ATP (Cambridge (CIE) A Level Biology)

ATP: Universal Energy Currency

• The energy released during the reactions of respiration is transferred to the molecule adenosine triphosphate (ATP)

• ATP is a small, 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

ATP structure diagram

ATP contains ribose, adenine and three phosphate groups

Hydrolysis of ATP

• When ATP is hydrolysed (broken down), ADP and phosphate are produced

• As ADP is formed, 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

Features of ATP table

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 (P_i) group

  • This is an energy-requiring reaction

  • Water is released as a waste product (therefore ATP synthesis is a condensation reaction)

ATP synthesis diagram

ATP is synthesised 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