NDA (II) 2018 General Ability Test · Question 58 of 146

Correct — B, lipid synthesis. The endoplasmic reticulum is a single membrane network with two faces. The rough part is studded with ribosomes and is the cell's protein arm — newly made polypeptides pass into it to be folded, processed and shipped on. The smooth part carries no ribosomes, and what it adds to the network is the manufacture of lipids: the fats and phospholipids that go into new membrane, and the steroid hormones built in cells of the adrenal cortex, the testis and the ovary. Set against the protein-handling work that defines the reticulum as a whole, lipid synthesis is the function the smooth portion contributes, which is why a cell that has lost the ability to build lipids points to a defective smooth endoplasmic reticulum — the way NDA framed the same fact in 2022. The official key marks (b).

• (a)protein synthesis — Protein synthesis belongs to the ribosomes, and within the reticulum to the ribosome-studded rough portion. The smooth endoplasmic reticulum carries no ribosomes at all, which is exactly what makes it smooth.
• (c)storage of biomolecules — Bulk storage of cell products is the work of vacuoles in plant cells and of specialised granules elsewhere. The smooth reticulum does store calcium ions in muscle fibres, where it is called the sarcoplasmic reticulum, but ion storage is not what this phrase describes and it is not the organelle's general added function.
• (d)detoxification of toxic substances — This is the closest of the three and deserves a straight answer. Detoxification is a genuine smooth-reticulum activity — in liver cells its enzymes chemically alter drugs, alcohol and poisons so the body can excrete them. But it is a role of particular tissues, whereas lipid manufacture is what the smooth reticulum does in cells generally, and lipid synthesis is what the official key marks.

The endoplasmic reticulum is a network of membranous tubules, vesicles and flattened sacs running through the cytoplasm and continuous with the outer membrane of the nuclear envelope. It divides the cell's interior into compartments, provides a large internal surface for enzymes to work on, and supplies the material for new membrane. Rough and smooth are not separate organelles but two regions of one continuous system, distinguished by whether ribosomes are attached.

The wording of this item is loose and it is worth saying so plainly, because a well-prepared student can talk himself into (d). Both lipid synthesis and detoxification are real smooth-reticulum functions, and school textbooks mention both. The key resolves it in favour of lipid synthesis, which is defensible on two grounds: measured against the reticulum's headline job of protein synthesis, lipid manufacture is what the smooth portion adds to the system; and it is universal across cell types, while detoxification is a speciality of liver cells. Answer to the key here, and store detoxification as the second fact rather than discarding it — NDA has asked for it too.

• The endoplasmic reticulum is a membrane network continuous with the nuclear envelope, present in all eukaryotic cells except mature red blood cells.
• Rough endoplasmic reticulum bears ribosomes and handles the synthesis, folding and transport of proteins.
• Smooth endoplasmic reticulum bears no ribosomes and is the site of lipid and steroid synthesis.
• Steroid-secreting cells, such as those of the adrenal cortex and the gonads, are packed with smooth reticulum.
• In liver cells the smooth reticulum also detoxifies drugs and poisons; in muscle, as the sarcoplasmic reticulum, it stores and releases calcium ions.

• Attaching protein synthesis to the smooth reticulum; ribosomes, and so the rough reticulum, do that.
• Assuming the smooth reticulum's only role is detoxification because that is the fact textbooks highlight.
• Treating rough and smooth reticulum as two separate organelles rather than two regions of one continuous network.

NDA usually approaches this backwards — a cell cannot make lipids, or cannot handle a drug, and you are asked which organelle is at fault.

• practice — not a real PYQ

  Cells of the adrenal cortex, which secrete steroid hormones, are especially rich in

  • (a)rough endoplasmic reticulum
  • (b)smooth endoplasmic reticulum
  • (c)lysosomes
  • (d)centrioles

  Answer(b) smooth endoplasmic reticulum — steroids are lipids, and the smooth reticulum is where lipid and steroid synthesis takes place.

• practice — not a real PYQ

  The endoplasmic reticulum is described as rough when it bears

  • (a)lysosomes on its surface
  • (b)ribosomes on its surface
  • (c)a thicker membrane
  • (d)stored glycogen granules

  Answer(b) ribosomes on its surface — the attached ribosomes give it the granular look under the electron microscope and make it the protein-handling region.