Name
CHAPTER 9
Class
Date
Photosynthesis and Cellular Respiration
SECTION
1 Energy in Living Systems
KEY IDEAS
As you read this section, keep these questions in mind:
• What type of energy is used in cells?
• How is an organism’s metabolism related to the carbon cycle?
• How is energy released in a cell?
READING TOOLBOX
Ask Questions Read this
section quietly to yourself.
As you read, write down any
questions you have. When
you finish reading, work in a
small group to figure out the
answers to your questions.
Background
Recall that homeostasis is
the process of maintaining
consistent internal
conditions, even if the
outside environment
changes.
8g^i^XVa I]^c`^c\
1. Predict What would
happen to life on Earth if all
the autotrophs disappeared?
Where Do Organisms Get Energy?
Why do you shiver when you are cold? Your body is
trying to maintain homeostasis. When you shiver, your
muscles produce heat to help warm you up. You and
all other organisms need chemical energy to maintain
homeostasis. This chemical energy comes from carbon
compounds in food.
Almost all of the energy in carbon compounds comes
from the sun. Plants, algae, and some prokaryotes use
energy from sunlight to change carbon dioxide and
water into carbon compounds. This process is called
photosynthesis. During photosynthesis, the energy
in sunlight is stored in chemical bonds in carbon
compounds. Organisms that carry out photosynthesis are
called autotrophs.
Other organisms, including all animals and fungi,
cannot perform photosynthesis. They get energy by
absorbing carbon compounds from other organisms or by
eating other organisms.
Watermelon plants, like
other autotrophs, produce
their own food through
photosynthesis. Humans,
like many other animals, get
energy by eating autotrophs.
EHHDBG@<EHL>K
2. Identify Where do
autotrophs get food?
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Photosynthesis and Cellular Respiration
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Class
SECTION 1
Date
Energy in Living Systems continued
How Is Metabolism Part of the Carbon
Cycle?
Metabolism involves either using energy to build
carbon compounds or breaking down carbon compounds
to release the energy in them. Therefore, metabolism
is part of Earth’s carbon cycle. The processes of the
carbon cycle move carbon compounds between and
within ecosystems. Because organisms obtain energy
from carbon compounds, the carbon cycle also delivers
chemical energy to organisms.
How Do Cells Use the Energy from Food?
The main organic compound that cells use is glucose.
Glucose is produced during photosynthesis. The overall
chemical reaction that occurs during photosynthesis is
shown below.
Background
Recall that metabolism is all
of the chemical reactions that
occur within an organism.
READING CHECK
3. Explain How is
metabolism part of the
carbon cycle?
Photosynthesis: Overall Chemical Reaction
Carbon
dioxide
6CO2
Energy
from
sunlight
Water
+
6H2O
+
Glucose
energy
Oxygen
C6H12O6
+
6O2
In order to get energy from glucose molecules, most
organisms use cellular respiration. During cellular
respiration, glucose combines with oxygen to produce
carbon dioxide, water, and energy, as shown below.
EHHDBG@<EHL>K
4. Compare How is the
overall chemical reaction for
cellular respiration related to
the overall chemical reaction
for photosynthesis?
Cellular Respiration: Overall Chemical Reaction
Glucose
C6H12O6 +
Oxygen
Carbon
dioxide
6O2
6CO2
Energy for
metabolism
Water
+
6H2O
+
energy
If glucose and oxygen combined in a single step, all of
the energy would be released as heat. Therefore, within a
cell, the reaction between glucose and oxygen is broken
up into several steps. The energy that is released at each
step is stored as chemical energy in a molecule called
adenosine triphosphate, or ATP.
ATP is the main energy source for cell processes.
The figure at the top of the next page shows how ATP is
produced.
READING CHECK
5. Identify What molecule
is the main energy source for
cell processes?
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Photosynthesis and Cellular Respiration
Name
SECTION 1
Class
Date
Energy in Living Systems continued
Hydrogen ions move across the
membrane through an enzyme called
ATP synthase. The enzyme uses the
energy from the moving hydrogen
ions to produce ATP from ADP and a
phosphate ion.
KXcb8Yflk@k
Summarize Review this
figure by yourself. Then,
describe what is shown in
the figure to a partner.
H
+
The area outside
the membrane
has a high
concentration of
hydrogen ions, H+.
+
H
H
+
H
+
H+ (high concentration)
Membrane
The area inside the
membrane has a
low concentration
of H+ ions.
