Cellular Respiration

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Cellular Respiration

• Chapter 7

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Where Is the Energy in Food?

• Electrons pass from atoms or molecules to one
  another as part of many energy reactions.
• Oxidation is when an atom or molecule loses an
  electron.
• Reduction is when an atom or molecule gains an
  electron.
• These reactions always occur together
• Oxidation-reduction (redox) reactions

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Where Is the Energy in Food?

• Redox reactions involve transfers of energy
  because the electrons retain their potential
  energy.
• The reduced form of an organic molecule has a
  higher level of energy than the oxidized form.

Loss of electron (oxidation)


o
o
e
A
B

A
B
A

B
Gain of electron (reduction)
Low energy
High energy
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Where Is the Energy in Food?

• The energy for living is obtained by breaking
  down the organic molecules originally produced in
  plants.
• The ATP energy and reducing power invested in
  building the organic molecules are stripped away
  as the chemical bonds are broken and used to make
  ATP.
• The oxidation of food stuffs to obtain energy is
  called cellular respiration.

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Where Is the Energy in Food?

• Cellular respiration is the harvesting of energy
  from breakdown of organic molecules produced by
  plants
• The overall process may be summarized as

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Cellular Respiration

• Cellular respiration takes place in two stages
• Glycolysis
• Occurs in the cytoplasm.
• Does not require O2 to generate ATP.

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Cellular Respiration

• Krebs cycle
• Occurs within the mitochondrion.
• Harvests energy-rich electrons through a cycle of
  oxidation reactions.
• The electrons are passed to an electron transport
  chain in order to power the production of ATP.

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Using Coupled Reactions to Make ATP

• Glycolysis is a sequence of reactions that form a
  biochemical pathway.
• In ten enzyme-catalyzed reactions, the six-carbon
  sugar glucose is broken into two three-carbon
  pyruvate molecules.

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Using Coupled Reactions to Make ATP

• The breaking of the bonds yields energy that is
  used to phosphorylate ADP to ATP.
• This process is called substrate-level
  phosphorylation.
• In addition, electrons and hydrogen atoms are
  donated to NAD to form NADH.

http//www.youtube.com/watch?v3GTjQTqUuOwlistFL
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Glucose
Glucose
1
ATP
1
Phosphorylation of glucose by ATP.
Glycolysis
ADP
P
Glucose 6-phosphate
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Pyruvate oxidation
2
Rearrangement, followed by a second ATP
phosphorylation.
P
Krebs cycle
Fructose 6-phosphate
ATP
3
ADP
P
P
Electron transport chain
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The six-carbon molecule is split into two
three-carbon molecules of G3P.
Fructose 1,6-bisphosphate
4,5
P
P
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Glyceraldehyde 3- phosphate (G3P)
Glyceraldehyde 3- phosphate (G3P)
Oxidation followed by phosphorylation
produces two NADH molecules and gives two
molecules of BPG, each with one
high-energy phosphate bond.
NAD
NAD
Pi
Pi
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NADH
NADH
P
P
P
P
1,3-bisphosphoglycerate (BPG)
1,3-bisphosphoglycerate (BPG)
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Removal of high-energy phosphate by two
ADP molecules produces two ATP molecules and
gives two 3PG molecules.
ADP
ADP
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ATP
ATP
P
P
3-phosphoglycerate (3PG)
3-phosphoglycerate (3PG)
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P
P
89
Removal of water gives two PEP molecules,
each with a chemically reactive phosphate bond.
2-phosphoglycerate (2PG)
2-phosphoglycerate (2PG)
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P
P
Phosphoenolpyruvate (PEP)
Phosphoenolpyruvate (PEP)
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Removal of high-energy phosphate by two
ADP molecules produces two ATP molecules and
gives two pyruvate molecules.
ADP
ADP
10
ATP
ATP
Pyruvate
Pyruvate
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Using Coupled Reactions to Make ATP

• Glycolysis yields only a small amount of ATP.
• Only two ATP are made for each molecule of
  glucose.
• This is the only way organisms can derive energy
  from food in the absence of oxygen.
• All organisms are capable of carrying out
  glycolysis.
• This biochemical process was probably one of the
  earliest to evolve.

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Harvesting Electrons from Chemical Bonds

• In the presence of oxygen, the first step of
  oxidative respiration in the mitochondrion is the
  oxidation of pyruvate.
• Pyruvate still contains considerable stored
  energy at the end of glycolysis.
• Pyruvate is oxidized to form acetyl-CoA.

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Acetyl-Coa

• When pyruvate is oxidized, one of its three
  carbons is cleaved.
• This carbon leaves as part of a CO2 molecule.
• In addition, a hydrogen and electrons are removed
  from pyruvate and donated to NAD to form NADH.

• The remaining two-carbon fragment of pyruvate is
  joined to a cofactor called coenzyme A (CoA).
• The final compound is called acetyl-CoA.

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Key Biological Process Transfer of H atoms

• NADH and NAD are used by cells to carry hydrogen
  atoms and energetic electrons.

