1.6.2 Hydrolysis & Synthesis of ATP

• Energy released during the reactions of respiration is transferred to the molecule adenosine triphosphate (ATP)
• ATP is a small and soluble molecule that provides a short-term store of chemical energy that cells can use to do work
• It is vital in linking energy-requiring and energy-yielding reactions
• ATP is described as a universal energy currency
  • Universal: It is used in all organisms
  • Currency: Like money, it can be used for different purposes (reactions) and is reused countless times

• The use of ATP as an ‘energy-currency’ is beneficial for many reasons:
  • The hydrolysis of ATP can be carried out quickly and easily wherever energy is required within the cell by the action of just one enzyme, ATPase
  • A useful (not too small, not too large) quantity of energy is released from the hydrolysis of one ATP molecule - this is beneficial as it reduces waste but also gives the cell control over what processes occur
  • ATP is relatively stable at cellular pH levels

Hydrolysis of ATP

• Hydrolysis of ATP to adenosine diphosphate (ADP) and an inorganic phosphate group (Pi) is catalysed by the enzyme ATP hydrolase sometimes called 'ATPase'
• The hydrolysis of ATP can be coupled to energy-requiring reactions within cells such as:
  • The active transport of ions up a concentration gradient
  • Enzyme controlled reactions that require energy
  • Muscle contraction and muscle fibre movement

• As ADP forms free energy is released that can be used for processes within a cell eg. DNA synthesis
  • Removal of one phosphate group from ATP releases 30.8 kJ mol ^-1 of energy, forming ADP
  • Removal of a second phosphate group from ADP also releases 30.8 kJ mol^-1 of energy, forming AMP
  • Removal of the third and final phosphate group from AMP releases 14.2 kJ mol^-1 of energy, forming adenosine

• The inorganic phosphate released during the hydrolysis of ATP can be used to phosphorylate other compounds, often making them more reactive

Features of ATP Table

• On average humans use more than 50 kg of ATP in a day but only have a maximum of ~ 200g of ATP in their body at any given time
• Organisms cannot build up large stores of ATP and it rarely passes through the cell surface membrane
• This means the cells must make ATP as and when they need it
• ATP is formed when ADP is combined with an inorganic phosphate (Pi) group by the enzyme ATP synthase
  • This is an energy-requiring reaction
  • Water is released as a waste product (therefore ATP synthesis is a condensation reaction)

Types of ATP synthesis

• ATP is made during the reactions of respiration and photosynthesis
  • All of an animal's ATP comes from respiration

• ATP can be made in two different ways:
  • Substrate-linked phosphorylation (occurs in the glycolysis stage of respiration)
  • Chemiosmosis (occurs in the electron transport chain stage of respiration)