Question

Why can the inside of the endoplasmic reticulum functionally be the same as the outside of the cell

Answer 1

The endoplasmicreticulum [ER] is a large continuous membrane bound organelle spread throughout the the cytoplasm of eukaryotic cells. It has three distinct morphologies which are -- the nuclear envelop [NE] , the peripheral ER cisternae and an interconnected tubular network .

The ER is composed of one completely continuous membrane bilayer and has one continuous lumen . It is interesting to note that the lumen of the ER is in continuity with the lumen found between the inner and outer nuclear envelop.

The ER are of two types rough ER and smooth ER. The rough ER have ribosomes studded on the cytosol surface of its membrane and is used to synthesise proteins.

The smooth ER are without ribosomes and are mainly concerned with synthesis of lipids.

In muscle fibers, ER are called sarcoplasmic reticulum and are not involved in synthesis of proteins or lipids . Instead they act as a store house of Ca ions which they release into the sarcoplasm on stimulation. The Ca ions help the myofibrils to contract.

The structure of ER membrane is similar to that of the plasma membrane i.e both are made of a layer of protein sandwiched between lipid bilayers. Like the cell membrane the ER membrane too allows movement of molecules inside and outside of its lumen. The lumen is not in direct contact with the cytosol and can only secrete the molecules synthesised in it with the help of cisternae and tubules which are continuously being pinched off from it.

Since the lumen of the ER is not in contact with the cytosol , it is like the exterior of the cell. Just as molecules move in and out of the cytosol through the cell membrane so do they into and out of the ER lumen through its membrane. This makes the ER lumen somewhat similar to the exterior of the cell but of course with a differens because the lumen of the ER forms a continuous space with the lumen between the inner and outer nuclear membranes.