Which one of the following cell organelles is not lined by membrane ?

Cell organelles are specialized subunits within a cell that are either membrane-bound or non-membrane-bound. Ribosomes are non-membrane-bound organelles found in both prokaryotic and eukaryotic cells. They are complex molecular machines composed of ribosomal RNA (rRNA) and proteins, acting as the primary site for protein synthesis (translation).

• Lysosomes are single-membrane-bound sacs containing digestive enzymes.
• The Nucleus is enclosed by a double-membrane structure called the nuclear envelope.
• The Golgi Apparatus consists of a series of flattened, membrane-bound sacs known as cisternae.

Other examples of organelles or structures lacking a membrane include the nucleolus, centrosomes (centrioles), and the cytoskeleton. Understanding these distinctions is fundamental to cell biology as it dictates how different cellular processes are compartmentalized.