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CONCEPTS IN BIOLOGY
TWELFTH EDITION
Enger • Ross • Bailey
CHAPTER 6
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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6.1 Energy and organisms

Organisms are classified based on the kind of energy
they use.
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
Autotrophs
 Use the energy from sunlight to make organic molecules
(sugar) called photosynthetic autotroph
 Use the energy from inorganic chemical reaction to
make larger organic molecules called chemosynthetic
autotroph
Heterotrophs
 Obtain their energy from the chemical bonds of food
molecules, such as carbohydrates, fats, and proteins,
which they must obtain from their surroundings
All organisms use cellular respiration.
–
To harvest the energy from organic molecules and use it to
make ATP
Energy transformation
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6.2 Aerobic respiration: An overview

A series of enzyme controlled reactions
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Oxygen is used to oxidize glucose.
Glucose is oxidized to form carbon dioxide.
Oxygen is reduced to form water.
During the oxidation of glucose
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The C-H and O-H bonds will be broken.
The electrons will be transferred to electron carriers, NAD
and FAD.

–
The electrons will be passed through an electron transport
chain.
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Glycolysis and Kreb’s cycle
The energy from the electrons will be used to pump protons.
The energy from the diffusion of protons will be used to make
ATP.
Aerobic respiration and oxidationreduction reactions
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Aerobic cellular respiration:
Overview
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Glycolysis


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
The breakdown of glucose
into pyruvic acid
Two ATP molecules are
used to energize glucose.
As glucose is metabolized
enough energy is released
to
– make 4 ATP molecules.
 4 ATP made -2 ATP
used = net
production of 2 ATP
– reduce 2 NAD+ to make
2 NADH.
Occurs in the cytoplasm
Krebs cycle


Also known as the citric acid
cycle or the tricarboxylic
acid (TCA) cycle
The breakdown of pyruvic
acid
–

Enough energy is released
as one pyruvic acid
molecules is metabolized to
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
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Released as carbon dioxide
make 1 ATP.
reduce 4 NAD+ to form 4
NADH.
reduce 1 FAD to form 1
FADH2.
Occurs in the mitochondrial
matrix
Electron-transport system
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
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NADH and FADH2 release the electrons they
received during glycolysis and the Kreb’s
cycle to the electron transport chain (ETC).
The proteins of the ETC transfer the
electrons and use the energy released to
pump protons.
– Protons are pumped from the matrix to the
intermembrane space.
– Creates a concentration gradient
Electron-transport system

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Oxygen is the final electron
acceptor at the end of the
ETC.
– Oxygen accepts the
electrons, combines with
protons and become
water.
The accumulated protons
diffuse back into the matrix
through ATPase
 The energy released
from the diffusion
fuels the formation of
ATP.
6.4 Aerobic respiration in prokaryotes


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Very similar to aerobic respiration in eukaryotes.
Since prokaryotes have no mitochondria, it all occurs
in the cytoplasm.
Make 2 more ATP because there is a cost to the
eukaryotic cell of getting the electrons into the
mitochondrion
6.5 Anaerobic cellular respiration



Some organisms do not have the enzymes
for Kreb’s cycle or the electron transport
system.
Some organisms can metabolize glucose in
the absence of oxygen.
Metabolizing glucose in the absence of
oxygen is called anaerobic respiration.
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Involves the incomplete oxidation of glucose.
Fermentation is an anaerobic pathway that uses
an organic molecule as the final electron acceptor.
Anaerobic cellular respiration

Anaerobic respiration usually starts with
glycolysis.
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
The fermentation reactions oxidize NADH to
regenerate the NAD+ that is needed in
glycolysis.
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Glucose is metabolized into pyruvic acid.
2 ATP are made.
In the process, pyruvic acid is reduced to either
lactic acid or ethanol or another organic molecule.
Types of fermentation
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Alcoholic fermentation

Starts with glycolysis
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During alcoholic fermentation
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Pyruvic acid is reduced to form
ethanol.
Carbon dioxide is released.
Yeast do this
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Glucose is metabolized to
pyruvic acid.
A net of 2 ATP is made.
Leavened bread (p. 122)
Sparkling wine (p. 123)
Lactic acid fermentation

Starts with glycolysis
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During lactic acid fermentation
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Pyruvic acid is reduced to form lactic acid. (p. 123)
No carbon dioxide is released.
Muscle cells have the enzymes to do this, but brain
cells do not. (p. 123)
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Glucose is metabolized to pyruvic acid.
A net of 2 ATP is made.
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Muscle cells can survive brief periods of oxygen deprivation,
but brain cells cannot.
Lactic acid “burn” in muscles.
OUTLOOKS 6.1
Souring VS. Spoilage
Metabolizing other molecules
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Cells will use the energy in carbohydrates first.
– Complex carbohydrates are metabolized into
simple sugars.
Cells can use the energy in fats and proteins as well.
– Fats are digested into fatty acids and glycerol.
– Proteins are digested into amino acids.
Cells must convert fats and proteins into molecules
that can enter and be metabolized by the enzymes
of glycolysis or the Kreb’s cycle.
Fat respiration (See p. 124)

Fats are broken down into
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Glycerol
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Converted to acetylCoA
Enter the Kreb’s cycle
Each molecule of fat fuels the formation of many
more ATP than glucose. (p. 124)
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Converted to glyceraldehyde-3-phosphate
Enters glycolysis
Fatty acids
–

Glycerol
Fatty acids
This makes it a good energy storage molecule.
Outlooks 6.2
Lipid Metabolism and ketoacidosis
Protein respiration (p. 125)


Proteins are digested into amino acids.
Then amino acids have the amino group
removed.
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Generates a keto acid (acetic acid, pyruvic acid,
etc)
Enter the Kreb’s cycle at the appropriate place
The interconversion of fats,
carbohydrates and proteins
The bottom line

Carbohydrates, fats and proteins can all be
used for energy.
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Glycolysis and the Kreb’s cycle allow these types
of molecules to be interchanged.
If more calories are consumed than used
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The excess food will be stored.
Once the organism has all of the proteins it needs
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And its carbohydrate stores are full
The remainder will be converted to and stored as fat.
How Science works 6.1
Applying Knowledge of Biochemical Pathways