CHAPTER 9 CELLULAR RESPIRATION: HARVESTING CHEMICAL ENERGY

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CHAPTER 9CELLULAR RESPIRATION HARVESTING
CHEMICAL ENERGY
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Some background

• 1. Relocation of electrons during chemical
  reaction releases energy stored in food.
• 2. The chemical reactions that transfer an
  electron from one reactant to another is an
  oxidation-reduction rxn.
• 3. Redox reactions relocate electrons closer to
  O2
• 4. When electrons move closer to electronegative
  atoms, they release some chemical energy.

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Background.

• The main energy foods, carbohydrates fats, are
  essentially electron reservoirs associated with
  hydrogen.
• Only activation barriers hold back flood of
  electrons to lower energy state. (ie. With an
  electronegative atom)
• If there were no activation energy, sugar would
  spontaneously react with oxygenbreakdown.

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Oxidation and Reduction

• The loss of electrons is called oxidation.
• The addition of electrons is called reduction.
• LEO the Lion goes GER
• The formation of table salt from sodium and
  chloride is a redox reaction.
• Na Cl ? Na Cl-
• Here sodium is oxidized and chlorine is reduced

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• Oxidizing Reducing Agents
• In a chemical reaction Xe- Y ? X Ye-
• X, the electron donor reducing agent
• (reduces Y)
• Y, the electron recipient oxidizing agent
  (oxidizes X)
• Example In NaCl, _____is the oxidizing agent
  and ____is reducing agent.
• Na Cl ? Na Cl -

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• Redox reactions in organic molecules
• Change in the degree of electron sharing in
  covalent bonds (need not be complete transfer).
• Eg) Combustion of methane to form water and
  carbon dioxide
• non-polar covalent bonds of methane (C-H) and
  oxygen (OO) converted to polar covalent bonds in
  CO2 and H2O

e- move closer to O
Fig. 9.3
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In catabolic reactions

• Chemical bonds in the food are reorganized.
• Electrons move from positions equidistant between
  two atoms
• to a closer position to oxygen, or more
  electronegative atom.
• In the process, free energy is lost.
• This energy used to make ATP.

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Cellular Respiration many redox rxns

• Glucose is oxidized
• e- from glucose are transferred through NAD,
  electron carrier, and ultimately to oxygen
• C6H12O6 6O2 ? 6CO2 6H2O
• oxygen is reduced

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Keep in mind

• The overall process of cellular respiration is
  exergonic releases energy is spontaneous.
• Therefore, the only thing holding back the
  spontaneous breakdown of sugars and fats is their
  high activation energy.
• This is where enzymes come inthey lower the
  activation energy.
• AND enzymes can be regulated!

Cellular respiration is exergonic, E released
goes to make ATP so organsims can do work
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Cellular Respiration 3 steps

• 1) Glycolysis
• 2) Krebs cycle (citric acid cycle)
• 3) Electron transport chain (ETC)

Fig. 9.6
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Step 1 Split Glucose molecule

• Glycolysis occurs in the cytoplasm.
• Breakdown of glucose into two molecules
• of pyruvate.
• Generate ATP and molecules of NADH

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Glycolysis

• 10 steps in glycolysis.
• Each step is catalyzed by a specific enzyme
• 2 phases
• an energy investment phase energy is used
• an energy payoff phase energy is made

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1) ATP is produced By substrate-level
phosphorylation 2) NAD is reduced to
NADH. NAD is an electron carrier molecule,
remember e- associated w/ H. 3) ATP is made.
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• Substrate-level phosphorylation
• an enzyme transfers a phosphate group from an
  organic molecule (PEP) to ADP.
• Takes place in glycolysis and Krebs cycle

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• The net yield from glycolysis is per glucose
• Energy molecules 2 ATP
• Reduced molecules 2 NADH
• Since glycolysis occurs without O2 it is also
  known as anaerobic cycle
• If O2 is present, pyruvate moves to the Krebs
  cycle (CAC) in the form of acetyl-CoA

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• Krebs Cycle (Citric Acid Cycle)
• Occurs in mitochondrial matrix
• It degrades pyruvate to carbon dioxide.
• Generates ATP and reducing molecules NADH and
  FADH2

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• Pyruvate changed to acetyl-CoA
• 1) Carbon lost as CO2
• 2) NADH produced
• 3) The acetate reacts with CoA to form acetyl-CoA

Fig. 9.10
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• Acetyl-CoA combines with oxaloacetate to form
  citrate.
• The cycle regenerates oxaloacetate is and
• the acetate is broken down to CO2.

