Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum (smooth ER) is a network of membranous tubules in the cell, continuous with the rough ER but lacking ribosomes on its surface. The smooth ER is primarily involved in lipid synthesis (including phospholipids and steroid hormones), detoxification of drugs and poisons, metabolism of alcohol, and storage of calcium ions.

• Key functions: Smooth ER makes lipids and steroids (e.g. in liver cells and endocrine cells), and it contains enzymes that break down toxins - liver cells have extensive smooth ER for detoxification.
• In muscle cells, a specialized smooth ER called the sarcoplasmic reticulum stores Ca-- and releases it to trigger muscle contraction.
• Contrast with Rough ER: Smooth ER does not have ribosomes and does not directly synthesize secretory proteins. It focuses on lipid production and detox, whereas rough ER (with ribosomes) makes proteins.

• Common distinction - -Which organelle is responsible for synthesizing lipids and detoxifying chemicals-- Answer: Smooth ER (examiners expect you to differentiate smooth vs rough ER function).
• Cell type clue - If a question mentions a cell that produces steroid hormones (like an adrenal gland cell) or detoxifies drugs (like a liver cell), expect that these cells have abundant smooth ER to perform those functions.
• Terminology: If you see -sarcoplasmic reticulum- in muscle physiology questions, remember it-s a form of smooth ER specialized for calcium storage in muscle cells.