Which membranous network in the cytoplasm lacks ribosomes and participates in lipid and steroid synthesis?

• A. Smooth endoplasmic reticulum
• B. Rough endoplasmic reticulum
• C. Nucleolus
• D. Ribosomes

Answer: (A)

The key idea is how the endoplasmic reticulum specializes based on whether ribosomes are attached to its surface. The network described is the smooth endoplasmic reticulum, which lacks ribosomes on its cytoplasmic surface and forms a tubular, site-like network. This structure is dedicated to lipid and steroid synthesis, providing phospholipids and cholesterol for membranes and producing steroid hormones in steroidogenic tissues. It also participates in detoxification and calcium storage, the latter being especially important in muscle cells where the analogous sarcoplasmic reticulum handles calcium release. In contrast, the rough endoplasmic reticulum is studded with ribosomes and primarily handles synthesis of secreted and membrane proteins. The nucleolus is a nuclear substructure involved in ribosome biogenesis and is not a cytoplasmic membranous network, and ribosomes themselves are particles that can be free in the cytosol or bound to rough ER, not a membranous network.