Which term describes an organelle in the cytoplasm consisting of a folded membrane; increased space for metabolic reactions; some have knobs called rough ER?

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

Answer: (C)

The endoplasmic reticulum is an organelle in the cytoplasm made of a network of folded membranes, which greatly increases surface area for metabolic reactions such as protein and lipid synthesis. When ribosomes are attached to its surface, it appears rough and is specialized for protein synthesis and processing, while regions without ribosomes (smooth ER) handle lipid synthesis and detoxification. The other options don’t describe this folded-membrane system: the cytoplasm is the general fluid inside the cell, the nucleus is a separate membrane-bound compartment containing DNA, and mitochondria are energy-producing organelles with their own membranes and cristae rather than a single folded-membrane network with rough regions.