Which part of the endoplasmic reticulum is characterized by the presence of ribosomes?

The part of the endoplasmic reticulum characterized by the presence of ribosomes is the Rough ER. This structure is called "rough" because the ribosomes attached to its cytoplasmic surface give it a bumpy appearance under a microscope. These ribosomes are the sites of protein synthesis, allowing the Rough ER to play a critical role in the production and processing of proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle.

In contrast, the Smooth ER lacks ribosomes and is involved in the synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons. Lysosomes are membrane-bound organelles containing enzymes that break down waste materials and cellular debris, while vacuoles are storage structures in the cell, primarily for nutrients, water, or waste products. These differences among cellular organelles highlight the specialized functions that the Rough ER serves within the context of the endoplasmic reticulum.