Organelle in cytoplasm consisting of a folded membrane; increased space for metabolic reactions; some have knobs called rough ER.

• A. Nucleus
• B. Cytoplasm
• C. Ribosome
• D. Endoplasmic reticulum

Answer: (D)

Maximizing internal surface area through folding is what this description is getting at. The endoplasmic reticulum is a network of membranous tubules and sacs in the cytoplasm. Its folded membrane creates a large surface area, providing many sites for metabolic reactions, protein synthesis, and trafficking of molecules. When ribosomes stud the surface, forming the rough endoplasmic reticulum, those ribosomes actively synthesize proteins that enter the ER for folding and preparation for export or membrane insertion. This distinguishes it from the nucleus, which is a separate membrane-bound organelle housing DNA; the cytoplasm, which is the fluid outside organelles; and ribosomes, which are small particles that synthesize proteins rather than forming a folded membranous compartment.