CDS (I) 2020 General Knowledge · Question 18 of 120

Correct — B, ATP. The phrase in the question is lifted from NCERT, which states that ATP is the energy currency for most cellular processes. Respiration breaks glucose down and the energy released is used at once to build ATP from ADP and inorganic phosphate. Cells then spend that ATP wherever work is needed — muscle contraction, protein synthesis, nerve impulses — by breaking the terminal phosphate linkage, which releases about 30.5 kilojoules per mole. The comparison NCERT draws is with a battery: one store of energy that can be drawn on for many different kinds of task.

• (a)Glucose — Glucose is the fuel, not the currency. It holds far more energy than a cell can use in one step, and it must be broken down through respiration before that energy is available in usable amounts.
• (c)ADP — ADP is the spent form. It is what remains after ATP gives up its terminal phosphate, and respiration recharges it back to ATP.
• (d)Pyruvic acid — Pyruvate is an intermediate — the three-carbon product of glycolysis in the cytoplasm. It is on the way to energy release, not the molecule that delivers it.

Respiration does not hand energy straight to the processes that need it. Glucose is broken down in stages, first to pyruvate in the cytoplasm and then, if oxygen is available, to carbon dioxide and water in the mitochondria. The energy released at these steps is captured by attaching a third phosphate to ADP, making ATP. Wherever the cell needs energy, ATP is hydrolysed back to ADP and phosphate, and the energy of that bond drives the reaction.

Three of the four options belong to the same pathway, which is what makes the item worth setting: glucose goes in, pyruvate appears midway, ADP and ATP cycle between charged and spent. The word to hold on to is currency. A currency is not the wealth itself but the standard, small, universally accepted unit in which wealth is spent — so the answer must be the molecule every process accepts, not the fuel and not the discharged form. ADP is the near miss, and the way to keep it out is to remember which direction respiration runs: it makes ATP from ADP, so ATP is the charged form.

• NCERT states that ATP is the energy currency for most cellular processes.
• Energy released in respiration is used to make ATP from ADP and inorganic phosphate.
• Breaking the terminal phosphate linkage of ATP with water releases about 30.5 kilojoules per mole.
• Glycolysis breaks glucose into pyruvate in the cytoplasm; aerobic breakdown of pyruvate then occurs in the mitochondria.
• The same ATP drives muscle contraction, protein synthesis and the conduction of nerve impulses.

• Picking glucose because it is the molecule that carries the energy into the cell.
• Confusing ADP with ATP; respiration converts the first into the second.
• Treating pyruvic acid as an energy carrier rather than a pathway intermediate.

Either as the energy-currency phrase, or as a where-does-it-happen item on the stages of glucose breakdown.

• practice — not a real PYQ

  The breakdown of glucose to pyruvate in a cell takes place in the

  • (a)mitochondria
  • (b)cytoplasm
  • (c)nucleus
  • (d)Golgi apparatus

  Answer(b) cytoplasm — glycolysis is a cytoplasmic process; the further breakdown of pyruvate with oxygen occurs in the mitochondria.

• practice — not a real PYQ

  ATP is formed in a cell from

  • (a)ADP and inorganic phosphate
  • (b)AMP and glucose
  • (c)ADP and pyruvate
  • (d)glucose and oxygen directly

  Answer(a) ADP and inorganic phosphate — energy released during respiration drives that phosphate onto ADP.