Cellular Respiration Releases Energy from Organic Compounds
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Cellular Respiration: electrons (and hydrogen ions) are taken from glucose (oxidized), releasing energy
and produces carbon dioxide and water.
Reduction (gain of hydrogen ion and electrons)
Oxidation (loss of hydrogen ion and electrons)
Three pathways for releasing energy:
o Aerobic cellular respiration: organisms that live in oxic conditions (oxygen-containing) use
aerobic cellular respiration for energy.
 “aerobic”: the process that requires oxygen to produce ATP.
 An oxidation reaction in which a series of enzyme-catalyzed reactions transfer electrons
from high energy molecules (mainly glucose) to oxygen.
 Energy that is released is in the form of ATP.
o Anaerobic cellular respiration: organisms that live in anoxic (no oxygen) conditions.
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“anaerobic”: process does not require oxygen to produce ATP.
 In some organisms, oxygen can be lethal.
 Some bacteria, some members of archaea and chemosynthetic organisms (deep-ocean
producers) depend on anaerobic cellular respiration.
o Fermentation: an anaerobic process.
 Yeasts and the bacteria that causes milk to sour are examples of organisms that carry
out fermentation.
 Also occurs in the muscle cells of mammals.
Glycolysis
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Cellular respiration starts with glycolysis which occurs in the cytoplasm of all cells.
Glycolysis can occur without oxygen as it is an anaerobic process.
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For some cells, glycolysis is the only source of energy because of its ability to happen anaerobically.
o Yeast usually lives without oxygen, yet can use oxygen when it is available.
o Human’s muscles can function anaerobically for a bit, but must obtain oxygen eventually. When
muscles don’t have enough oxygen to function, lactic acid builds up and ceases the muscle.
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Role of glycolysis is to split glucose (6-carbon molecule) into two molecules of pyruvate (3-carbon
molecule)
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ATP (2 molecules) is used at the start of glycolysis (even though glucose is a high energy molecule, energy
is needed to start the series of reactions).
After energy starts the process to split glucose into 2 intermediate 3-carbon molecules, ATP is created (4
molecules) and a molecule NAD+ (nicotinamide adenine dinucleotide) is reduced to NADH.
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Because 4 molecules of ATP are created, and only 2 were used at the beginning, there is a net gain of 2
molecules of ATP.
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Glycolysis is complete when there are 2 identical 3-carbon molecules (C3H4O3) of pyruvate.
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When there is enough oxygen in the cell, pyruvate is transferred into the matrix of the mitochondria.
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http://www.mcgrawhill.ca/school/applets/abbio/quiz/ch05/how_glycolysis_works.swf
Krebs Cycle Preparation
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Happens in the Mitochondria
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After pyruvate is transferred into the matrix of the mitochondria, pyruvate loses a carbon atom in the
form of carbon dioxide. The remaining two carbon atoms are bonded to the molecule Coenzyme A (CoA).
o CoA now will take the 2-carbon compound (an acetyl group) that is attached and bring it to the
Krebs cycle.
o At this time, another NAD+ is reduced to NADH.
http://www.mcgrawhill.ca/school/applets/abbio/quiz/ch05/how_nad_works.swf
Krebs Cycle
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Inside the mitochondria
Starts with a 4-carbon molecule, and attaches with the 2-carbon compound (acetyl group) that is attached
to CoA creating a 6-carbon molecule.
2 carbon atoms are fully oxidized to carbon dioxide (waste product), creating another 4-carbon molecule.
The energy released when carbon atoms are oxidized is transformed into reducing power in the form of
reduced NADH and another molecule FADH2 (Flavin adenine dinucleotide).
o NAD+ → NADH, FAD →FADH2
o FADH2 is similar to NADH and NADPH (high energy)
An ATP molecule is also generated.
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http://www.mcgrawhill.ca/school/applets/abbio/quiz/ch05/how_the_krebs_cycle_wor.swf
Electron Transport
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Most of the ATP molecules in aerobic cellular respiration are produced during the electron transport
system.
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Similar to the ETS in the chloroplasts, now it is in the mitochondria.
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High-energy electrons are passed down this chain, and each time the electron is passed, a small amount
of energy is released
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The electron carrying molecules that pass on the electrons are attached to the inner membrane of the
mitochondria.
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The energy produced helps push the hydrogen ions across the membrane from the matrix (where most
of cellular respiration occurs) to the space between the inner and outer membrane (intermembrane
space).
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In the intermembrane space, the hydrogen ion concentration is built up. In order to go back to the matrix,
chemiosmosis occurs.
o ATP synthase (enzyme) uses the energy of the concentration gradient to bind a phosphate group
to ADP, forming ATP.
o The hydrogen ion goes down the concentration gradient using ATP synthase.
