COURSE READINESS ASSESSMENT
FOR
PHYSIOLOGY
CHEMICAL REACTIONS
Sections in this module
Energy
II. Chemical reactions
III. Enzymes
I.
I. Energy
What is energy?

Energy is defined as the capacity to do work

There are two basic types of energy:
◦ Kinetic energy is energy associated with movement
Examples:
Heat, which is produced by the movement of molecules
Light, which is produced by the movement of light particles
◦ Potential energy is stored energy

Potential energy can be:
◦ The energy stored by the position of an object
Example:
A child at the top of a playground slide has more potential
energy than a child at the bottom of a slide
◦ The energy stored in a chemical bond, like a covalent bond.
This type of potential energy is called chemical energy.
Thermodynamics: Energy can change form

Potential energy can be converted to kinetic energy
Example:
Carbohydrates contain potential energy (covalent bonds).
Your body digests these molecules to release the energy
and power your movement.

Kinetic energy can be converted to potential energy
Example:
Plants absorb sunlight (kinetic energy) to form covalent
bonds and build sugar molecules.
Energy and chemical reactions

In a living cell, chemical reactions are carried out to store
chemical energy by forming bonds or to release energy by
breaking them
◦ Your body digests carbohydrates, lipids, and proteins in
food. Chemical energy is released when covalent bonds in
these molecules are broken.
◦ Your cells are building molecules like fats, nucleic acids, and
proteins. The covalent bonds that form to build these
molecules store chemical energy.
II. Chemical reactions
Chemical reactions

Chemical reactions are written in a common format:
Reactants  Products

The reactants represent molecules that are the starting
material for the reaction

The products represent molecules that are made by the
reaction

The arrow shows the direction of the reaction

Chemical reactions are written so that they are balanced. The
number of atoms on the reactants side should equal the atoms
on the products side.
Example:
A reaction between hydrogen gas (H2) and oxygen gas (O2) to
form water (H2O)
2H2 + O2  2H2O
Notice that there are 4 hydrogen atoms and 2 oxygen atoms on
either side of the arrow. This is balanced chemical reaction.
Two common types of chemical reactions

In cells, some chemical reactions combine smaller molecules
to build a larger and more complex one:
A + B  AB
Example:
Amino acids are joined by covalent bonds to form a protein.

In these building, or anabolic, reactions, the reactants (A and
B) have less chemical energy than the product (AB). Anabolic
reactions require an investment of energy to occur.

Another type of chemical reaction breaks down a large and
complex molecule into smaller molecules:
CD  C + D
Example:
A protein is digested by your body, breaking covalent bonds
to separate amino acids from each other.

In these breaking, or catabolic, reactions, the reactant (CD)
has more chemical energy than the products (C and D).
Catabolic reactions release energy - chemical energy and
sometimes also heat or light.

Some reactions are at chemical equilibrium:
EF

The
E+F
represents two arrows pointing in opposite directions.
This means that the rate of the forward reaction (EF  E + F)
is equal to the rate of the reverse reaction (E + F  EF).

Both of these reactions are happening at the same time. This
means that at any time, EF, E, and F are all present.

Many factors can favor one reaction over the other, such as
temperature, molecule concentration, volume, and pressure
EF

E+F
Some examples for this reaction
◦ Temperature:
Increasing the temperature might favor the reverse reaction
(E + F  EF). High temperatures cause molecules to move
more quickly, increasing the likelihood of E and F interactions.
◦ Molecule concentration:
Increasing the amount of EF might favor the forward reaction
(EF  E + F). Larger amounts of E and F might favor the
reverse reaction.
EF

E+F
Some examples for this reaction
◦ Container volume and pressure:
If this reaction occurred in a smaller container, E and F might
interact more frequently, favoring the reverse reaction.
Increasing the pressure within a container might favor the
reverse reaction for the same reason.
How a chemical reaction occurs

In an anabolic chemical reaction like A + B  AB
A and B must collide with sufficient force and in the correct
orientation
A and B collide with glancing blow = No reaction
A and B collide with incorrect orientation = No reaction
A and B collide with enough force and correct
orientation = Reaction occurs,AB is produced!

The force of collision and molecule orientation have to be “just
right” for A and B to react. This requirements create a barrier
preventing the reaction from happening quickly or happening at
all.

This barrier is an energy barrier called the energy of
activation (EA)

Some proteins in cells act as catalysts. A catalyst helps a
chemical reaction occur faster by lowering the energy of
activation of the reaction. The catalyst itself is not consumed by
the reaction, so it can be reused.

Proteins that act as catalysts are called enzymes
III. Enzymes
Enzymes

An enzyme is a protein (usually)
that catalyzes a chemical reaction.

Enzymes increase the rate of a
reaction. They are also reusable.

The graph shows the chemical
energy in a reaction:
The red line represents the EA
(the barrier) without an enzyme
The blue line represents the EA
when the proper enzyme is
present. The enzyme lowers this
barrier!
© Jerry Crimson Mann/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL

Almost all chemical reactions in a cell are catalyzed by enzymes.

Basically, enzymes act like “on/off” switches for chemical
reactions. They determine where and when chemical reactions
take place in a cell.

Some enzymes catalyze anabolic (building) reactions, acting like
matchmakers that bring people together.

Other enzymes catalyze catabolic (breaking) reactions, acting
like divorce lawyers that break up marriages.
Enzyme structure

Proteins are chains of amino acids folded into a distinct
three-dimensional shape. A protein’s shape is critical for its
proper function.

There is a “pocket” in the shape of an enzyme where the
chemical reaction occurs. This part of an enzyme is called its
active site.
The active site of an
enzyme
© Aejahnke/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL
How enzymes work

The active site of an enzyme binds to one or more reactants
of a chemical reaction. The reactant molecule must have a
shape that fits the shape of the active.
The reactant molecule(s) that bind the enzyme’s active site
are called substrates.
Enzyme
Substrate
Enzyme-substrate
complex
© Aejahnke/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL

The substrate (or substrates, if more than one reactant) are
held in the active site by weak bonds

The enzyme then catalyzes the chemical reaction, forming
the products of the reaction

The products are released from the enzyme’s active site. The
enzyme is free to catalyze the reaction again.
Enzyme
Substrate
Enzyme-substrate
complex
© Aejahnke/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL
Enzyme
Products
Summary of Chemical Reactions

There are two basic types of energy: Kinetic and potential.
Chemical energy is a type of potential energy.

A chemical reaction converts reactants into products.

Chemical reactions form or break chemical bonds. When a
bond is formed, chemical energy is stored in the bond. The
energy is released when the bond is broken.

Anabolic reactions build larger molecules. Catabolic
reactions break molecules down into smaller ones.

Some reactions are in chemical equilibrium

An enzyme is a protein that acts as catalyst. It increases the
rate of a chemical reaction by lowering its energy of
activation. Enzymes can be reused.

An enzyme must have a specific 3D shape to function
properly. The active site of an enzyme binds to its
substrate(s), helping the chemical reaction take place.