What structure is involved in the synthesis of lipids and protein modification?

The endoplasmic reticulum plays a crucial role in the synthesis of lipids as well as the modification and folding of proteins. It is divided into two types: the rough endoplasmic reticulum, which is studded with ribosomes and involved in protein synthesis and modification, and the smooth endoplasmic reticulum, which is primarily responsible for the synthesis of lipids, including phospholipids and cholesterol.

Proteins synthesized in the rough endoplasmic reticulum undergo post-translational modifications, such as glycosylation or the addition of other functional groups, which are vital for their functionality and proper trafficking to their final destinations. Therefore, the endoplasmic reticulum is critical for cell function and homeostasis due to its dual role in lipid synthesis and protein processing.

Other structures mentioned, such as the nucleolus, primarily focus on the production of ribosomal RNA and assembly of ribosomes. The cytoskeleton provides support and structure to the cell but is not directly involved in lipid synthesis or protein modification. The plasma membrane serves as a barrier and mediator of transport but does not participate in the synthesis of lipids or modification of proteins.