Cellular Pathways that Harvest Chemical Energy

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Cellular Pathways that Harvest Chemical Energy
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Energy and Electrons from Glucose

• The sugar glucose (C6H12O6) is the most common
  form of energy molecule.
• Cells obtain energy from glucose by the chemical
  process of oxidation in a series of metabolic
  pathways.

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Energy and Electrons from Glucose

• The equation for the metabolic use of glucose
• C6H12O6 6 O2 6 CO2 6 H2O energy
• About half of the energy from glucose is
  collected in ATP.
• ?G for the complete conversion of glucose is
  negative.
• The reaction is therefore highly exergonic, and
  it drives the endergonic formation of ATP.

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Energy and Electrons from Glucose

• Three metabolic processes are used in the
  breakdown of glucose for energy
• Glycolysis
• Cellular respiration
• Fermentation

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Figure 7.1 Energy for Life
Glucose
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Energy and Electrons from Glucose

• Glycolysis produces some usable energy and two
  molecules of a three-carbon sugar called
  pyruvate.
• Glycolysis begins glucose metabolism in all
  cells.
• Glycolysis does not require O2 it is an
  anaerobic metabolic process.

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Energy and Electrons from Glucose

• Cellular respiration uses O2 and occurs in
  aerobic (oxygen-containing) environments.
• Pyruvate is converted to CO2 and H2O.
• The energy stored in covalent bonds of pyruvate
  is used to make ATP molecules.

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Energy and Electrons from Glucose

• Fermentation does not involve O2. It is an
  anaerobic process.
• Pyruvate is converted into lactic acid or
  ethanol.
• Breakdown of glucose is incomplete less energy
  is released than by cellular respiration.

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Energy and Electrons from Glucose

• Redox reactions transfer the energy of electrons.
• A gain of one or more electrons or hydrogen atoms
  is called reduction.
• The loss of one or more electrons or hydrogen
  atoms is called oxidation.
• Whenever one material is reduced, another is
  oxidized.

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Figure 7.2 Oxidation and Reduction Are Coupled
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Energy and Electrons from Glucose

• An oxidizing agent accepts an electron or a
  hydrogen atom (it itself is reduced).
• A reducing agent donates an electron or a
  hydrogen atom (it itself is oxidized).
• During the metabolism of glucose, glucose is the
  reducing agent (and is oxidized), while oxygen is
  the oxidizing agent (and is reduced).

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Energy and Electrons from Glucose

• The coenzyme NAD is an essential electron carrier
  in cellular redox reactions.
• NAD exists in an oxidized form, NAD, and a
  reduced form, NADH H.
• The reduction reaction requires an input of
  energy
• NAD 2H NADH H
• The oxidation reaction is exergonic
• NADH H ½ O2 NAD H2O

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Figure 7.3 NAD Is an Energy Carrier
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Energy and Electrons from Glucose

• The energy-harvesting processes in cells use
  different combinations of metabolic pathways.
• With O2 present, four major pathways operate
• Glycolysis
• Pyruvate oxidation
• The citric acid cycle
• The respiratory chain (electron transport chain)
• When no O2 is available, glycolysis is followed
  by fermentation.

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Table 7.1 Cellular Locations for Energy Pathways
in Eukaryotes and Prokaryotes
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Glycolysis From Glucose to Pyruvate

• Glycolysis can be divided into two stages
• Energy-investing reactions that use ATP
• Energy-harvesting reactions that produce ATP

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Glycolysis From Glucose to Pyruvate

• The energy-investing reactions of glycolysis
• In separate reactions, two ATP molecules are used
  to make modifications to glucose.
• Phosphates from each ATP are added to the glucose
  molecule.
• The molecule is split into two 3-C molecules that
  become glyceraldehyde 3-phosphate (G3P).

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Figure 7.6 Glycolysis Converts Glucose to
Pyruvate (Part1)
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Figure 7.6 Glycolysis Converts Glucose to
Pyruvate (Part2)
Glycerladehyde 3-phosphate (G3P) 2 molecules
Dihydroxyacetone phosphate (DAP)
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Glycolysis From Glucose to Pyruvate

• The energy-harvesting reactions of glycolysis
• The first reaction (an oxidation) releases free
  energy that is used to make two molecules of NADH
  H, one for each of the two G3P molecules.
• Two other reactions each yield one ATP per G3P
  molecule.
• The final product is two 3-carbon molecules of
  pyruvate.

