endoplasmic reticulum is an extensive network of membranes consisting of tubules and sacs called cisternae. It separates the internal compartment (lumen) from the cytosol. The space between the nuclear envelope is continuous with the ER lumen.
Smooth ER lacks ribosomes, and functions in diverse metabolic processes depending on the cell type and speciality. Functions include the synthesis of lipids, metabolism of carbohydrates and detoxification. Ensymes of smooth ER are used for the synthesis of lipids (oils, phospholipids and steroids), or may be used in the detoxification process. Smooth ER also stores calcium ions (e.g. muscle cells) and may synthesise or store steroids such as sex hormones (testes and ovaries contain cells abundant in smooth ER).
rough ER is embedded with proteins and secretes the proteins produced. Most secretory proteins are glycoproteins (have carbohydrates covalently bonded). Secretory proteins depart from the ER in vesicles that bud from the transitional ER (transport vesicles).