HOW CELLS RELEASE STORED ENERGY

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HOW CELLS RELEASE STORED ENERGY

• Chapter 7

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Energy-releasing Pathways

• Sun ? producers ? consumers (heterotrophs)
• Producers harvest the suns energy to make
  glucose
• Consumers eat the producers (and other consumers)
  to obtain energy (glucose)
• Consumers break down glucose to convert its
  energy to ATP using aerobic (with O2) or
  anaerobic (without O2) respiration
• The chemical equation for aerobic respiration
  shows its relationship to photosynthesis
• C6H12O6 6O2 ?? 6CO2 6H2O ATP
• See the small figure on page 120 showing the
  relationship between photosynthesis and aerobic
  respiration

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Energy-releasing Pathways

• Both aerobic and anaerobic respiration start with
  glycolysis in the cytoplasm
• Produces a small amount of ATP
• Aerobic respiration continues in the mitochondria
  with the Krebs cycle and electron transfer
  phosphorylation (This is the same as electron
  transfer chain. The book uses phosphorylation, I
  will refer to it as the electron transfer chain.)
• Produces a large amount of ATP

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Fig. 7-2b, p.108
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Fig. 7-3, p.109
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Questions

• What are heterotrophs?
• Which type of respiration occurs in the presence
  of oxygen?
• What metabolic pathway is used by both aerobic
  and anaerobic respiration?
• Where does glycolysis occur?
• Where does the Krebs cycle occur?
• Which type of respiration produces the most ATP?

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Glycolysis

• Glycolysis is a metabolic pathway that occurs in
  the cytoplasm
• Glucose is broken down through a series of
  intermediates to two pyruvate molecules
• Study unit 7.3 and figure 7.5
• The following slide highlights some important
  steps of the pathway

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Glycolysis

• Two ATP molecules are used to energize the
  rearrangement of glucose into two 3-carbon
  molecules called PGAL
• For glycolysis you have to spend some energy to
  earn some energy
• Both PGAL molecules are rearranged through
  several intermediates
• Electrons are stripped from PGAL producing NADH
• ATP is produced when phosphate groups are
  transferred to ADP (phosphorylation)
• The final product is two pyruvate molecules

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Glycolysis

• Results per glucose molecule
• 2 ATP total
• Used 2 ATP
• Made 4 ATP
• 2 NADH molecules
• Co-enzymes carrying electrons and H
• Will be shuttled to the electron transport chain
• 2 pyruvate molecules

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Questions

• What starting molecule is broken down during
  glycolysis?
• How much energy is required to get glycolysis
  started?
• After the input of energy what 3-carbon
  intermediate is formed?
• What is the final product of glycolysis?
• How much ATP is generated (gross and net)?
• How much NADH is generated?

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Krebs Cycle

• The Krebs cycle is a cyclic metabolic pathway
  that occurs in the mitochondria
• Pyruvate is rearranged to form Acetyl-CoA which
  then enters the Krebs cycle where it is broken
  down in a series of steps to CO2
• Study unit 7.4 and figure 7.7
• The following slide highlights some important
  steps of the pathway

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Krebs Cycle

• Pyruvate enters the mitochondrion (remember there
  are two pyruvate molecules for every glucose
  molecule)
• Pyruvate is changed to Acetyl-CoA by the
  following reactions
• Electrons and H are stripped to make NADH
• CO2 is released
• Co-enzyme A is attached
• Acetyl-CoA combines with oxaloacetate to form
  citrate
• Several rearrangements and intermediates result
  in
• Two more CO2 molecules are released
• Three more NADH (and one FADH2, which is another
  electron carrying co-enzyme) form by stripping
  electrons and H from the intermediates
• One molecule of ATP is formed
• Ultimately oxaloacetate is reformed to start the
  cycle again

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Fig. 7-6a, p.113
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Krebs Cycle

• Results per glucose molecule (remember there are
  two pyruvates produced per molecule of glucose)
• Six CO2 molecules released
• Accounts for all six carbons found in glucose
  C6H12O6
• Eight NADH plus two FADH2 molecules
• Co-enzymes carrying electrons and H
• Will be shuttled to the electron transport chain
• Two ATP molecules

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Questions

• Where does the Krebs cycle occur?
• Prior to starting the Krebs cycle pyruvate is
  changed to what molecule?
• Acetyl-CoA is bound to ______ to form _____.
• During the Krebs cycle electrons and hydrogen
  atoms are stripped and carried by what two
  co-enzymes?
• How many ATP molecules are formed (per one
  glucose)?
• How many NADH molecules are formed (per one
  glucose)?

