CIE A Level Biology (9700) 2019-2021

Revision Notes

12.1.2 ATP

ATP: Universal Energy Currency

  • 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

Structure of ATP

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

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 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

Features of ATP table

Features of ATP table, downloadable AS & A Level Biology revision notes

Exam Tip

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

For example,  saying that “energy is used in muscles” in the exam won’t get you marks for describing how the muscles work as it is too vague an answer.

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)

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
    • Chemiosmosis

Substrate-linked phosphorylation

  • ATP is formed by transferring a phosphate directly from a substrate molecule to ADP

ADP + Pi —> ATP

  • The energy required for the reaction is provided directly by another chemical reaction
  • This type of ATP synthesis occurs in the cell cytoplasm and in the matrix of the mitochondria
  • It only accounts for a small amount of the ATP synthesised during aerobic respiration
    • ~ 4 / 6 ATP per glucose molecule
  • This type of ATP synthesis takes place in glycolysis


  • This specific type of ATP synthesis involves a proton (hydrogen ion) gradient across a membrane
  • It takes place across the inner membrane of the mitochondria and the thylakoid membrane of chloroplasts
  • An electron transport chain helps to establish the proton concentration gradient
    • High energy electrons move from carrier to carrier releasing energy that is used to pump protons (up a concentration gradient) across the inner membrane into the intermembrane space
    • Protons are pumped from a low concentration in the mitochondrial matrix to a high concentration in the intermembrane space
  • The protons then move down the concentration gradient into the matrix which releases energy
  • The protons move through the ATP synthase complex which uses the released energy to drive the phosphorylation of ATP
  • Oxygen acts as the final electron and proton acceptor to form water
  • Most of the ATP made during respiration is synthesised via chemiosmosis
    • ~ 32 / 34 ATP per glucose molecule

ATP synthesis table

ATP Synthesis Table, downloadable AS & A Level Biology revision notes

Exam Tip

You may be asked to identify which type of ATP synthesis is occurring at different stages of respiration and photosynthesis. Remember that chemiosmosis involves a proton gradient that has been created by an electron transport chain and it takes place across an inner membrane.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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