Which one of the following functions is not carried out by smooth endoplasmic reticulum (SER) in eukaryotic cells?

Introduction / Context:
The endoplasmic reticulum (ER) is a network of membranes inside eukaryotic cells and exists in two main forms: rough ER and smooth ER. Each form performs distinct functions. Rough ER is associated with ribosomes and specialises in protein synthesis, while smooth ER lacks ribosomes and is involved in lipid metabolism, steroid synthesis, and detoxification. Exam questions often ask you to distinguish which functions belong to smooth ER and which do not. This question focuses on identifying a function that is not carried out by smooth ER.

Given Data / Assumptions:
• The organelle in focus is smooth endoplasmic reticulum (SER).• The options include transport of materials, lipid synthesis, protein synthesis, steroid hormone synthesis, and detoxification.• We assume a typical animal or plant cell with both smooth and rough ER present.• We must choose the function that is not characteristic of SER.

Concept / Approach:
Smooth ER is distinguished by the absence of ribosomes on its surface. It is involved in the synthesis of lipids, including phospholipids and cholesterol, and in the synthesis of steroid hormones in certain endocrine cells. In liver cells, smooth ER plays an important role in detoxifying drugs and harmful substances. It also contributes to transport of materials within the cell by forming part of the endomembrane system. In contrast, protein synthesis connected with membrane bound ribosomes is a function of rough ER, not smooth ER. Therefore, synthesis of protein on attached ribosomes is not a function of smooth ER.

Step-by-Step Solution:
Step 1: Recall that the key difference between rough and smooth ER is the presence or absence of ribosomes on the surface.Step 2: Recognise that rough ER, with ribosomes, is specialised for synthesis of proteins that are secreted or inserted into membranes.Step 3: Remember that smooth ER is more involved in lipid synthesis, steroid hormone production, and detoxification reactions.Step 4: Examine each option and check whether it is associated with smooth ER.Step 5: Note that transport of materials can involve both types of ER and the Golgi apparatus, so it is not excluded from smooth ER.Step 6: Identify synthesis of protein on attached ribosomes as a function of rough ER, not smooth ER, making it the correct answer.

Verification / Alternative check:
Textbook diagrams and descriptions of ER always show rough ER with ribosomes and label it as the site of protein synthesis for secretory and membrane proteins. Smooth ER is depicted without ribosomes and is noted for lipid metabolic functions and detoxification, particularly in liver and adrenal cortex cells. Experiments show high levels of smooth ER in cells actively producing steroid hormones, such as cells of the adrenal cortex and gonads. These consistent descriptions confirm that protein synthesis on attached ribosomes is not a function of smooth ER but of rough ER.

Why Other Options Are Wrong:
Transport of materials within the cell is facilitated by the continuous lumen of ER and its connection with Golgi apparatus, and smooth ER participates in this system. Synthesis of lipids such as phospholipids is a classic function of smooth ER, especially in liver and membrane forming cells. Synthesis of steroid hormones in certain cells, such as adrenal cortex and gonadal cells, takes place largely in smooth ER. Detoxification of some drugs and poisons in liver cells is a well known role of smooth ER, where enzymes like cytochrome P450 are located. These options describe proper functions of smooth ER and therefore are not the correct choice for this question.

Common Pitfalls:
Students sometimes loosely associate all forms of ER with protein synthesis because they know ER is near the nucleus and involved in processing. This can lead them to forget the important structural difference between rough and smooth ER. Another pitfall is to overlook the detoxification role of smooth ER and mistakenly doubt that option. To avoid errors, remember the simple rule: rough ER is for proteins with ribosomes attached, smooth ER is for lipids and detoxification without ribosomes. This clear division helps you quickly spot that protein synthesis on attached ribosomes is not a function of smooth ER.

Final Answer:
The function that is not carried out by smooth endoplasmic reticulum is Synthesis of protein on attached ribosomes, which is a role of rough ER instead.