3
2
1
Substrate
Product

e
H
e

H

e
H
NAD
NAD
H
NAD
NAD
H
NAD
NADH then diffuses away and is available to
donate the hydrogen to other molecules.
Enzymes that harvest hydrogen atoms have a
binding site for NAD located near the substrate
binding site.
In an oxidation-reduction reaction, the hydrogen
atom and an electron are transferred to NAD,
forming NADH.
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Harvesting Electrons from Chemical Bonds

• The fate of acetyl-CoA depends on the
  availability of ATP in the cell.
• If there is insufficient ATP, then the acetyl-CoA
  heads to the Krebs cycle.
• If there is plentiful ATP, then the acetyl-CoA is
  diverted to fat synthesis for energy storage.

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Krebs Cycle

• The second step of oxidative respiration is
  called the Krebs cycle.
• The Krebs cycle is a series of 9 reactions that
  can be broken down into three stages
• Acetyl-CoA enters the cycle and binds to a
  four-carbon molecule, forming a 6-C molecule.
• Two carbons are removed as CO2 and their
  electrons donated to NAD. In addition, an ATP is
  produced.
• The four-carbon molecule is recycled and more
  electrons are extracted, forming NADH and FADH2.

http//www.youtube.com/watch?v-cDFYXc9Wko
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The Krebs cycle
Oxidation of pyruvate
Glucose
Pyruvate
CO2
Glycolysis
NAD
Pyruvate oxidation
Coenzyme A
NADH
CoA
Krebs cycle
Acetyl-CoA
Electron transport chain

• Note A single glucose molecule produces two
  turns of the cycle, one for each of the two
  pyruvate molecules generated by glycolysis.

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1
The cycle begins when a C2 unit reacts with a
C4 molecule to give citrate (C6).
Mitochondrial membrane
Krebs cycle
CoA
2-4
1
Oxidative decarboxylation produces NADH with
the release of CO2.
(4 C) Oxaloacetate
Citrate (6 C)
NADH
2
8-9
9
The dehydrogenation of malate produces
a third NADH, and the cycle returns to
its starting point.
NAD
3
(4 C) Malate
Isocitrate (6 C)
NAD
4
8
NADH
H2O
CO2
(4 C) Fumarate
?-Ketoglutarate (5 C)
FADH2
NAD
7
CO2
5
NADH
CoA
CoA-SH
FAD
S
(4 C) Succinate
Succinyl-CoA (4 C)
6
6-7
A molecule of ATP is produced and the
oxidation of succinate produces FADH2.
CoA-SH
5
A second oxidative decarboxylation produces a
second NADH with the release of a second CO2.
ADP
ATP
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Harvesting Electrons from Chemical Bonds

• In the process of cellular respiration, the
  glucose is entirely consumed.
• The energy from its chemical bonds has been
  transformed into
• 4 ATP molecules.
• 10 NADH electron carriers.
• 2 FADH2 electron carriers.

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Using the Electrons toMake ATP

• NADH and FADH2 transfer their electrons to a
  series of membrane-associated molecules called
  the electron transport chain.
• Some protein complexes in the electron transport
  chain act as proton pumps.
• The last transport protein donates the electrons
  to hydrogen and oxygen in order to form water.
• The supply of oxygen able to accept electrons
  makes oxidative respiration possible.

http//www.youtube.com/watch?vkN5MtqAB_YclistFL
9N_Px072WuVorSwDfqf-9windex2featureplpp
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The electron transport chain
Glucose
Intermembrane space
Glycolysis
H
H
H
Inner mitochondrial membrane
Pyruvate oxidation
Krebs cycle
e
e
e
Electron transport chain
FADH2
NADH
NAD
H
H2O
2H
O2
1
2
Protein complex III
Protein complex II
Protein complex I
Krebs
Mitochondrial matrix
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Using the Electrons toMake ATP

• Chemiosmosis is integrated with electron
  transport.
• Electrons harvested from reduced carriers (NADH
  and FADH2) are used to drive proton pumps and
  concentrate protons in the intermembrane space.
• The re-entry of the protons into the matrix
  across ATP synthase drives the synthesis of ATP
  by chemiosmosis.

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Cells Can Metabolize Food Without Oxygen

• In the absence of oxygen, organisms must rely
  exclusively on glycolysis to produce ATP.
• In a process called fermentation, the hydrogen
  atoms from the NADH generated by glycolysis are
  donated to organic molecules, and NAD is
  regenerated.
• With the recycling of NAD, glycolysis is allowed
  to continue.

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Fermentation

• Bacteria can perform more than a dozen different
  kinds of fermentation.
• Eukaryotic cells are only capable of a few types
  of fermentation.

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Fermentation

• In yeasts (single-celled fungi), pyruvate is
  converted into acetaldehyde, which then accepts a
  hydrogen from NADH, producing NAD and ethanol.
• In animals, such as ourselves, pyruvate accepts a
  hydrogen atom from NADH, producing NAD and
  lactate.

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Glucose Is Not the OnlyFood Molecule

• Cells also get energy from foods other than
  sugars.
• These complex molecules are first digested into
  simpler subunits, which are then chemically
  modified into intermediates.
• These intermediates enter cellular respiration at
  different steps.

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