Cellular Respiration: Harvesting Chemical Energy

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Chapter 9

• Cellular Respiration Harvesting Chemical Energy

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Living is work

• Work requires energy
• Human body maintenance voluntary activities
  requires 2200kcal/day
• Need ATP to start reactions (activation energy)

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Conservation of Energy

• 2nd law of thermodynamics Conservation of Energy
• Energy in MOST ecosystems comes from sunlight ?
  trapped as organic molecules by plants
• Organic molecules store energy by arrangement of
  molecules (potential)
• Enzymes release energy when catabolize orgs.
• Some released used for work rest wasted as heat

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Catabolism

• Fermentation leads to partial breakdown of
  sugars in absence of oxygen
• Cellular Respiration catabolicuses oxygen to
  break down organic molecules
• Most efficient and widespread process
• Organic compounds O2 ? CO2 H2O Energy
• Carbs, fats, proteins can all be used as
  fuelmost common with glucose
• C6H12O6 6O2 ? 6CO2 6H2O Energy (ATP Heat)
• ExergonicYields ?G -686kcal per mole glucose

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Energy in Cells ATP

• ATPadenosine triphosphate energy in cells
• loaded springclose pack 3 phosphate groups to
  store energy
• Remove phosphate?relax spring
• Price of MOST cellular work is ATP?ADP Pi
• ATP works by phosphorylating (transferring Pi to)
  another molecule
• This will change shape of the molecule,
  performing work

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ATP ADP

• ATP can be regenerated from ADP by
  phosphorylationput the Pi back on
• Oxidative Phosphorylation made in ETCmakes 90
  of ATP
• Substrate-level phosphorylation enzymes transfer
  P directly to ADPoccurs in Glycolysis and Krebs

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Substrate Level Phosphorylation
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Reactions involved

• Rearrangements of electrons Redox
• Oxidation loss of electrons
• Reduction addition of electrons
• Reducing agent is the electron donor
• Oxidizing agent is the electron recipient
• Na Cl ? Na Cl-
• (Sodium is oxidized and Chlorine is reduced)
• Oxygen is one of the most potent oxidizing agents
• Electron loses energy as it shifts from a less
  electronegative to a more electronegative atom

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Redox in Glycolysis

• In Cell Resp. Glucose is oxidized, releasing
  energy
• Glucose oxidized, oxygen reduced, electrons lose
  potential energy
• Molecules with lots of H are excellent fuels have
  hilltop electrons which fall closer to Oxygen

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Pathways

• Cell Resp. does not work in a single stepit
  would be explosive(LINK)
• Uses pathways (Electron Transport Chain) with
  many steps to transfer Hs one at a time, each
  step has its own enzyme

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Figure 9.5 An introduction to electron transport
chains
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Key Steps

• H removed from glucose, given to NAD
  (nicotinamide adenine dinucleotide)
• Uses dehydrogenase to strip 2 Hs from the
  fuelpasses 2 electrons and one proton to NAD
  and releases H
• H-C-OH NAD ? CO NADH H
• NAD is oxidizing agent in many Redox steps
• Electrons in NADH lose very little
  energyeventually used to make ATP as electrons
  fall to oxygen
• NADH shuttles electrons to top of the ETC at the
  bottom, they are captured by oxygen and release
  their energy.

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Process of Cell Respiration

• 3 stages
• Glycolysis
• Krebs Cycle
• Electron Transport Chain

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Figure 9.6 An overview of cellular respiration
(Layer 3)
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Glycolysis

• Occurs in cytoplasm
• Anaerobic does not require oxygen
• Begins catabolism of glucose into 2
  pyruvatesplit 6C sugar into 2-3C sugars
• Ten stepseach has its own enzyme
• Two phases
• Energy Investment provides activation energy by
  using 2 ATP to phosphorylate glucose
• Energy Payoff produces ATP by substrate level
  phosphorylation and NAD ? NADH
• Yields a total of 4ATP and 2 NADH per glucose
• Net yield of 2 ATP

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Figure 9.8 The energy input and output of
glycolysis
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Krebs Cycle

• Named after Hans Krebs who discovered the
  pathways
• Occurs in mitochondrial matrix
• Aerobic requires oxygen
• Pyruvate ?carbon dioxide
• More than ¾ of the energy of glucose is still in
  pyruvate

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Entering Krebs

• Pyruvate must be converted before it can enter
  the Krebs Cycle adds a coenzyme coA to make
  acetyl coA. CO2 is removed, 1 NADH is made
• Acetyl coA joins the Krebs Cycle as it combines
  with oxaloacetate to form citrate

