Cellular Respiration
Lecture 6
http://upload.wikimedia.org
Much of the text material in the lecture notes is from our textbook,
“Essential Biology with Physiology” by Neil A. Campbell, Jane B.
Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship.
Other sources were sometimes used, and are noted.
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Outline
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Intense exercise
Producers and consumers
Food web
Chemical cycle in ecosystems
Cellular respiration
Versatility of cellular respiration
Anaerobic effort
Fermentation in foods
Life on an anaerobic earth
Words and terms to know
Possible test items
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Starting Line
http://eerc.ra.utk.edu
Running can require both aerobic and anaerobic effort.
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Aerobic and Anaerobic
Muscles need a steady supply of oxygen to perform work and continue
functioning.
• The metabolism of muscle is ‘aerobic’ when the muscle cells receive an
adequate supply of oxygen.
• Breathing becomes faster and deeper as muscles work progressively
harder.
• The metabolism becomes ‘anaerobic’ when the demand for oxygen is
greater than the body’s ability to deliver it.
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Intense Exercise
In anaerobic conditions, muscles break-down glucose, producing lactic
acid as a by-product.
• If too much lactic acid builds-up, the burning sensation is accompanied
by an inability of the muscles to function.
• We will be discussing the metabolic pathways for aerobic and anaerobic
work.
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Skeletal Muscle
Microscopic view
http://www.ivy-rose.co.uk
Muscle fiber
http://www.brown.edu
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The Longest Yards
http://www.abbagav.com
Endurance runners learn to stay within their aerobic limits
until the final dash.
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Solar Energy and Food
Food molecules are the storage of energy from the sun in an indirect
form.
• Animals rely on plants to convert the energy of sunlight to the chemical
energy of sugars and other organic molecules.
• Humans depend on plants for cotton, lumber, paper, and other products.
• The conversion of solar energy to chemical energy (photosynthesis) will
be discussed next week.
http://solar-center.stanford.edu
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Autotrophs and Heterotrophs
Most plants are autotrophs (self-feeders) that make all their organic
matter from inorganic nutrients including carbon dioxide, water, and
minerals from the soil.
• Animals are heterotrophs (other-feeders) that cannot make organic
molecules from inorganic ones—they must eat to obtain nutrients
from food.
• Heterotrophs depend on autotrophs for organic materials for growth
and repair.
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Producers and Consumers
Biologists refer to autotrophs as producers and heterotrophs as
consumers.
• Almost all ecosystems depend on sunlight and photosynthesis for
food.
• An example of a food web in a forest ecosystem is shown on the
next slide.
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http://www.biologycorner.com
Food Web
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http://www.nyitotkert.com
Human ‘Food Web’
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Photosynthesis and Cellular Respiration
Photosynthesis is dependent upon carbon dioxide absorbed by leaves,
and water absorbed by roots.
• The chloroplasts found in plants use CO2 and H2O to form glucose and
O2.
• Animals and plants use the glucose and O2 to obtain chemical energy
through cellular respiration.
• Cellular respiration produces the by-products, CO2 and H2O, completing
the chemical cycle.
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The oxygen present in the atmosphere is a result of photosynthesis,
and it enabled life as we know it to develop and flourish.
The atmosphere is not very deep, and most humans live at
elevations of 10,000 feet our less.
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Shallow Ocean of Air
Russian Space Station Mir
http://solarsystem.nasa.gov
The shallow depth of the Earth’s atmosphere can be seen
along the horizon.
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Chemical Cycle in Ecosystems
Sunlight
http://img.dailymail.co.uk
Photosynthesis
Chloroplasts in plants
http://pws.byu.edu
C6H12O6 (glucose)
+ O2 (oxygen)
CO2 (carbon dioxide)
+ H2O (water)
Cellular Respiration
Mitochondria in animals and plants
http://www.soquel.org
ATP
Cellular Work
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Where We Are Headed
http://therreddragonm.com
The rest of this lecture is about the
production of chemical energy in
consumers. Next week we will discuss
producers and photosynthesis.
http://www,kipapanursery.com
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Cellular Respiration
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Cellular respiration is a form of metabolism, the term for the sum of all
chemical processes that occurs in cells.
A large part of cellular respiration occurs in the mitochondria.
The potential energy in food is converted to chemical energy that can
be used by cells.
Over two dozen chemical reactions are involved in cellular respiration.
A specific enzyme catalyzes each reaction in the metabolic pathways.
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High-Level View
We will examine cellular respiration at a fairly high level—some of the
illustrations were chosen to convey the complexity of the biochemical
processes.
• I will point out the illustrations that you don’t need to know in detail for
this course.
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Potential Energy of Food
http://oregon.4h.oregonstate.edu
Food is a form of potential energy that can be converted
to chemical energy.
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Adenosine Triphosphate
As we discussed last week, ATP has a tail consisting of three phosphate
groups.
