CHAPTER 8

Cellular Energy
1 How Organisms Obtain Energy MAINIDEA
All living organisms use
3(F), 4(B), 9(B), 9(D) energy to carry out all
biological processes.
Before You Read
Think about the objects in your home that use energy. On the What You’ll Learn
lines below, describe the ways that these objects get energy.
Then read about how organisms obtain energy. • the two laws of
thermodynamics
Stoves get energy from natural gas and power outlets • the difference between
get energy from electric currents from metal wires anabolic and catabolic
pathways
• how ATP works in a cell

Focus
After you read this section, create
a quiz based on what you have
learned. Then be sure to answer
the quiz questions.
Read to Learn
Transformation of Energy
Cells need energy. They need energy to move molecules
across membranes and to make and break down molecules.
Energy is the ability to do work. Thermodynamics is the study
of how energy flows and changes in the universe.
What are the laws of thermodynamics?
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Two laws of thermodynamics explain the flow of energy. The

first law states that energy can change form, but it cannot be
created or destroyed. For example, your body changes the
chemical energy in food into a more useable form. Then when Think it Over
you move, your body changes that energy into mechanical
1. State the first law of
energy.
thermodynamics.
The second law of thermodynamics states that systems change
from states of order to states of disorder on their own. This
disorder is known as entropy (EN truh pee). Entropy is always
increasing. This means that when your body changes forms of
energy, some of the energy is lost as heat. The energy is still
present, but it can no longer be used.
How do organisms get energy from the Sun?
Nearly all the energy for life on Earth comes from the Sun.
Some organisms make their own food. Some autotrophs use Energy can change
inorganic substances as a source of energy. Other autotrophs form, but it cannot be
change light energy from the Sun into chemical energy. Plants created or destroyed
and some bacteria are autotrophs.

Reading Essentials • Cellular

Energy
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 Heterotrophs get their energy by eating food. Heterotrophs get
energy from the Sun indirectly. They do this by eating
autotrophs. Animals are heterotrophs. The figure below shows
the relationship between autotrophs and heterotrophs.
Take a Look
2. Circle the organism that
makes its own food.

Autotroph

The Autotroph Heterotroph Heterotroph

Sun
Metabolism
All of the chemical reactions that go on inside a cell are known
as the cell’s metabolism. A series of reactions in which the
product of one reaction becomes the reactant for the next
reaction is called a metabolic pathway.
What are the two metabolic pathways?
Think it Over There are two types of metabolic pathways: catabolic (ka
3. Compare the energy usage tuh BAH lik) pathways and anabolic (a nuh BAH lik)
in anabolic and catabolic pathways. In catabolic pathways, energy is released by
pathways. breaking larger

use.
Copyright
©
Catabolic pathways break molecules into smaller molecules. In anabolic pathways, the

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down larger molecules into
smaller ones and anabolic
energy released by catabolic pathways is used to build larger
path ways make larger
molecules from smaller molecules.
molecules from smaller Energy flows between the metabolic pathways of organisms in
ones an ecosystem. Photosynthesis is an anabolic pathway. Cellular
respiration is a catabolic pathway. These pathways work together
to meet the energy needs of cells.
How is energy changed during photosynthesis?
Photosynthesis is a series of reactions that change light energy
from the Sun into chemical energy that can be used by the cell.
During photosynthesis, light energy, carbon dioxide, and water
are changed into organic molecules and oxygen. The energy
stored in organic molecules made during photosynthesis can be
passed to other organisms. When an animal eats a plant, the
plant’s stored energy is passed to the animal.

Reading Essentials • Cellular

Energy
84
 What happens during cellular respiration?
Cellular respiration is a series of reactions that break
down organic molecules into carbon dioxide, water, and
energy. The energy is used by the cell. The processes of
cellular respiration and photosynthesis form a cycle, which is
shown in the figure below. The products of photosynthesis
are the reactants for cellular respiration, and the products of Take a Look
cellular respiration are the reactants for photosynthesis. 4. Identify Draw a circle
around the anabolic process
Sunlight and a square around the
catabolic process.

Photosynthesis

CO2 + H2O O2 + Glucose

Cellular respiration

ATP: The Unit of Cellular Energy

Cells store chemical energy in biological molecules. The most
important biological molecule is adenosine triphosphate (uh
DEN uh seen • tri FAHS fayt), or ATP.
How does ATP store energy?
ATP is the most abundant energy-storing molecule. It is found
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in all kinds of organisms. The structure of ATP is shown below.

It is made of an adenine base, a ribose sugar, and three
phosphate groups.
ATP releases energy when the bond between the second and
third phosphate groups is broken, forming a molecule called
adenosine diphosphate (ADP). ADP can be changed back into
ATP by adding a phosphate group. Take a Look
Adenine
5. Identify Circle the high-
Triphosphat H
e group 2
energy bond that is broken
when ATP is converted to
P P P
ADP.
Ribose
ATP

Adenine
Diphosphate
group
P P P Energy
Ribose Phosphate
ADP

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Energy
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