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Adenosine Triphosphate (ATP) (HL IB Biology)

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Adenosine Triphosphate (ATP)

  • 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 DiagramATP Structure, downloadable AS & A Level Biology revision notes

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

Features of ATP Table

Feature Benefit
Releases a small but sufficient quantity of energy This is enough energy to drive important metabolic reactions while keeping energy wastage low
Exists as a stable molecule It doesn't break down unless a catalyst (ATPase) is present so energy won't be wasted
Can be recycled The breakdown of ATP is a reversible reaction, ATP can be reformed from ADP and Pi. This means the same molecule can be reused elsewhere in the cell for different reactions
Hydrolysis is quick and easy Allows cells to respond to a sudden increase in energy demand
Soluble and moves easily within cells Can transport energy to different areas of the cell
Forms phosphorylated intermediates This can make metabolites more reactive and lower the activation energy required for a reaction

Examiner Tip

Be careful not to use the terms energy and ATP interchangeably. Energy is the capacity or power to do work while ATP is a molecule which carries energy to places in the cell that need it in order to do work. 

Life Processes Reliant On ATP

  • 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 (Pi), 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

Interconversions Between ATP & ADP

  • 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

Cycling of ATP

The constant cycling of ATP and ADP + Pi 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

Hydrolysis of atp flowchart

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

ATP Synthesis

Energy-requiring synthesis of ATP from ADP and Phosphate

Examiner Tip

Note that you are not required to know the exact quantity of energy in kilojoules that are involved with the interconversions between ATP and ADP, but you should appreciate that it is sufficient for performing tasks within the cell.

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.