• Chemical energy is released when a phosphate group is split from the tail
forming ADP.
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ATP Molecule
http://biology.clc.uc.edu
Tail containing
three phosphate groups
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Release of Chemical Energy
Some of the energy is used to recycle ATP by adding a phosphate group
to ADP.
• The splitting-off of a phosphate group from ATP also contributes electrons
to NAD+ to form NADH.
• NADH provides additional energy for performing cellular work through the
electron transport chain.
• Energy release is used by the cell’s organelles to perform their functions.
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Basic Chemical Equation
The basic chemical equation for cellular respiration is shown on the next
slide.
• The left- and right-hand sides of the equation were seen in the ‘Chemical
Cycling in Ecosystems’ slide of the interrelationship between producers
and consumers.
• The overall chemical reaction is a form of redox reaction, which we won’t
discuss—you can read more about it in your textbook if you are interested.
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The Equation
(glucose)
Glucose
molecule
http://eurekalert.org
+
6O2
6CO2
Cellular
Respiration
+
6H20
+
ATP
(chemical
energy)
ATP
molecule
http://biology.clc.uc.edu
C6H12O6
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Chemical Cycle in Ecosystems, Redux
Sunlight
http://img.dailymail.co.uk
Photosynthesis
Chloroplasts in plants
http://pws.byu.edu
C6H12O6 (glucose)
+ O2 (oxygen)
CO2 (carbon dioxide)
+ H2O (water)
Cellular Respiration
Mitochondria in animals and plants
http://www.soquel.org
ATP
Cellular Work
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Cellular Respiration
1. Glycolysis
A human lymphocyte cell and the
visible organelles including a number
of mitochondria.
2. Krebs Cycle
3. Electron Transport Chain
The three processes involved
in cellular respiration
http://www.sinauer.com
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Glycolysis
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Glycolysis takes place in the cytoplasm of eukaryotic and prokaryotic
cells.
The process is anaerobic, that is, it does not use oxygen.
Glycolysis breaks six-carbon glucose molecules into two, three-carbon
molecules of pyruvic acid.
Two molecules of ATP are produced—pyruvic acid also donates two
high-energy electrons to the molecule NAD+ for the electron transport
chain.
NAD+ is converted to NADH once the high-energy electron is donated.
Pyruvic acid contains most of the chemical energy of glucose, which is
harvested in the Krebs cycle.
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http://staff.jccc.net
Biochemistry of Glycolysis
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Krebs Cycle
The Krebs cycle takes place in mitochondria of plant and animal eukaryotic cells.
• The process is also known by other names including the citric acid cycle.
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Chemical Reactions
Pyruvic acid is first converted to acetic acid before attaching to the carrier
molecule, coenzyme A, for entry into the Krebs cycle.
• The Krebs cycle extracts chemical energy until CO2 is formed, a major byproduct of cellular respiration.
• The cycle produces two molecules of ATP, and donates six high-energy
electrons to NAD+ for the electron transport chain.
• The cycle also donates two high-energy electrons to another molecule,
FADH2 for the electron transport chain.
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Biochemistry of the Krebs Cycle
http://upload.wikimedia.org
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Electron Transport Chain
The molecules of the electron transport chain are in the inner membrane
of the mitochondria.
• Hydrogen ions (H+) ‘fall’ toward oxygen molecules that entered the mitochondria through diffusion along their concentration gradient.
• The electron transport chain uses the high-energy electrons in NADH to
pump hydrogen ions (H+) against their concentration gradient across the
membrane of the mitochondria.
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Electron Transport Chain, Continued
The hydrogen ions diffuse along their concentration gradient (from high
to low concentration) back into the mitochondria.
• The H+ inflow turns tiny ‘turbines’ made of protein molecules located in
the membrane.
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Analogy
Boulder Dam, Nevada and Arizona
http://www.mcnarybergeron.com
Turbines connected to generators
produce electrical energy from the
downhill flow of water.
Powerhouse turbines
http://www.bossanova.com
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ATP Synthase
Energy from each turbine, known as ATP synthase, is used in attaching a
phosphate group to an ADP molecule to regenerate an ATP molecule.
• Up to 34 ATP molecules are produced by the ATP synthase—compare
with the much smaller ATP output from glycolysis and the Krebs cycle.
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http://www.sparknotes.com
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http://safety.more4kids.com
When Things Go Wrong
Carbon monoxide (CO) and cyanide block the transfer of the high-energy
electrons to oxygen in the electron transport chain.
• The mitochondria can no longer harvest food energy to convert ADP to
ATP.
• The cells stop working and the organism can die, often very rapidly.
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Versatility of Cellular Respiration
We have focused on glucose as a fuel source for cellular respiration.
• Cellular respiration also uses other carbohydrates, fats, and proteins.
• The digestive process hydrolyzes large food molecules into monomers
that can be used by glycolysis and the Krebs cycle.
