Chemical Reactions
Reactants and Products
 A chemical reaction is the
process by which atoms or
groups of atoms in substances
are reorganized into different
substances  bonds are
formed or broken.
 Clues that a chemical reaction
has taken place include the
production of heat or light, and
formation of a gas, liquid, or
solid (a new substance).
Chemical reaction
Physical change
Chemical Equations
 Chemical formulas describe the substances in
the reaction and arrows indicate the process of
change.
 Reactants are the starting substances, on the left
side of the arrow.
 Products are the substances formed during the
reaction, on the right side of the arrow.
A Chemical Equation
REACTANTS
PRODUCTS
+ ENERGY
Glucose
from food
Oxygen
from air
Carbon
dioxide
Water
 This equation shows cellular respiration -- a series of chemical
reactions that require glucose and oxygen as reactants.
 Glucose sugar molecules are broken down and the energy from
its covalent bonds is released.
 Carbon dioxide and water are produced.
 In a chemical reaction, the atoms in the reactants are rearranged
to form new compounds (products).
Chemical Reactions
Balanced Equations
 The law of conservation of mass states matter
cannot be created or destroyed.
 The number of atoms of each element on the
reactant side must equal the number of atoms of
the same element on the product side.
This equation is balanced.
Energy is needed to form covalent bonds.
Breaking covalent bonds releases energy.
Chemical Reactions
Energy of Reactions
AB
A+B
Z
 The activation energy
is the minimum
amount of energy
needed for reactants
to form products in a
chemical reaction.
Chemical Reactions
AB
A+B
Z
 This reaction is
exergonic and
released heat energy
(exothermic).
Ex.: hand warm packs,
metabolic reactions
that warm up the body
 The energy of the
product is lower than
the energy of the
reactants.
Chemical Reactions
 This reaction is
endergonic and
absorbed heat energy
(endothermic).
Ex.: “ice” packs used in
sports injuries
 The energy of the
products is higher than
the energy of the
reactants.
AB
A+B
Chemical Reactions & Enzymes
Enzymes
 A catalyst is a substance
that lowers the activation
energy needed to start a
chemical reaction.
• It does not increase how
much product is made
and it does not get used
up in the reaction.
• Enzymes are biological
catalysts; they are
proteins.
AB
A+B
Chemical Reactions & Enzymes
 The reactants that bind to the enzyme are called
substrates; enzymes are specific for their
substrates/reactions.
 The specific location where a substrate binds on an
enzyme is called the active site.
What determines the specificity of an enzyme?
Chemical Reactions & Enzymes
 The active site changes shape and forms the enzymesubstrate complex, which helps chemical bonds in the
reactants to be broken and new bonds to form.
 Factors such as pH, temperature, and other substances affect
enzyme activity.
 All the chemical reactions happening inside an organism
make up its metabolism  enzymes control metabolic
reactions
Two Models of Enzyme Function

Lock & Key – the enzyme’s active site has a
shape that is specific for the substrate; the
substrate “fits” like a key into the active site

Induced Fit – as the substrate binds, the enzyme
changes shape and the active site conforms to
the substrate
Metabolic Reactions


Anabolic reactions = building up organic molecules; usually
endergonic
Ex.: Synthesis / Condensation – putting together building blocks
to make a larger molecule
Catabolic reactions = taking apart organic molecules; usually
exergonic
Ex.: Digestion, breaking down proteins and other large molecules
Which arrow represents an anabolic reaction? Catabolic reaction?
Link to Virtual Lab – Effect of Substrate Concentration and pH on Enzyme Activity:
http://glencoe.mcgraw-hill.com/sites/dl/free/0078759864/383930/BL_11.html