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Krebs Cycle

• 8 steps to cycle
• Each cycle produces 1 ATP by substrate-level
  phosphorylation, 3NADH, and 1 FADH2 per acetyl
  coA (X2 per glucose)
• Recycles oxaloacetate to begin again

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Electron Transport Chain

• Occurs in the cristae or membranes inside of the
  mitochondria
• Components of the chain are proteins in the
  membrane (cytochromes)
• Majority of ATP comes from energy in the
  electrons of NADH (and FADH2)
• Electrons lose free energy as the move down the
  chain
• Last electron acceptor is oxygenfor every 2
  electron carriers (4 electrons) one O2 is reduced
  to H2O
• ETC does NOT make ATPhelps ATP synthase and
  chemiosmosis by producing proton gradientuse
  energy from electrons to pump H from the matrix
  to the intermembrane space

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Chemiosmosis

• ATP synthase in cristae enzyme that makes ATP
  from ADP and Pi
• Chemiosmosis coupling of Redox reactions of ETC
  to ATP synthesisuses energy stored in H
  gradient to drive cellular work (found in
  mitochondria and chloroplasts)
• Uses a proton gradient between intermembrane
  space and matrix to power ATP synthesisproton
  motive force
• Protons diffuse down the gradient into the matrix
  through the ATP synthase enzymeexergonicpowers
  the generation of ATP

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TOTALS

• Energy Flow glucose?NADH?ETC?Proton Motive
  Force?ATP
• One 6-C glucose? 6 CO2
• Each NADH from Krebs can make 3 ATP, each FADH2
  can make 2 ATP
• NADH from glycolysis can make 2 ATP (needs to use
  energy to get into mitochondria)
• MAX yield 34 ATP by oxidative phosphorylation
  4 ATP by substrate-level phosphorylation TOTAL
  38 ATP
• Only 40 efficient, 60 of energy is lost as heat

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Fermentation

• Occurs in cytoplasm
• Anaerobic Occurs were there is NO oxygen
• In glycolysis, glucose is oxidized to two
  pyruvate molecules with NAD as oxidizing agent.
  Produces 2 ATP (net)
• If no oxygen is present, the ETC will not work.
  ?NAD can get used up and glycolysis will stop.
• Need to keep glycolysis running for energy?must
  find a way to regenerate NAD

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Alcoholic Fermentation

• Pyruvate is converted to ethanol releases CO2,
  regenerates NAD from NADH
• Occurs only in
• yeast used in
• alcohol (wine,
• beer, etc) making,
• and bread rising

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Lactic Acid Fermentation

• Pyruvate is converted to lactate (or lactic
  acid), regenerates NAD from NADH, makes NO ATP
• Occurs in some bacteria and fungi?used in yogurt
  and cheese making
• Occurs in muscle cellslactate makes muscles
  sore but is reconverted back to pyruvate by the
  liver when oxygen becomes available

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Anaerobic Totals

• ONLY get 2 ATP from glycolysis
• Facultative Anaerobes organisms that can survive
  doing either fermentation or respiration (yeast,
  bacteria) (muscles cells act as f.a.s but the
  rest of the body is not)

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Evolution

• Evidence of Glycolysis is found in oldest
  bacterial fossils 3.5billion yrs oldno oxygen
  was available.
• Glycolysis is the most widespread metabolic
  pathway and occurs in cytosol (not in membrane)
  suggests that it was evolved VERY early

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Other Foodswe dont eat just glucose!

• Polysaccharides most are broken down to
  monosaccharides, then converted into glucose or
  can enter glycolysis at an intermediate step
• Proteins digested into amino acids and the amine
  group is removed (excreted) carbon skeleton is
  modified and enters as an intermediate of
  glycolysis or Krebs

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Other Foods, cont.

• Fats broken down into glycerol and fatty acids.
  Fatty acids are broken down and enter as acetyl
  coA
• One gram of fat generates 2X ATP than a gram of
  carbs
• Not all food is catabolized completely for
  ATPsome is used for anabolism into parts of the
  body

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Controls

• Feedback regulation based on the amount of ATP in
  the cellif ATP levels drop, cell will speed up
  catabolism
• Phosphofructokinase enzyme for 3rd step of
  glycolysis can be allosterically regulated
• Shut off by ATP
• Turned on by AMP
• Synchronizes glycolysis with Krebs citrate also
  shuts off reaction

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