The membranous organelles such as endoplasmic reticulum (ER), Golgi apparatus, lysosomes and vacuoles are together referred to as the endomembrane system, because their functions are coordinated and they help in biosynthesis, processing, packaging, transport and secretion of proteins and lipids as well as in storage of wastes.
The ER synthesises, processes and transports proteins and lipids to the other organelles as well as metabolises carbohydrates in the liver cells and detoxifies the drugs present in the body. ER occurs as a network of tiny tubular structures, dividing the intracellular space into two distinct compartments—the luminal compartment and the extra luminal compartment.
There are two types of ER—the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The RER has ribosomes on its surface and is involved in protein synthesis and secretion. The SER lacks ribosomes and synthesises lipids and lipid-like steroidal hormones such as oestrogens in the case of animal cells. The SER in muscle cells is called the sarcoplasmic reticulum and it stores and pumps calcium ions.
The Golgi apparatus, named after Camillo Golgi, consists of a series of flat, disc-shaped cisternae, stacked one above the other. The cisternae, concentrically arranged near the nucleus have two distinct faces – the convex-shaped cis that faces the nuclear envelope, and the concave-shaped trans that lies towards the plasma membrane.
The vesicles containing proteins, synthesised by the RER, get pinched off from its surface and fuse with the cis face of the Golgi apparatus. Here, they are modified in the cisternae before they are released from the trans face. It aids in the formation of glycoproteins and glycolipids that form a part of the plasma membrane.
Lysosomes are single-membrane vesicular structures, formed during the packaging process in the Golgi apparatus. They contain hydrolytic enzymes such as lipases, proteases and carbohydrases that digest lipids, proteins, carbohydrates and nucleic acids.
Vacuoles contain water, sap, excretory products and waste materials and are covered by a membrane called tonoplast. In plant cells, vacuoles can make up 90 per cent of the volume of the cell.