2 carbon
6carbon
4 carbon
5 carbon
4carbon
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• Each cycle produces
• 1) 1 ATP (via substrate-level phosphorylation)
• 2) 3 NADH, 1 FADH2
• NADH and FADH2 enter electron transport system
• 3) Carbon from organic molecule lost as CO2

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Summation so far

• We have few ATP made from glycolysis and CAC.
• We have lots of NADH and one FADH2
• We have disposed of the carbon in the form of CO2
• BUT we still do not have LOTS of ATP.
• And where is the oxygen we breath and water that
  we exhale?

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• The electrons from NADH and FADH2
• move along the electron transport chain
• until they combine with oxygen and hydrogen ions
  to form water.
• In the process energy released is used to make ATP

Electron Transport Chain
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• Proteins bound on inner mitochondrial membrane
  act as electron shuttles.
• As electron is passed from one protein complex to
  the next, energy is released.

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Electron shuttle releases energy
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• Oxygen is the ultimate electron acceptor
• Due to its electronegative character, oxygens
  drive to bind electrons is the driving force
  behind electron transport chain.

Fig. 9.13
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• What is the purpose of ETC?
• The electron transport chain does not generate
  ATP directly.
• BUT,
• It breaks the large free energy drop from food to
  oxygen
• into a series of smaller steps that release
  energy in manageable amounts.
• 3 proteins in the ETC are proton shuttles!
• This sets up a electrochemical gradient across
  the inner mitochondrial membrane!

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Proton shuttling
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Chemiosmosis

• Chemiosmosis is an energy-coupling mechanism.
• Uses energy stored in the form of an H gradient
  across a membrane
• To generate ATP.

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• The protons (H) are unequally distributed across
  the intermembranous space and mitochondrial
  matrix
• intermembranous space has higher H ion
  concentration pH7
• Matrix has lower H ion concentration pH 8

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• H can easily diffuse into matrix back because of
  concentration gradient.
• However,
• The flow of protons is highly regulated with the
  help of ATP synthetase
• The ATP synthase molecules are the only place
  that will allow H to diffuse back to the matrix.
• This flow of H in a controlled manner
  generates energy.
• This energy is used by the enzyme ATP synthetase
  to synthesize ATP.

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• Chemiosmosis
• Using energy in H gradient set up by ETC, the
  ATP synthetase enzyme phosphorylates ADP to renew
  ATP supply.

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• Remember, there are two ways to generate ATP
• Substrate level phosporylation
• Oxidative phosphorylation

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• substrate-level phosphorylation
• enzyme transfers a phosphate group from an
  organic molecule (substrate) to ADP, forming
  ATP.
• Takes place in glycolysis and Krebs cycle

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• Oxidative phosphorylation
• A protein complex, ATP synthase, in the cristae
  actually makes ATP from ADP and Pi.
• using energy of an proton gradient

Fig. 9.14
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The result of respiration

• electrons loose potential energy resulting in the
  
• Generation of ATP by oxidative phosphorylation.
• A molecule of glucose
• a. Under aerobic respiration, yields 38 ATP,
• b. Under anaerobic respiration, yields 2 ATP.

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Anaerobic respiration/ fermentation

• It does not require O2
• It is an incomplete breakdown of glucose
• In humans it takes place in muscle cells

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• Fermentation can generate ATP from glucose
• by substrate-level phosphorylation
• recycle NAD by transferring electrons from NADH
  to pyruvate
• or derivatives of pyruvate.

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• Lactic acid fermentation
• pyruvate is reduced directly by NADH to form
  lactate (ionized form of lactic acid).

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• Muscle cells use anaerobic respiration when
  oxygen is scarce.
• Lactate is transported to liver where its
  converted to pyruvate
• Lactic acid fermentation by some fungi and
  bacteria is used to make cheese and yogurt.

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• Alcoholic fermentation
• pyruvate is converted to ethanol in 2 steps.
• 1) pyruvate is converted to a two-carbon
  compound, acetaldehyde by the removal of CO2.
• 2) acetaldehyde is reduced by NADH to ethanol.

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• Catabolism of sugar breakdown of sugar into
  simpler molecules can take place by two processes

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• Besides Carbohydrates,
• fats, and
• proteins can all be catabolized
• through the same respiratory pathways.