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Figure 7.6 Glycolysis Converts Glucose to
Pyruvate (Part3)
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Figure 7.6 Glycolysis Converts Glucose to
Pyruvate (Part 4)
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Pyruvate Oxidation

• Pyruvate is oxidized to acetate which is
  converted to acetyl CoA.
• Pyruvate oxidation is a multistep reaction
  catalyzed by an enzyme complex attached to the
  inner mitochondrial membrane.
• One NADH is generated during this reaction.

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Figure 7.8 Pyruvate Oxidation and the Citric
Acid Cycle (Part 1)
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The Citric Acid Cycle

• The citric acid cycle begins when the two carbons
  from the acetate are added to oxaloacetate, a 4-C
  molecule, to generate citrate, a 6-C molecule.
• A series of reactions oxidize two carbons from
  the citrate. With molecular rearrangements,
  oxaloacetate is reformed, which can be used for
  the next cycle.
• For each turn of the cycle, three molecules of
  NADH H, one molecule of ATP, one molecule of
  FADH2, and two molecules of CO2 are generated.

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Figure 7.8 Pyruvate Oxidation and the Citric
Acid Cycle (Part 2)
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The Respiratory ChainElectrons, Protons, and
ATP Production

• The respiratory chain uses the reducing agents
  generated by pyruvate oxidation and the citric
  acid cycle (i.e. NADH and FADH2).
• The electrons flow through a series of redox
  reactions.
• ATP synthesis by electron transport is called
  oxidative phosphorylation.

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Figure 7.10 The Oxidation of NADH H
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The Respiratory ChainElectrons, Protons, and
ATP Production

• As electrons pass through the respiratory chain,
  protons are pumped by active transport into the
  intermembrane space against their concentration
  gradient.
• This transport results in a difference in
  electric charge across the membrane.
• The potential energy generated is called the
  proton-motive force.

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The Respiratory ChainElectrons, Protons, and
ATP Production

• Chemiosmosis is the coupling of the proton-motive
  force and ATP synthesis.
• NADH or FADH2 yield energy upon oxidation.
• The energy is used to pump protons into the
  intermembrane space, contributing to the
  proton-motive force.
• The potential energy from the proton-motive force
  is harnessed by ATP synthase to synthesize ATP
  from ADP.

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Figure 7.12 A Chemiosmotic Mechanism Produces
ATP (Part 1)
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Figure 7.12 A Chemiosmotic Mechanism Produces
ATP (Part 2)
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Fermentation ATP from Glucose, without O2

• Some cells under anaerobic conditions continue
  glycolysis and produce a limited amount of ATP if
  fermentation regenerates the NAD to keep
  glycolysis going.
• Fermentation uses NADH H to reduce pyruvate,
  and consequently NAD is regenerated.
• Lactic acid fermentation occurs in some
  microorganisms and in muscle cells when they are
  starved for oxygen.

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Figure 7.14 Lactic Acid Fermentation
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Fermentation ATP from Glucose, without O2

• Alcoholic fermentation involves the use of
  enzymes to metabolize pyruvate, producing
  acetaldehyde.
• Then acetaldehyde is reduced by NADH H,
  producing NAD and ethanol (a waste product).

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Figure 7.15 Alcoholic Fermentation
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Figure 7.16 Cellular Respiration Yields More
Energy Than Glycolysis Does (Part 1)
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Figure 7.16 Cellular Respiration Yields More
Energy Than Glycolysis Does (Part 2)
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Figure 7.17 Relationships Among the Major
Metabolic Pathways of the Cell
Intermediate chemicals are generated that are
substrates for the synthesis of lipids, amino
acids, nucleic acids, and other biological
molecules.
Glucose utilization pathways can yield more than
just energy. They are interchanges for diverse
biochemical traffic.
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Regulating Energy Pathways

• Metabolic pathways work together to provide cell
  homeostasis.
• Control points regulated by enzymes use both
  positive and negative feedback mechanisms.
• For example, some enzymes are inhibited by ATP
  and activated by ADP and AMP.