NDA (II) 2022 General Ability Test · Question 113 of 150

Correct — C, lysosomes. The lysosome is the membrane-bound sac in which a cell keeps its hydrolytic, or digestive, enzymes — more than fifty distinct hydrolases that between them break down proteins, carbohydrates, lipids and nucleic acids. The interior is deliberately kept acidic, at roughly pH 4·5 to 5·0 against a cytoplasm of about pH 7·2, because these enzymes need acid to work; that arrangement also means a small leak into the neutral cytoplasm does little harm. Christian de Duve identified the organelle in the 1950s and was awarded the Nobel Prize in Physiology or Medicine in 1974 for the work.

• (a)mitochondria — The mitochondrion is the site of aerobic respiration and ATP synthesis. Its enzymes oxidise pyruvate and drive the electron transport chain — they release energy from food that has already been digested, rather than doing the digesting.
• (b)vacuoles — Vacuoles are storage compartments — for water, sap, pigments and wastes — and in single-celled organisms the contractile vacuole also expels excess water. A food vacuole in such a cell is digested only after a lysosome fuses with it and supplies the enzymes, so the enzymes are still the lysosome's.
• (d)ribosomes — Ribosomes are the sites of protein synthesis — they build polypeptide chains from amino acids. They assemble proteins rather than break them down, which is the opposite of digestion, and they are not even membrane-bound.

A eukaryotic cell divides its chemistry between organelles, each with a matching set of enzymes. Lysosomes are the cell's disposal and recycling system: they digest worn-out organelles, material brought in by endocytosis, and invading bacteria taken up by white blood cells. Because a rupture of the lysosomal membrane can release those enzymes and destroy the cell from inside, lysosomes are popularly nicknamed the suicide bags of the cell.

The word 'digestive' is what makes this question look harder than it is — many candidates read it as being about the digestive system and reach for an enzyme name rather than an organelle. Here it means intracellular digestion, and the sorting rule is simply one job per organelle: mitochondria release energy, ribosomes build proteins, vacuoles store, lysosomes digest. One honest nuance is worth carrying: in plant cells the large central vacuole does take over part of the lytic work, and lysosomes are rare there. The organelle that holds the digestive enzymes as a matter of definition, and the answer the key marks, is still the lysosome.

• Lysosomes are membrane-bound organelles containing over fifty kinds of hydrolytic enzyme, found in animal cells and only rarely in plant cells.
• The lysosomal interior is acidic, about pH 4·5 to 5·0, while the surrounding cytosol is near pH 7·2.
• Christian de Duve discovered lysosomes in the 1950s and shared the 1974 Nobel Prize in Physiology or Medicine.
• Lysosomes are called suicide bags because a burst lysosome can digest the cell that contains it.
• Mitochondria carry out respiration, ribosomes carry out protein synthesis, and vacuoles store water, sap and wastes.

• Reading 'digestive enzymes' as a question about the human digestive system and hunting for pepsin or trypsin.
• Choosing vacuoles because food vacuoles are where intracellular digestion is seen — the enzymes still arrive from a lysosome.
• Mixing up the ribosome, which builds proteins, with the lysosome, which breaks them down.

As this one-word association, as a 'which statement about lysosomes is NOT correct' item, or as a scenario in which one cell function has failed and you must name the defective organelle.

• practice — not a real PYQ

  Lysosomes are called the 'suicide bags' of the cell because

  • (a)they store the cell's genetic material
  • (b)their rupture releases hydrolytic enzymes that digest the cell itself
  • (c)they consume oxygen and release carbon dioxide
  • (d)they divide independently of the cell

  Answer(b) their rupture releases hydrolytic enzymes that digest the cell itself — which is why the enzymes are kept sealed inside a membrane.

• practice — not a real PYQ

  The organelle chiefly responsible for the synthesis of proteins in a cell is the

  • (a)lysosome
  • (b)ribosome
  • (c)mitochondrion
  • (d)vacuole

  Answer(b) ribosome — it links amino acids into polypeptide chains, whether free in the cytoplasm or attached to the rough endoplasmic reticulum.