In vertebrates, smooth endoplasmic reticulum in cells of which organ plays a crucial role in detoxification of toxic compounds?

Correct — D, Liver. Hepatocytes carry an unusually extensive smooth endoplasmic reticulum, and embedded in its membranes is the cytochrome P450 family of enzymes. These add oxygen or hydroxyl groups to drugs, alcohol, pesticides and other foreign compounds, and a second set of enzymes then couples the altered molecule to glucuronic acid or sulphate. The point of both steps is solubility: a fat-soluble poison that would otherwise sit in the body's membranes is converted into a water-soluble derivative the kidney can excrete. Two things make the liver rather than any other organ the site of this work. Blood from the stomach and intestine reaches it first, through the hepatic portal vein, so it screens what has been absorbed before the rest of the body sees it. And its smooth endoplasmic reticulum is inducible — sustained exposure to a drug such as phenobarbital makes hepatocytes build more of it, which is one reason tolerance develops and why one drug can speed up the clearance of another.

• (a)Spleen — The spleen filters blood mechanically and immunologically. It removes worn-out red cells and mounts responses to blood-borne organisms; it does not run the chemical conversions that neutralise foreign compounds.
• (b)Intestine — Intestinal cells do carry some of the same enzymes and contribute to first-pass metabolism, but their smooth endoplasmic reticulum is nothing like as developed. The intestine is where absorption happens; the chemistry that follows is done downstream.
• (c)Kidney — The kidney is the exit, not the workshop. It filters and excretes the water-soluble products, which is a different job from converting a fat-soluble toxin into something excretable in the first place.

The endoplasmic reticulum comes in two forms. The rough kind is studded with ribosomes and makes proteins destined for secretion or for membranes. The smooth kind has no ribosomes and does chemistry instead: it synthesises lipids and steroid hormones, stores calcium ions, and in liver cells breaks down foreign compounds. The same smooth membranes in hepatocytes also carry glucose-6-phosphatase, the enzyme that lets the liver release free glucose into the blood, so one organelle serves both detoxification and blood-sugar regulation.

The question can be answered without any cell biology if you already think of the liver as the body's chemical plant, and the reasoning is worth rehearsing because it recurs. Everything absorbed from the gut goes to the liver first. Alcohol is handled there. Ammonia from protein breakdown is turned into urea there. Bilirubin from dead red cells is conjugated there. Drug doses are calculated around what the liver will do to them. Given all that, an organ specialising in chemical conversion is exactly where you would expect the organelle that performs chemical conversion to be most abundant. Two present-day details are worth attaching to the concept: this is why paracetamol overdose damages the liver rather than the kidney, and why grapefruit juice, which inhibits one P450 enzyme, changes the effective dose of several common medicines.

• Smooth endoplasmic reticulum has no ribosomes; rough endoplasmic reticulum does.
• Cytochrome P450 enzymes sit in the smooth endoplasmic reticulum and begin the conversion of fat-soluble foreign compounds into water-soluble ones.
• The hepatic portal vein carries blood from the gut to the liver, so absorbed material is processed before it enters the general circulation.
• Repeated exposure to some drugs makes hepatocytes build more smooth endoplasmic reticulum, which raises tolerance and can alter the clearance of other drugs.
• The same organelle in the liver stores calcium and releases glucose, and in the gonads and adrenal cortex it makes steroid hormones.

• Choosing the kidney because it is the organ associated with removing waste — excretion and detoxification are different steps.
• Assuming the smooth endoplasmic reticulum does the same job everywhere; in the gonads it makes steroid hormones, in muscle it stores calcium.
• Reading the spleen as a filter of toxins rather than of cells.

Asked as a structure-to-organ mapping, where the organelle is named in the stem and the mark depends on knowing which tissue is richest in it.

• practice — not a real PYQ

  In which of the following does the smooth endoplasmic reticulum chiefly synthesise steroid hormones?

  • (a)Cells of the adrenal cortex
  • (b)Red blood cells
  • (c)Neurons
  • (d)Epidermal cells

  Answer(a) Cells of the adrenal cortex — steroid-secreting tissues, the adrenal cortex and the gonads, are packed with smooth endoplasmic reticulum for exactly this purpose.

• practice — not a real PYQ

  Blood carrying absorbed nutrients from the small intestine reaches the liver through the

  • (a)hepatic artery
  • (b)hepatic vein
  • (c)hepatic portal vein
  • (d)inferior vena cava

  Answer(c) hepatic portal vein — it is the reason the liver, and not some other organ, gets first sight of everything absorbed from the gut.