Biology 12

reactant

product

energy is

released

reactant

product

energy

must be

supplied

Energy released

Energy supplied

A Exothermic B Endothermic

40 MHR • Unit 1 Metabolic Processes

some is waste thermal energy. This means that the

products (carbon dioxide and water) contain less

energy than the reactants (glucose and oxygen).

In contrast, an endothermic reactioninvolves an

input of energy. For example, the synthesis of glucose

by plants during photosynthesis is as follows:

Synthesis involves building molecules — it uses

more energy than it gives off (see Figure 2.3). For

example, green plants require energy in the form

of sunlight to produce glucose. Because this

endothermic reaction stores chemical energy in

molecules, there is a gain in energy.

As you can see in the two equations above,

oxidation and synthesis of glucose are two

reactions that are the reverse of each other. If two

reactions are the reverse of each other, one reaction

is endothermic and the other is exothermic.

Exothermic and endothermic reactions both

involve energy transformations. How do cells

control the flow of energy so that they do not

overheat and destroy themselves? In the next

section, you will learn how cells are able to lower

the amount of activation energy necessary to carry

out a variety of metabolic reactions.

6CO 2 + 6H 2 O+ energy

(sunlight)

→C 6 H 12 O 6 + 6O 2

carbon glucose oxygen

dioxide

water

Figure 2.3In an

exothermic reaction (A),

the reactants contain

more chemical energy

than do the products, so

there is a release of

energy. In an endothermic

reaction (B), the reactants

contain less energy than

do the products, so

energy must be added for

the reaction to proceed.

SECTION REVIEW

1. How is energy important to a living organism?


2. A molecule of glucose has potential energy.
   (a)What does this mean to a cell?
   (b)Where is the energy in the glucose molecule?


3. Why is a living cell considered to be part of
   an open system?


4. Draw diagrams to show how energy flows in
   an open system and in a closed system.


5. Define the term “free energy.”


6. You turn on a CD player and the disc begins
   to rotate. How is this an example of energy
   transformation?


7. Define and provide examples of exothermic and
   endothermic reactions.


8. An endothermic reaction is often referred to as an
   “uphill” process. Use an analogy from everyday life to
   explain why this is a good description.


9. Make a chart to contrast exothermic and
   endothermic reactions in terms of energy.
   10. Suppose you start a fire by using a magnifying
   glass to focus the Sun’s image on a piece of paper.
   What is the source of activation energy for this
   reaction? What is the source of activation energy after
   the fire has started and the magnifier is taken away?
   11. A research scientist wishes to study the
   breakdown of glucose in a solution to form carbon
   dioxide and water. Design an investigation to
   determine if this is an exothermic or an endothermic
   reaction.
   12. Create a flowchart that shows the series of steps
   that take place starting with reactants and ending
   with products formed in a chemical reaction. Indicate
   where energy is involved in the reaction.

In the investigation at the end of the unit, you will

design your own experiment. Be familiar with correct

experimental procedure. How will you form your

hypothesis? How will you plan your procedure? How will

you determine your dependent and independent variables?

UNIT INVESTIGATION PREP

C

I

K/U

C

C

K/U

MC

K/U

C

K/U

K/U

K/U