Chemical Reactions
and Enzymes
What is a Chemical Reaction?
� Chemical reactions change substances into different ones by breaking and forming bonds.
� Reactions that allow living organisms to obtain energy, grow, reproduce, and adapt are called biochemical reactions.
� Examples: � Photosynthesis, cellular respiration, digestion,
muscle contraction, nerve signals.
What is a Chemical Reaction?
� Chemical reactions can be represented by a chemical equation.
� Example: photosynthesis
H2O + CO2
C6H12O6 + O2
Reactants
Products
“Yields”
Chemical Reactions and Energy � The amount of energy that is needed to
start the reaction is called activation energy.
� There are 2 types of reactions: 1. Endergonic 2. Exergonic
Endergonic Reactions
� Must ABSORB energy in order for the reaction to begin.
� Absorbs heat = endothermic
� Example: � Photosynthesis – requires sunlight to initiate
the reaction
Endergonic Reactions
Exergonic Reactions
� Release energy and they can occur spontaneously.
� Releases heat = exothermic
� Examples: � Cellular respiration � bioluminescence
Exergonic Reactions
Factors that Affect Reactions
� Four things affect the rate of a reaction (i.e. how quickly the reaction proceeds)
1. Temperature 2. pH 3. Catalysts 4. Concentration of reactants
Factors that Affect Reactions
� The higher the temperature, the faster the reaction rate.
� A small change in pH can disrupt a reaction. � This problem is solved by buffers.
� The more concentrated the chemicals, the faster the reaction rate.
Catalysts
� A catalyst is a substance that speeds up the rate of a chemical reaction
� A catalyst works by lowering the activation energy of the reaction, allowing the reaction to occur at a lower-than-normal temperature
� A catalyst is not “used-up” or changed during the reaction
� Catalysts are reusable
Enzymes
� Enzymes are catalysts in living organisms
� Enzymes are a type of protein
� Involved in many biochemical reactions that are necessary for organisms to obtain and utilize energy, grow, move, develop, and reproduce
� Enzymes are very specific; only one particular enzyme can catalyze a certain reaction.
Enzymes at Work
� The way that enzymes operate can be modeled as a “Lock and Key” system.
� In most cases, only one key only fits a lock, just as… one enzyme only works with one reactant (which is called a substrate).
� On the surface of an enzyme is an area called the active site that has a shape that fits only one specific substrate.
“Lock and Key” Model
1. The substrate binds to the enzyme.
2. The enzyme-substrate complex is formed.
3. The reaction occurs, and the products are released.
4. The enzyme can be reused again.
What Affects Enzymes?
� The structure of an enzyme can be altered by three things, which denature (destroys) the enzyme.
1. pH 2. Temperature 3. Heavy Metals (i.e. Gold, Lead, etc)
� An enzyme is designed to operate at a certain pH and temperature. If either of these factors is changed too much, the enzyme can become irreversibly damaged.
What Affects Enzymes? � If the shape of the enzyme’s active site is changed,
it can no longer bind to the substrate and the enzyme will not work and it becomes denatured.
What is Catalase? � The enzyme catalase is found inside the cells of
many living tissues. � It catalyzes (speeds up the chemical reaction that
breaks down hydrogen peroxide (H2O2)
� Hydrogen peroxide is a normal byproduct of metabolism � However, it is poisonous to cells
� Catalase breaks down hydrogen peroxide into two harmless substances: water and oxygen gas.
What is Catalase? � This decomposition reaction is represented by the
following chemical equation:
2 H2O2 2 H2O + O2