2.  Cells store and retrieve energy by making and breaking chemical bonds in metabolic reactions  Some reactions require a net input of energy – others end with a net release of energy

3.  Reaction Process of chemical change  Reactant Molecule that enters a reaction  Product A molecule remaining at the end of a reaction

5.  Chemical bonds hold energy – the amount depends on which elements take part in the bond  Cells store energy in chemical bonds by running energy-requiring reactions, and access energy by running energy- releasing reactions

7.  Molecules of life release energy when combined with oxygen – but not spontaneously – energy is required to start even energy-releasing reactions  Activation energy Minimum amount of energy required to start a reaction

8. Fig. 4-4, p. 65 Reactants: 2 H 2 + O 2 Activation energy Energy Difference in energy between reactants and products Products: 2 H 2 O Tim e Stepped Art Activation Energy

9.  Enzymes make chemical reactions proceed much faster than they would on their own  Enzyme Protein or RNA that speeds a reaction without being changed by it

10.  An enzyme’s particular substrates bind at its active site  Substrate A reactant molecule that is specifically acted upon by an enzyme

11.  Active site Pocket in an enzyme where substrates bind and a reaction occurs

12.  Each enzyme works best within a characteristic range of temperature, pH, and salt concentration  When conditions break hydrogen bonds, an enzyme changes its characteristic shape (denatures), and stops working

14. Fig. 4-6a, p. 66 normal tyrosinase temperature- sensitive tyrosinase Enzyme activity 20°C (68°F)30°C (86°F)40°C (104°F) A Temperature

15. Fig. 4-6b, p. 66 glycogen phosphorylase trypsin pepsin Enzyme activity 1234567891011 B pH

16.  Cells concentrate, convert, and dispose of most substances in enzyme-mediated reaction sequences  Metabolic pathway Series of enzyme-mediated reactions by which cells build, remodel, or break down an organic molecule