12.1.2 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

Structure of ATP

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

Structure of ATP

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 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

The hydrolysis of ATP

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

Energy-requiring synthesis of ATP from ADP and Phosphate

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 + P_i —> 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

Chemiosmosis

• 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