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Oxygen is the final electron-accepting molecule throughout the ETS which accepts electrons and
hydrogen ions, creating a water molecule.
http://www.mcgrawhill.ca/school/applets/abbio/ch05/electrontrans_electron.swf
http://www.mcgrawhill.ca/school/applets/abbio/quiz/ch05/electron_transport_syst38.swf
Aerobic Cellular Respiration and the Role of Oxygen
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Oxygen is not used until the last step in cellular respiration (the final molecule that accepts the electron
and hydrogen ions to create water).
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If there was no oxygen, then the electrons would not be able to be passed along and ultimately, NADH
and FADH2 would not be able to lose their electrons. So the compounds would remain in their reduced,
energized state. There would be no oxidized NAD+ and FAD to pick up electrons from the Krebs cycle.
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Glycolysis would still occur; however, the amount of energy that is created in glycolysis is not enough to
sustain the needs of most eukaryotic cells.
Anaerobic cellular respiration
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Prokaryotes (bacteria, Archaea) live in anoxic (no oxygen) environments and do not need oxygen for their
energy needs.
o Environments such as swampy sediments and your colon (large intestine).
Anaerobic cellular respiration is not as efficient as aerobic because this process provides less energy
(fewer ATP’s are created) during the ETS and the synthesis of ATP using ATP synthase.
Because oxygen is not available as the final accepting electron molecule, usually an inorganic chemical
(sulfate, nitrate, carbon dioxide) is used instead.
So, instead of water as a by-product from the final acceptor, common by-products would be elemental
sulfur, nitrite, elemental nitrogen or methane.
Fermentation
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Occurs in the cytoplasm of the cell.
A metabolic pathway that includes the stage of glycolysis and a couple of reactions in which NADH is
oxidized to NAD+ by reducing pyruvate to other compounds.
The only energy generated is that which is produced through glycolysis (ATP).
2 types of fermentation:
o Lactate Fermentation:
 Occurs in single-celled organisms and animal cells that are temporarily without oxygen.
 Use NADH to convert pyruvate to a lactate molecule (aka lactic acid) in a single step.
 NAD+ is left over and is recycled to continue the process.
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Occurs when muscle cells are overworking and the demand for energy exceeds what can
be produced aerobically.
Glycolysis is increased to a point where it creates a condition called “oxygen debt”.
(exceeds the oxygen supply).
Because pyruvate can’t be broken down fast enough in the Krebs Cycle and the ETS,
pyruvate starts to accumulate after glycolysis occurs.
For glycolysis to be sustained, the muscle will remove the excess pyruvate by converting
it to lactate (which is stored in the muscle cells).
When lactate builds up, the muscle fatigues and cramps.
When enough oxygen is present again, the lactate is converted back to pyruvate, and
the whole process can continue.
Ethanol Fermentation
 Organisms that can function both aerobically and anaerobically, they carry out ethanol
fermentation when they are in the anaerobic stage.
 Pyruvate is converted to ethanol and carbon dioxide through ethanol fermentation.
 Pyruvate is converted to a 2-carbon molecule and then to ethanol.
 NADH is also oxidized to NAD+.
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Depending on the substance being fermented, the variety of yeast used, and whether
carbon dioxide is allowed to escape during the process, yeast fermentation can produce:
 Wine or champagne from grapes
 Mead (syrupy drink) from honey
 Cider from apples
 Beer is brewed by fermenting sugars in grains such as barley, rice, hops or corn.
Besides lactate and ethanol, acetone (can make smokeless gunpowder called cordite) and butanol (can
make artificial rubber for tires) can also be fermented.
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Fuel Production
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In organisms that carry out ethanol fermentation, the ethanol produced is released as a waste product.
o This waste burns, so ethanol was used for fuel for lamps and early internal combustion engines
and cars in the 1800’s.
Ethanol is toxic to yeast.
Gasohol (10% ethanol, 90% gasoline) is becoming more common. Cars that are manufactured after 1980
can use gasohol without any modifications.
Auto companies are creating engines that can use fuels with ethanol that have a higher percentage than
10%.
Brazil is the world’s leading producer of ethanol and its use as a fuel. Cars manufactured in Brazil can burn
pure ethanol.
In Canada, the most common source of ethanol is the fermentation of corn and wheat. (p. 193 – Ethanol
Production)
Ethanol burns more slowly.
Ethanol reduces the amount of carbon monoxide in the exhaust.
Burning pure ethanol also eliminates the release of partially burned hydrocarbons and volatile organic
compounds (VOC’s) which contribute to smog.
Questions:
Page 194 #2, 3, 8, 9, 10, 11
Unit 3 Review p.198
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