1. 2–4 Chemical Reactions and Enzymes
Photo Credit: Richard Megna/Fundamental Photographs
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Chemical Reactions
Chemical Reactions
A chemical reaction is a process that changes one set of chemicals into another set of chemicals.
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Chemical Reactions
The elements or compounds that enter into a chemical reaction are known as reactants.
The elements or compounds produced by a chemical reaction are known as products.
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Energy in Reactions
Energy Changes
Chemical reactions that release energy often occur spontaneously. These are exergonic reactions.
Chemical reactions that absorb energy will not occur without a source of energy. These are endergonic reactions.
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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
How would you reverse this reaction?
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Cellular respiration is the process through which organisms energy from food.
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Energy in Reactions
Activation Energy
Chemists call the energy that is needed to get a reaction started the activation energy.
Chemical reactions that release energy often occur spontaneously. Chemical reactions that absorb energy will occur only with a source of energy. The peak of each graph represents the energy needed for the reaction to go forward. The difference between this required energy and the energy of the reactants is the activation energy.
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Enzymes
Enzymes
Some chemical reactions that make life possible are too slow or have activation energies.
These chemical reactions are made possible by catalysts.
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Catalysts
Catalysts are substances that make chemical reactions happen faster.
Enzymes are biological catalysts.
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Enzymes
Enzymes speed up chemical reactions that take place in cells.
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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
An Enzyme-Catalyzed Reaction
The enzyme hexokinase converts the substrates glucose and ATP into glucose-6-phosphate and ADP.
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Enzyme Action
Regulation of Enzyme Activity
Enzymes can be affected by any variable that influences a chemical reaction.
pH values
Changes in temperature
Enzyme or substrate concentrations
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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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