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Biology
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2. 2–4 Chemical Reactions and Enzymes
Photo Credit: Richard Megna/Fundamental Photographs
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Chemical Reactions
Chemical Reactions
*Everything that happens in an organism is based on chemical reactions.
Def: A chemical reaction is a process that changes one set of chemicals into another set of chemicals.
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Chemical Reactions
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Chemical Reactions
What happens to chemical bonds during chemical reactions?
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Chemical Reactions
Chemical reactions always involve changes in the chemical bonds that join atoms in compounds.
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Energy in Reactions
Energy in Reactions
*Energy is released or absorbed whenever chemical bonds form or are broken.
Because chemical reactions involve breaking and forming bonds, they involve changes in energy.
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Energy in Reactions
How do energy changes affect whether a chemical reaction will occur?
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Energy in Reactions
Energy Changes
Chemical reactions that release energy often occur spontaneously. Chemical reactions that absorb energy will not occur without a source of energy.
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Energy in Reactions
*When hydrogen gas reacts with oxygen to produce water vapor, it is an energy-releasing reaction in which energy is given off as heat.
2H2 + O2 2H2O + Energy
2H2O H2 + O2
*Reversing this reaction would require the addition of large amounts of energy.
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Energy in Reactions
*In order to stay alive, organisms need to carry out reactions that require energy.
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Energy in Reactions
Activation Energy
Chemical reactions that release energy do not always occur spontaneously.
Def: Chemists call the energy that is needed to get a reaction started the activation energy.
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Enzymes
Enzymes
*Some chemical reactions that make life possible are too slow.
Def: A catalyst is a substance that speeds up the rate of a chemical reaction. (enzymes act as catalysts)
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Enzymes
Why are enzymes important to living things?
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Enzymes
1.Enzymes are proteins that act as biological catalysts.
Enzymes speed up chemical reactions that take place in cells.
2.Enzymes act by lowering the activation energy.
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Enzymes
3.Lowering the activation energy has a dramatic effect on how quickly the reaction is completed.
Enzymes speed up chemical reactions that take place in cells. Notice how the addition of an enzyme lowers the activation energy in this reaction. This action speeds up the reaction.
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Enzymes
*Enzymes are very specific, generally catalyzing only one chemical reaction.
*For this reason, part of an enzyme’s name is usually derived from the reaction it catalyzes(works on).
EX: CO2 + H  H2CO3(carbonic acid)
*Left to itself, the reaction is very slow & CO2 builds up in blood. When your blood makes carbonic anhydrase(enzyme)it speeds up reaction so CO2 is removed immediatley.
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Enzyme Action
The Enzyme-Substrate Complex
*Enzymes provide a site where reactants can be brought together to react, reducing the energy needed for reaction.
*The reactants of enzyme-catalyzed reactions are known as substrates.
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Enzyme Action
STEPS:
1.The substrates bind to the active site on the enzyme, forming an enzyme-substrate complex.
*The fit is so precise that the active site and substrates are often compared to a lock and key.
2.The enzyme and substrates remain bound together until the reaction is done and the substrates are converted to products.
3.The products of the reaction are released and the enzyme is free to start the process again.
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Enzyme Action
An Enzyme-Catalyzed Reaction
The enzyme hexokinase converts the substrates glucose and ATP into glucose-6-phosphate and ADP.
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2–4
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2–4
The elements or compounds produced by a chemical reaction are known as
reactants.
enzymes.
products.
waste.
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2–4
Chemical reactions always involve
changes in energy.
enzymes.
catalysts.
changes in the atomic number of the reactants.
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2–4
The factor that prevents many energy-releasing reactions from occurring at relatively low temperatures is called
catalytic energy.
chemical bond energy.
enzyme energy.
activation energy.
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2–4
Which of the following statements is true?
All proteins are enzymes.
All catalysts are enzymes.
All enzymes are catalysts.
All catalysts are proteins.
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2–4
What happens to an enzyme after the reaction it catalyzes has taken place?
The enzyme is destroyed, and the cell must make another.
The enzyme holds on to the product until another enzyme removes it.
The enzyme is unchanged and ready to accept substrate molecules.
The enzyme changes shape so it can accept a different kind of substrate.
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