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Energy and Respiration: Question 1

Syllabus 12.1

Multiple choice A2 1 mark

All living cells need a continuous supply of energy, and ATP (adenosine triphosphate) acts as the universal energy currency that transfers this energy to wherever it is needed in the cell.

Which statement best explains why ATP, rather than glucose, is used directly to supply energy for most cellular reactions?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall why cells need a “usable” form of energy

Complete oxidation of a molecule of glucose releases a large amount of energy in one go. If a cell released all of this energy in a single reaction, most of it would be lost as heat, and it would be very difficult to capture in a controlled, useful way for individual reactions such as active transport or muscle contraction.

ATP solves this problem. Hydrolysis of the terminal phosphate bond of ATP (ATP + water → ADP + inorganic phosphate) releases a comparatively small, manageable “packet” of energy, well matched to the requirements of a single step in a metabolic pathway. Because only a small amount of energy is released and used at a time, ATP can be hydrolysed and resynthesised very rapidly and repeatedly, so a cell only needs to maintain a small working pool of ATP rather than storing a large reserve of directly usable chemical energy. Cells instead keep their larger, long-term energy reserves as glucose, glycogen or lipid, and only convert this energy into ATP as it is required, mainly through respiration.

Because essentially every type of living cell uses the same ATP–ADP system to couple energy-releasing reactions to energy-requiring reactions, ATP is described as the universal energy currency.

Step 2: Evaluate each option

  • A: correct. This describes exactly why ATP is useful: it releases energy in small, controllable amounts on hydrolysis, and is quickly regenerated from ADP and phosphate, so cells do not need to release or store large amounts of energy all at once.
  • B: incorrect. This is the wrong way round. Glucose releases far more energy overall when fully oxidised than a single ATP molecule releases on hydrolysis; ATP’s advantage is releasing a small amount of energy at a time, not a large one.
  • C: incorrect. ATP is not a stable long-term store; the opposite is true, as it is readily and repeatedly hydrolysed to release energy. Glycogen and lipid, not ATP, act as the cell’s longer-term energy stores.
  • D: incorrect. ATP is generated mainly by respiration (which happens in essentially all living cells, including in the dark and in non-photosynthetic organisms), not only during photosynthesis, and respiration does not exist to convert ATP back into glucose.

Final answer

  • ATP is well suited to be the universal energy currency because its hydrolysis releases a small, manageable amount of energy and it can be rapidly resynthesised as it is used, option A.