Adenosine Triphosphate (ATP) (SL IB Biology)

• Living organisms require energy to perform and maintain life processes such as movement, nutrition and excretion
• This energy is released by the process of cell respiration
• Energy released during the reactions of respiration is transferred to the molecule adenosine triphosphate (ATP)
  • The energy is transferred in a series of small steps 
  • Heat is lost at each step, which is used to regulate body temperature in endotherms
• ATP is a small and soluble molecule that provides a short-term store of chemical energy that cells can use to do work
  • Its solubility and size enables it to move easily in cells and living organisms by facilitated diffusion
• 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

Structure of ATP

• ATP is a phosphorylated nucleotide
• It is made up of:
  • Ribose sugar
  • Adenine base
  • Three phosphate groups

ATP Diagram

Structure of ATP contains ribose sugar, an adenine base and three phosphate groups

Features of ATP Table

• Some of the life processes that are reliant on ATP as a source of energy include:
  • In anabolic reactions to synthesise larger molecules (macromolecules) from smaller ones
  • To move molecules across the cell membrane against their concentration gradient during active transport
  • Enabling movement of the entire cell
  • The move cell components, such as chromosomes, within the cell
• ATP is readily converted to adenosine diphosphate (ADP) and a phosphate ion (P_i), during which energy is released
  • Since ATP is a very reactive molecule, it is not stored in living organisms
• Molecules such as glucose and fatty acids are used as short-term stores of energy, while glycogen, starch and triglycerides act as long-term storage molecules of energy

• ATP is a very reactive molecule and is readily converted to ADP and phosphate when releasing its energy
  • ADP and phosphate can then be re-converted to ATP during respiration
• Organisms require a constant supply of ATP because much of the energy is lost to the surroundings as heat

ATP Cycle Diagram

The constant cycling of ATP and ADP + P_i within a cell

Hydrolysis of ATP

• When ATP is hydrolysed (broken down), ADP and phosphate are produced
• As ADP forms, free energy is released that can be used for processes within a cell e.g. DNA synthesis
  • Removal of one phosphate group from ATP releases approximately 30.5 kJ mol ^-1 of energy, forming ADP
  • Removal of a second phosphate group from ADP also releases approximately 30.5 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

Hydrolysis of ATP Diagram

The hydrolysis of ATP

ATP synthesis

• 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
  • This is an energy-requiring reaction
  • Water is released as a waste product (therefore ATP synthesis is a condensation reaction)

Synthesis of ATP Diagram

Energy-requiring synthesis of ATP from ADP and Phosphate