2 4 Chemical Reactions and Enzymes Chemical Reactions

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1 Chemical Reactions A chemical reaction occurs when chemical bonds are broken and reformed. Rust forms very slowly, while rocket fuel combustion is explosive! The significance of this comparison is that some reaction rates are too slow and others too fast to function as part of an organism s metabolism. The reactions must be controlled by ENZYMES! The formation of iron oxide (rust) 4Fe + 3O 2 2Fe 2 O Rocket Fuel Combustion 2H 2 + O 2 2H 2 O 37

2 Chemical Reactions Chemical reactions break and reform chemical bonds. Elements or compounds that enter into chemical reactions are called reactants and are usually found on the left side of a chemical equation. The elements or compounds produced by a chemical reaction are called products and are usually found on the right side of a chemical equation. In the example reaction, the reactants are hydrogen and oxygen, and the product is water. The numbers in front of the elements and compounds balance the equation, so there are the same number of atoms of each element on each side. 38

3 Energy in Reactions Breaking and forming bonds requires changes in energy. An exergonic reaction is one that releases energy, Ex. Aerobic Cellular Respiration. An endergonic reaction is one that absorbs energy, Ex. Photosynthesis. In order to stay alive, organisms must perform endergonic reactions. The ultimate source of energy for these reactions in most organisms is the sun. The overall chemical equations for photosynthesis and cellular respiration show that the products of one are the reactants of the other. Energy arrives on earth as sunlight and is transformed during photosynthesis to chemical energy in glucose. Glucose is broken down during aerobic cellular respiration into another form of chemical energy that cells can use ATP. ATP 39

4 Energy in Reactions Endergonic Exergonic The energy available for a reaction (y-axis) is thermal energy (heat). This increases the kinetic energy (movement) of reactants. Note that BOTH endergonic and exergonic reactions require activation energy, which is the minimum energy necessary to start a reaction. An endergonic reaction can be identified from this type of graph when the products have more energy than the reactants (uphill). An exergonic reaction can be identified when the products have less energy than the reactants (downhill). 40

5 Energy in Reactions Enzymes are biological catalysts that reduce the activation energy required for reactions that would normally never happen or run too slowly to function in living systems. Enzymes are very specific to the reactions they catalyze and their name usually relates to the reaction they catalyze and end in ase. For example, catalase is the enzyme that catalyses the break down of hydrogen peroxide to water and oxygen gas, a reaction that normally would happen too slowly, allowing the build up of toxic hydrogen peroxide. DNA polymerase is the enzyme that catalyzes the polymerization of DNA during its replication. 60C 37C Thermal 41

6 Enzyme Activity Glucose + ATP Glucose-6-phosphate + ADP hexokinase The reactants of an enzyme catalyzed reaction are called substrates. The region of the enzyme where the substrates fit is called the active site. The enzyme will not function if the active site is not exactly the right shape. The way the substrates fit in the active site is known as lock and key. The enzyme-substrate complex forms when the substrates bind in the active site. Note that the diagram is cyclical, illustrating that enzymes are REUSABLE! 42

7 Enzyme Activity Temperature and ph can change the shape of the enzyme's active site so that it can not work? This is called denaturation. Living cells can regulate enzymes with proteins that bind to the enzyme and change the shape of the active site (see diagram). E 43