ADP +
H+ (low concentration)
ATP
synthase
H
+
ATP
EHHDBG@<EHL>K
6. Identify What two
molecules combine to form
ATP?
P
ATP contains
three phosphate
groups. Adenosine
diphosphate, or ADP,
contains only two.
Energy is required
to add a phosphate
group to ADP to
form ATP.
The H+ ions flow from the area of high concentration (outside the membrane) to the
area of low concentration (inside the membrane). They can flow only through the ATP
synthase molecule.
READING CHECK
7. Describe How is energy
released in an electron
transport chain?
The diagram above shows that the concentration of H+
ions outside the membrane is higher than that inside
the membrane. Remember that ions flow from areas of
high concentration to low concentration. How, then, is
the area of high concentration outside the membrane
produced? The cell must use energy to move the
hydrogen ions from inside the membrane to outside. This
energy comes from an electron transport chain.
In an electron transport chain, energy is released
as electrons move between different molecules.
This released energy is used to move hydrogen ions
from areas of low concentration to areas of high
concentration.
There are three main molecules that are part of
electron transport chains: NADH, FADH2, and NADPH.
Each of these molecules can release energy by losing
electrons. The energy can then be used to move hydrogen
ions across the membrane.
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Photosynthesis and Cellular Respiration
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Section 1 Review
SECTION VOCABULARY
electron transport chain a series of molecules,
found in the inner membranes of mitochondria
and chloroplasts, through which electrons pass
in a process that causes protons to build up on
one side of the membrane
photosynthesis the process by which plants,
algae, and some bacteria use sunlight, carbon
dioxide, and water to produce carbohydrates
and oxygen
ATP adenosine triphosphate, an organic
molecule that acts as the main energy source
for cell processes; composed of a nitrogenous
base, a sugar, and three phosphate groups
ATP synthase an enzyme that catalyzes the
synthesis of ATP
cellular respiration the process by which
cells produce energy from carbohydrates;
atmospheric oxygen combines with glucose to
form water and carbon dioxide
1. Describe How does ATP synthase produce ATP?
2. Explain How does the carbon cycle deliver energy to organisms?
3. Identify How do organisms that are not autotrophs get energy?
4. Explain In cells, glucose is combined with oxygen in a series of steps instead of
all at once. What is the reason for this?
5. Describe What happens in an electron transport chain?
6. Identify Name two molecules that can release energy as part of the electron
transport chain.
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Photosynthesis and Cellular Respiration
Name
CHAPTER 9
Class
Date
Photosynthesis and Cellular Respiration
SECTION
3 Cellular Respiration
KEY IDEAS
As you read this section, keep these questions in mind:
• How does glycolysis produce ATP?
• How is ATP produced in aerobic respiration?
• Why is fermentation important?
READING TOOLBOX
Summarize As you read,
underline the important
ideas in this section. When
you are finished reading,
write a one- or twoparagraph summary of the
section, using the underlined
ideas.
What Is Glycolysis?
There are three main parts of cellular respiration:
glycolysis, the Krebs cycle, and the electron transport
chain. During glycolysis, glucose is broken down into
two molecules of a three-carbon compound called
pyruvate. The figure below shows how this occurs.
'LYCOLYSIS
Background
Recall that cellular
respiration is the process by
which organisms get energy
from organic compounds.
EHHDBG@<EHL>K
1. Identify How many
molecules of ATP does the
overall process of glycolysis
produce?
1 Enzymes add two phosphate molecules
'LUCOSE
# # # # # #
!40È
from ATP to a glucose molecule and
break the glucose apart. This forms two
three-carbon molecules, each of which
contains a phosphate group.
0
2 Each three-carbon molecule reacts
with another phosphate group. During
this reaction, two hydrogen atoms are
transferred to two molecules of NAD+.
This forms two molecules of NADH, an
electron carrier.
3 Each three-carbon molecule is then
converted into a molecule of pyruvate.
This process produces 4 ATP molecules.
Because the first part of glycolysis
requires two ATP molecules, the overall
process produces two molecules of ATP.
!$0È
È 0
# # #
# # #
0
È.!$
È.!$(ÈÈ(
0
# # #
0
0
# # #
0
!$0È
ÈPYRUVATES
!40È
# # #
# # #
What Happens During Aerobic Respiration?
READING CHECK
2. Define What does
aerobic mean?
Glycolysis is an anaerobic process. That means it
does not require oxygen. Most organisms use oxygen to
release even more energy from glucose. This process is
called aerobic respiration, because it requires oxygen.