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http://www.borderfoodsinc.com
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Anaerobic Effort
Cells can work for short periods of time without oxygen through the
anaerobic conversion of glucose to chemical energy.
• Anaerobic, a type of fermentation, means ‘without oxygen.’
• Muscles cells have an adequate number of ATP molecules to support
anaerobic activity for about five seconds.
• They have a secondary supply of the molecule creatine phosphate to
provide an additional ten seconds of energy reserve.
http://blog.beyou.tv
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Glycolysis and Anaerobic Effort
Glycolysis is the anaerobic pathway that can provide a limited number
of ATP molecules for cellular work in the absence of oxygen.
• The process isn’t efficient since two ATP molecules are produced for
every glucose molecule compared to 36 ATP molecules in the Krebs
cycle and electron transport chain.
• To provide energy for a final dash to the finish line, cells compensate
by consuming more glucose molecules per second, provided the body
has an adequate supply of carbohydrates.
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Lactic Acid
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Pyruvic acid is the by-product of glycolysis, which is converted (metabolized) to lactic acid.
When exercising vigorously, lactic acid accumulates in muscles, often
leading to a burning sensation and soreness.
The muscles may temporarily shut down (‘hitting the wall’) if lactic acid
accumulates in high concentrations.
The lactic acid is eventually transported to the liver and converted to
pyruvic acid.
This process requires oxygen, and this is why we may breathe so hard
even after we stop exercising.
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Fermentation in Foods
Some fungi and bacteria produce lactic acid as a by-product of fermentation.
• Microbes have been domesticated to transform milk into cheese, sour
cream, and yogurt—the sharp flavor of some of these products is due to
lactic acid.
• Fermentation is used in producing soy sauce, pickles, olives, pepperoni,
and salami, among other foods.
http://www.talamospizza1.com
http://www.sks-bottle.com
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Yeast
Yeast is a fungus capable of both cellular respiration and fermentation.
• If kept without oxygen, yeast produces sugars and the by-products, CO2
and ethyl alcohol.
• The carbon dioxide bubbles produced by yeast enable bread dough to
rise.
• Ethyl alcohol is a major component and active ingredient of alcoholic
beverages.
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Budding Yeast
Electron micrograph of budding yeast
http://upload.wikimedia.org
Yeast in the process of reproducing.
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Bread
http://biology.clc.uc.edu
Bread, with pockets resulting from CO2 bubbles
during fermentation.
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Alcoholic Beverages
http://www.auark.edu
Wine grapes covered in a yeast growth known as
‘blush.’
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http://www.answers.com
Wine Production
The fermentation process in the 16th century—the basic
principles remain the same.
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Anaerobes
Yeast are known as facultative anaerobes since they can harvest food
energy through either cellular respiration or fermentation.
• Obligate anaerobes, such as some bacteria living in stagnant ponds or
in the soil, are poisoned by oxygen.
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Clostridium difficile, an obligate anaerobe
http://www.theguardians.com
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A Much Younger Earth
http://www.zo.utexas.edu
Volcanoes released many gases containing molecules necessary for
the formation of early life—they include methane, carbon dioxide,
water, and ammonia.
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Earliest Life
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The Krebs cycle and electron transport chain operate only in aerobic
conditions (when O2 is present).
Glycolysis can operate under anaerobic and aerobic conditions since it
is not dependent upon oxygen.
Bacteria first formed about 3.5 billion years ago, and used glycolysis to
produce ATP.
Oxygen, a by-product of certain photosynthetic bacteria, did not begin
accumulating in the atmosphere until about 2.5 billion years ago.
The accumulation paved the way for the use of O2 by eukaryotic cells.
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Anaerobic Origins
Glycolysis continues to be the most widespread metabolic pathway for
ATP production.
• Glycolysis, which takes place in the cytoplasm of all cells, had an early
origin compared to the mitochondria in eukaryotic cells, which evolved
much later.
• Its much older ancestry suggests it evolved early in ancestors common
to all kingdoms of life.
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Words and Terms to Know
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Aerobic
Anaerobic
ATP synthase
Autotroph
Cellular respiration
Chemical cycle
Consumer
Electron transport chain
Facultative anaerobe
Fermentation
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Food web
Glycolysis
Heterotroph
Krebs cycle
Lactic acid
Metabolism
Obligate anaerobe
Producer
Pyruvic acid
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Possible Test Items
1. How do athletes make the best use of aerobic and anaerobic effort?
2. Describe the roles of producers and consumers in a food web of your
choosing.
3. Describe the chemical cycle in all ecosystems, and write the chemical
equation for cellular respiration.
4. Give a brief description of the three major stages of cellular respiration.
5. Describe why cyanide and carbon monoxide are poisonous and can be
deadly.
6. How could life survive on a much younger earth without oxygen in the
atmosphere?
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