Question 1

Functions of the smooth endoplasmic reticulum (SER) Which biosynthetic activity is a primary role of the SER?

Accepted Answer

Introduction / Context: The endoplasmic reticulum is partitioned into rough (RER) and smooth (SER) subcompartments, each with distinct tasks. Knowing their roles is essential for cell biology and pathology. Given Data / Assumptions: SER lacks ribosomes; RER bears ribosomes. Lipid and steroid biosynthesis are SER-centered. Protein synthesis targeted to secretion/membranes mainly occurs on RER. Concept / Approach: Match cellular processes to ER subtypes: SER handles phospholipid, cholesterol, and steroid synthesis, plus detoxification and Ca2+ storage (for example, sarcoplasmic reticulum). RER performs co-translational translocation of secretory/membrane proteins. Step-by-Step Solution: Identify SER hallmark: lipid/steroid biosynthesis.Exclude RER feature: ribosome-dependent protein synthesis.Choose the option consistent with SER function: phospholipid synthesis. Verification / Alternative check: Cells rich in SER (for example, hepatocytes, adrenal cortex) correlate with lipid synthesis and detoxification roles. Why Other Options Are Wrong: Amino acid synthesis: Cytosolic pathways dominate; not a primary SER function. Protein synthesis: RER role. Carbohydrate synthesis: Glycosylation mainly occurs in ER/Golgi but is not the defining SER hallmark. Common Pitfalls: Equating all ER with protein synthesis; only RER is ribosome-studded. Final Answer: Phospholipid synthesis (and other lipid/steroid biosynthesis).

Question 2

Classes of membrane lipids Which of the following lipid classes are found in cellular membranes?

Accepted Answer

Introduction / Context: Membrane architecture relies on a mixture of amphipathic lipids. Recognizing the principal classes helps explain curvature, signaling, and raft formation. Given Data / Assumptions: Phospholipids (glycerophospholipids) are the dominant bilayer constituents. Sphingolipids contribute to microdomains and recognition (for example, sphingomyelin, glycosphingolipids). Sterols (cholesterol) modulate fluidity and permeability in eukaryotes. Concept / Approach: All three classes co-exist in many eukaryotic membranes, with proportions varying by organelle and organism. Their combined properties generate membrane order, thickness, and platforms for protein function. Step-by-Step Solution: List known membrane lipids: glycerophospholipids, sphingolipids, sterols.Confirm they are amphipathic and assemble into bilayers.Select the inclusive option “All of these.” Verification / Alternative check: Analytical lipidomics of plasma membranes consistently identifies all three classes. Why Other Options Are Wrong: Single-class answers: Incomplete view of real membranes. Triglycerides only: Triacylglycerols are storage lipids, not bilayer-forming membrane staples. Common Pitfalls: Assuming one lipid type explains all membrane behavior; composition is heterogeneous and dynamic. Final Answer: All of these.

Membrane Structure and Functions Questions