metabolic processes,
which vary in different cells. Enzymes insides are important in the synthesis of lipids including oils, phospholipids and steroids. Steroids produced are the sex hormones and the various steroid hormones secreted by the adrenal glands. Sexual reproductive organs like testes and ovaries are rich of smooth endoplasmic reticulum due to its functions.
It also involves in carbohydrate metabolism and detoxification of toxins and poisons. Detoxification involves adding hydroxyl groups to drug molecules, making them more soluble in water and easier to be excreted from the body. For instance, sedative phenobarbital are metabolized in the smooth endoplasmic reticulum in liver cells. Alcohol induces the proliferation of smooth endoplasmic reticulum and increases the enzyme. Therefore, it increases the rate of detoxification.
Calcium ions are also stored in smooth endoplasmic reticulum. It pumps calcium ions from cytoplasm into the its lumen. When muscle cells are stimulated by electric impulse, calcium ions are cross back into cytoplasm and the trigger the muscle contraction. Sarcoplasmic reticulum is an example of smooth endoplasmic reticulum function in skeletal muscle contraction.