CBSE Class 9 Answered

The endoplasmic reticulum (ER) is the site for the synthesis of proteins and lipids destined for the plasma membrane, secretory vesicles or endocytic vesicles. There are two models which explain the transport of lipids and proteins from the ER to the plasma membrane and other cell organelles.

• Membrane budding model: According to this model, a patch of the membrane pinches off the ER and forms a vesicle that finds its way to its destination.
• Phospholipid exchange model: In this model, water-soluble proteins remove the lipid from the ER membrane and release it into the target membrane.

However, it is believed that the proteins synthesised on the ER are transported from the ER membranes by small vesicles that pinch off the surface and enter the Golgi membrane stack. From the Golgi, the proteins are transported to the cell surface or to other organelles.

Role of proteins and lipids in plasma membrane:

• They help to maintain the integrity of the plasma membrane
• They help to maintain the fluidity of the plasma membrane
• Proteins with extracellular domains are usually involved in cell-to-cell interactions.
• Proteins that reside mainly within a membrane usually form channels or pores to allow molecules to cross the membrane.
• Proteins with cytosolic domains can act as anchors for cytoskeletal elements involved in intracellular signaling.