Endoplasmic Reticulum

Eukaryotic cells comprise of a highly convoluted network of flattened sacs and branching tubules known as Endoplasmic reticulum (ER). They are double layered membraneous organelles provding a large surface area for a number of biochemical activities. The internal space of ER is known as lumen. The ER is found to be continious with the double layered nuclear envelope, providing passage between the cytoplasm and nucleus. It is involved in the manufacturing, packaging and export of lipids and proteins to other organelles. It functions closely in association with the Golgi complex, ribososmes and RNA molecules. The ER bound to ribosomes are known as rough ER, owing to the rugged structure, and those devoid of ribosomes are known as smooth ER.

Structure and function of smooth ER
The smooth ER (SER) is tubular in form and is primarily involved in synthesis of carbohydrate and lipids. Two major lipid components synthesized by SER are phospholipids, the main constituent of cell membrane (also involved in membrane biogenesis), and steroidal hormones. Apart from synthesis, SER also serves as a strorage compartment for ions which are used during cellular metabolism. Release of calcium ions from SER in the muscles causes muscle contraction. SER also assists the break down of harmful toxins in liver cells and conversion of cholesterol to steroid

hormones. SER serves as a transportation media, tranferring products from the rough endoplasmic reticulum to other cytoplasmic organelles (eg. Golgi) or between the cytoplasm and the nucleus.

Structure and function of rough ER
The rough endoplasmic reticulum (RER) is a connection of flattened membraneous sacs (called cisternae). The difference between the SER and RER is the presence of large number of ribosomes on its outer membrane. Due to the presence of large number of ribosomes, RER, is involved in protein synthesis and packaging. Along with protein synthesis, RER also secretes serum proteins (such as albumin) in the liver, and hormones (such as insulin) and other substances (such as milk) in the glands. After synthesis, the proteins are transported through vesicles which moves towards the cell membrane or the Golgi apparatus.