Which cellular structure is often not considered an organelle?

Ribosomes are often not considered organelles because they lack a surrounding membrane, which is a defining characteristic of organelles. While they are crucial for protein synthesis, being the site where amino acids are assembled into proteins, they exist as free particles in the cytoplasm or can be attached to the endoplasmic reticulum. In contrast, structures like mitochondria, the nucleus, and chloroplasts have distinct membranes and perform specialized functions, such as energy production or containing genetic material. This distinction is key in the classification of cellular structures, as organelles typically consist of compartmentalized areas within a cell that are enclosed by lipid bilayers, whereas ribosomes are more of a molecular complex involved in a specific process rather than a compartmentalized structure.