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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