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Chapter 9
Date
Cellular Respiration
• Summary
9-i Chemica’ Pathways
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Food is the energy source for cells. The energy in food is measured
in calories. A calorie is the amount of energy needed to raise the
temperature of 1 gram of water 1 degree Celsius. The Calorie
(capital C) used on food labels is equal to 1000 calories.
Cells do not burn glucose or other food compounds. They
gradually rel?ase thç energy. The process begins with a pathway
called glycolysis.
Glycolysis is the process in which a glucose molecule is split
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in half. This forms two molecules of pyruvic acid, a 3-carbon
compound. Glycolysis takes place in the cytoplasm of a cell.
Through glycolysis, the cell gains 2ATP molecules. In addition,
the electron carrier NAD~ accepts a pair of high-energy electrons,
producing NADH. By doing this, NAD~ helps passenergy from
glucose to other pathways in the cell.
When oxygen is not present, fermentation follows glycolysis.
Fermentation releases energy from food molecules by forming
ATE Fermentation• does not need oxygen, so it is said to be
anaerobic. During fermentation, cells convert NADH back into
the electron carrier NAD~ that is needed for glycolysis. This lets
glycolysis continue to make a steady supply of ATP. The two
types of fermentation are alcoholic fermentation and lactic
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acid fermentation.
• Yeasts and a few other microorganisms carry out alcoholic
fermentation. The equation for alcoholic fermentation after
glycolysis is:
pyruvic acid + NADH
alcohol + CO2 + NAD~
• Lactic acid fermentation occurs inmuscles during rapid
exercise. The equation for lactic acid fermentation after
glycolysis is:
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pyruvic acid + NADH lactic acid + NAD~
If oxygen is present, the Krebs cycle and electron transport
chain follow glycolysis. Together, these pathways make up
cellular respiration. Cellular respiration is the process that
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releases energy by breaking down glucose and other food
molecules in the presence of oxygen. Cellular respiration takes
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place in niltochondria. The equation for cellular respiration is:
6O~ + C6H1206 -4 6C02 + £H2O + Energy
oxygen + glucose
carbon dioxide + water + Energy
-‘--+
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9-2 The Krebs Cyde and flectron Transport
Cellular respiration requires oxygen, soit is said to be aerobic.
The Krebs cycle is the second stage ofiDellular respiration.
During the Krebs cycle, pyruvicacid is broken down into
carbon dioxide in a series of energy-extracting reactions.
The Krebs cycle is also known as the citric acid cycle, because•
citric acid is one of its first products.
Here are the stages of the Krebs cycle.
The Krebs cycle starts when pyruvic acid formed by
glycolysis enters the mitochondrion.
• The pyruvic acid is broken down iflto carbon dioxide and
a 2-carbon acetyl group.
• The two carbons of the acetyl group join a 4-carbon compound to produáe citric acid. The Kr.ebs ~cle continues in
a series of reactions~ In these reactions, two energy carriers
accept high-energy electrons. NAD~ is changed to NADH,
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and FAD is changed to FADH2. These molecules carry the
high-energy electrons to the electron trañspôtt chain. The
carbon dioxide is released as a waste product.
The electron transport chain uses the high-energy electrons
to change ADPinto ATP. Tn the electron transport chain, highenergy elçctrons move from one carrier protein to the next. At
the end of the chain, oxygen pulls electrons from the final carrier
molecule. These~ electrons join with hydrogen ions, forming water.
Each transfer along the chain releases a small amount of energy
ATP synthase uses..the energy to produce ATE
Glycolysis produces 2 ATP molecules from one molecule
of glucose The Krebs cycle and the electron transport chain let the
cell form 34 ATP moleculeá ~er glucose molecule. The total, then,
for cellular respiration is36 ATP molecules per glucose molecule
The energy flows in photosynthesis and cellular respiration
occur in opposite directions On a global level, photosynthesis and
cellular respiration are also opposites Photosynthesis removes
carbon dioxide from the atmosphere and puts back oxygen
Cellular respiration removes oxygen from the atmosphere and
puts back carbon dioxide
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The M~tochondr~on
In plant and animal cells, the final stages of cellular respiration
take place in mitochondria. A mitochondrion has two membranes.
The inner membrane is folded up inside the outer membrane. The
space between the inner and outer membranes is called the intermembrane space. The space inside the inner membrane is called
the matrix.
Label the inner membrane, intermembrane space, matrix, and
outer membrane~
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Answer the questions. Circle the correct answer.
1. In which membrane is the electron transport chain located?
outer membrane
iiuier membrane
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CeUubr Respfrat~on Overview
Cellular respiration is the process that rejeases energy from food
-in the presence of oxygen.
Glucose
Use the words below to label the diagram of cellular respiration on the
lines provided.
ATP
electron transport chain
.
glycolysis
Krebs cycle
niitochondrion
1.
2.
3.
4.
5.
Use the diagram to answer the questions.
1. Where does glycolysis take place?
2. Where do the Kreb cycle and electron transport chain take place?
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Ce~hsDar ~espfrat~o~i and Photosynthesis
Cellular respiration and photosynthesis can be thought of as
opposite processes. Energy flows in opposite directions in the
two processes.
Complete the table using the words below. Some cells have been
completed for you. Some words may. be used more than once.
carbon dioxide
energy release
mitochondria
Photosynthesis
0
Function
energy capture
Location
chioroplasts
Reactants
Products
water
Cellular Respiration
glucose; oxygen
oxygen; glucose
Use the table to answer the quçstions.
1. Which process releases energy for the cell? Circle th?
correct answer.
cellular respiration
photosynthesis
2. For which reaction is 6C02 + 6H20 —. C6H1206
equation? Circle the correct answer.
cellular respiration
+
6O~ the correct
photosynthesis
3. How do the products of phobsynthesis compare to the
reactants of cellular respiration?
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