1. Energy and Chemical Reactions Living things undergo thousands of chemical reactions as part of the life process The collection of these reactions is called metabolism

2. Energy Transfer Much of the energy organisms need is provided by sugar (food) Undergoes a series of chemical reactions in which energy is released (cellular respiration) The net release of free energy is called an exergonic (exothermic) reaction ~ energy or heat given off.

3. Reactions that involve an input of free energy are called endergonic (endothermic) reactions Photosynthesis is an example Most reactions in living organisms are endergonic; therefore living organisms require a constant source of energy Energy Transfer

4. Most chemical reactions require energy to begin The amount of energy needed to start the reaction is called activation energy Energy Transfer

5. Certain chemical substances (catalysts) reduce the amount of activation energy required Biological catalysts are called enzymes Enzymes

6. Enzymes are an important class of catalysts in living organisms – Mostly protein – Thousands of different kinds – Each specific for a different reaction (1 enzyme : 1 substrate) Enzymes

7. Enzyme Structure Enzymes work on substances called substrates Substrates must fit into a place on an enzyme called the active site Enzymes are reusable!