 Chemical reactions either store or release energy

 Endergonic reactions  Absorb energy and yield products rich in potential energy Potential energy of molecules Reactants Energy required Products Amount of energy required

 Exergonic reactions ▪ Release energy and yield products that contain less potential energy than their reactants Reactants Energy released Products Amount of energy released Potential energy of molecules

 ATP shuttles chemical energy and drives cellular work  ATP powers nearly all forms of work within living things

 The energy in an ATP molecule ▪ Lies in the bonds between its phosphate groups Phosphate groups ATP Energy PPP P PP Hydrolysis Adenine Ribose H2OH2O Adenosine diphosphate Adenosine Triphosphate + + ADP

 ATP drives endergonic reactions by phosphorylation ▪ Transferring a phosphate group to make molecules more reactive Figure 5.4B ATP Chemical work Mechanical work Transport work P P P P P P P Molecule formedProtein moved Solute transported ADP + Product Reactants Motor protein Membrane protein Solute +

ATP ADP + P Energy for endergonic reactions Energy from exergonic reactions Phosphorylation Hydrolysis  Cellular work can be sustained ▪ Because ATP is a renewable resource that cells regenerate Figure 5.4C

 Enzymes speed up the cell’s chemical reactions by lowering energy barriers

 For a chemical reaction to begin ▪ Reactants must absorb some energy, called the energy of activation E A barrier Reactants Products 12 Enzyme

 A protein catalyst called an enzyme can decrease the energy of activation needed to begin a reaction Reactants E A without enzyme E A with enzyme Net change in energy Products Energy Progress of the reaction

 A specific enzyme catalyzes each reaction  Enzymes have unique three-dimensional shapes ▪ That determine which chemical reactions occur in living things

Enzyme (sucrase) Glucose Fructose Active site Substrate (sucrose) H2OH2O 1 Enzyme available with empty active site 2 Substrate binds to enzyme with induced fit 4 Products are released 3 Substrate is converted to products

 We often think of the enzymes and their substrates as a lock & a key.  Enzyme = lock  Substrate = key  Only certain substrates will fit into an enzyme like only certain keys will fit into a lock. Because of this we say enzymes are specific.

 There are many factors that affect enzyme activity. ▪ Temperature ▪ Salinity ▪ pH ▪ Concentration of substrate & its surface area ▪ Concentration of enzyme  Enzymes are very important but also very fragile.  When an enzyme’s structure is damaged and it no longer functions we say the enzyme has been denatured.