The Krebs cycle and the electron transport chain are
the two parts of aerobic respiration. They occur within
mitochondria. During the Krebs cycle, pyruvate is
broken down to produce energy. This energy is stored
in the molecules NADH, FADH2 , and ATP. During the
electron transport chain, ATP and water are produced.
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Photosynthesis and Cellular Respiration
Name
Class
SECTION 3
Date
Cellular Respiration continued
4HEÈ+REBSÈ#YCLE
.!$(ÈÈÈ(
.!$
Summarize After you read
this page, describe what
happens during the Krebs
cycle and the electron
transport chain to a partner.
Put the information into your
own words.
# # #
# # #
# ASÈ#/
# # #
#
#
# # #
# #
0YRUVATE
FROMÈGLYCOLYSISÈ
KXcb8Yflk@k
# ASÈ#/
.!$
# ASÈ#/
#
# # #
.!$(ÈÈÈ(
.!$
.!$(ÈÈÈ(
.!$
.!$(ÈÈÈ(
8g^i^XVa I]^c`^c\
#
# # #
&!$
&!$(
3. Apply Concepts How
many molecules of ATP,
NADH, and FADH2 are
produced by the Krebs cycle
for every molecule of glucose
that enters glycolysis?
!$0È 0
!40
A pyruvate molecule that is produced during glycolysis enters the Krebs cycle. It
reacts with other compounds through a series of steps. At each step, energy is
released. This energy is then stored in the compounds ATP, NADH, and FADH2.
For each molecule of pyruvate that enters the Krebs cycle, one molecule of ATP,
three molecules of NADH, and one molecule of FADH2 are produced.
(Hint: How many molecules
of pyruvate are produced by
each molecule of glucose?)
The Electron Transport Chain in Cellular Respiration
1
Electrons from NADH and FADH2 move through the electron
transport chain. Proteins use the energy from the electrons to
move hydrogen ions across the membrane of the mitochondrion.
H
H
+
H
Outer compartment
+
H
+
+
H
e-
H
+
+
ATP synthase
eH
+
+
NADH +
H
+
H
FADH2
NAD+
FAD
Membrane
+
H
+
4H + O2
Inner compartment
2
At the end of the electron
transport chain, the electrons
have lost most of their energy.
They combine with oxygen atoms
and hydrogen ions to form water.
2H2O
+
EHHDBG@<EHL>K
H
ADP +
P
ATP
3
The hydrogen ions move through
ATP synthase molecules, producing
ATP molecules. The electron
transport chain can produce up to 32
molecules of ATP for each molecule
of glucose that enters glycolysis.
4. Identify How many
molecules of ATP can be
produced by the electron
transport chain for every
molecule of glucose?
FERMENTATION
Some organisms get all of their energy from glycolysis.
They use a process called fermentation to produce the
NAD+ that accepts electrons during glycolysis. During
fermentation, electrons move from NADH to pyruvate.
Fermentation allows cells in aerobic organisms to
continue to carry out glycolysis when there is no oxygen
around. For example, your muscles can use fermentation
to produce energy when you exercise too hard.
READING CHECK
5. Explain How is
NAD+ generated during
fermentation?
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Section 3 Review
SECTION VOCABULARY
aerobic describes a process that requires oxygen
anaerobic describes a process that does not
require oxygen
fermentation the breakdown of carbohydrates
by enzymes, bacteria, yeasts, or mold in the
absence of oxygen
glycolysis the anaerobic breakdown of glucose
to pyruvic acid, which makes a small amount
of energy available to cells in the form of ATP
Krebs cycle a series of biochemical reactions
that convert pyruvic acid into carbon dioxide
and water; it is the major pathway of oxidation
in animal, bacterial, and plant cells, and it
releases energy
1. Explain Why is glycolysis considered an anaerobic process, but the electron
transport chain is not?
2. Describe Fill in the blank spaces in the table below.
Process
Description
Overall number of ATP
molecules produced per
molecule of glucose
Glucose is broken down into
two pyruvate molecules.
Pyruvate is used to produce
NADH, ATP, and FADH2;
carbon dioxide is produced as
pyruvate breaks down.
Energy from electrons in
NADH and FADH2 is used
to produce ATP; water is
produced as hydrogen and
oxygen accept electrons.
3. Compare Organism A can carry out cellular respiration. Organism B can carry
out only glycolysis. Which organism will be able to use more of the energy in a
molecule of glucose? Explain your answer. (Hint: Remember that ATP is the main
source of energy for cellular processes.)
4. Describe Why is fermentation important?
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