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Energy & ATP

Biology: Cambridge International A Level, topic 12.1.1

Hydrolysis of ATP

ATP is known as the "universal energy currency" of cells. Hydrolysis of ATP happens when a cell needs energy.

ATP hydrolysis reaction

ATP hydrolysis reaction

  • The equation for the hydrolysis of ATP is:
  • ATP + H2O → ADP + Pi (+ energy)
  • Hydrolysis of ATP to adenosine diphosphate (ADP) and an inorganic phosphate group (Pi) is catalysed by the enzyme ATP hydrolase.
  • This reaction uses a water molecule so is called a hydrolysis reaction.
Energy release

Energy release

  • Energy is released during the hydrolysis of ATP.
  • This energy is used to perform work inside the cell.
Energy coupling

Energy coupling

  • The hydrolysis of ATP can be coupled to other reactions within cells that need energy.
  • An example of energy coupling using ATP is the transmembrane sodium-potassium pump (Na+/K+ pump):
  • Energy is released from the hydrolysis of ATP.
  • Energy is needed to power the pump that drives Na+ out of the cell and K+ into the cell.
Phosphorylation

Phosphorylation

  • The inorganic phosphate released in ATP hydrolysis can be used to phosphorylate other compounds.
  • This often makes the compounds more reactive.

Resynthesis of ATP

Like most chemical reactions, the hydrolysis of ATP to ADP is reversible.

ATP resynthesis reaction

ATP resynthesis reaction

  • The equation for the resynthesis of ATP is:
  • ADP + Pi (+ energy) → ATP + H2O
  • ATP is resynthesised by the condensation of ADP and an inorganic phosphate.
  • This reaction is catalysed by the enzyme ATP synthase during photosynthesis or respiration.
Formation of ATP requires energy

Formation of ATP requires energy

  • Cells rely on the regeneration of ATP just as people rely on the regeneration of spent money through some sort of income.
  • Since ATP hydrolysis releases energy, ATP regeneration must require an input of energy (just like earning money requires hard work!).

Test yourself on Energy & ATP

  • Which enzyme catalyses ATP hydrolysis?

  • How does ATP's inorganic phosphate make other compounds more reactive?

  • ATP is resynthesised by what type of reaction between ADP and Pi?

  • Which enzyme is used during the resynthesis of ATP?

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