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Electron Transfer Chain(aka electron transfer
phosphorylation)

• Electron transfer is an energy producing process
  that occurs over the inner mitochondrial membrane
• Study unit 7.5 and figure 7.8
• The following slides highlight some important
  steps of the transfers

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Electron Transfer Chain(aka electron transfer
phosphorylation)

1. NADH and FADH2 carry the electrons and H to the
   electron transfer chain
2. As the electrons move through the chain they
   release small amounts of energy allowing the
   transfer chain to shuttle H over the membrane

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Fig. 7-7b, p.114
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Electron Transfer Chain(aka electron transfer
phosphorylation)

• An H gradient forms in the outer mitochondrial
  compartment (in between the two membranes)
• The resulting gradient propels H across the
  mitochondrial membrane through ATP synthases
• The flow has enough force to cause the synthases
  to attach phosphate to ADP, forming ATP
• The process is called chemiosmosis or H
  electrochemical gradient

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Fig. 7-7b, p.114
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Electron Transfer Chain(aka electron transfer
phosphorylation)

• Once the electrons have moved through the
  electron transfer chain, they are accepted by O2
  which is the terminal electron acceptor
• O2 combines the electrons and H to form water
• O2 H electrons ? H2O

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Fig. 7-7b, p.114
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Electron Transfer Chain(aka electron transfer
phosphorylation)

• Results per glucose molecule
• 32 molecules of ATP!
• Depending on the needs of the cell the amounts
  can fluctuate
• Shifting concentrations of reactant,
  intermediates and products
• Shuttling mechanisms for moving NADH may use some
  ATP

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Questions

• Where is the electron transfer chain?
• What carries the electrons to the chain?
• What happens when energy is released by electrons
  moving through the electron transfer chain?
• The build up of a H concentration gradient
  provides the force to cause what important event?
• For aerobic respiration what is the terminal
  electron acceptor?
• How many molecules of ATP can be formed by the
  electron transfer chain (per glucose molecule)?

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Energy-releasing Pathways

• Total possible for one molecule of glucose (cells
  dont always harvest this much as they may use
  the intermediates in other processes)
• Glycolysis 2 ATP
• Krebs 2 ATP
• Electron transfer 32 ATP
• Total 36 ATP
• See figure 7.9 for a summary of aerobic
  respiration

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Fig. 7-8, p.115
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Energy-releasing Pathways

• Anaerobic respiration uses glycolysis, but due to
  the lack of oxygen Krebs and electron transfer
  are not used
• Instead of oxygen being the terminal electron
  acceptor, an organic substance is used for NADH
  to donate the electrons (NADH must be recycled to
  NAD to be used in glycolysis again)
• Alcoholic fermentation (fig 7.10)
• Pyruvate is converted to acetaldehyde which
  accepts electrons from NADH producing ethanol and
  CO2
• Used to produce yeast breads and alcoholic
  beverages
• Lactate fermentation (figure 7.11)
• Pyruvate accepts electrons from NADH producing
  lactate
• Muscle cells use this pathway when they are not
  receiving enough O2
• Anaerobic pathways are referred to as
  fermentation pathways

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Fig. 7-9b, p.116
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Fig. 7-10a, p.117
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Fig. 7-10b, p.117
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Fig. 7-10c, p.117
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Fig. 7-9c, p.116
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Fig. 7-11, p.117
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Alternative Energy Sources

• Glucose is a type of carbohydrate
• Carbohydrates are an important part of the diet
  to provide energy
• Proteins and lipids (fats) can also be used for
  energy
• The molecules are broken down to form PGAL or
  Krebs cycle intermediates
• They can then be used to produce ATP
• See figure 7.12

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Fig. 7-12, p.119
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Questions

• Anaerobic respiration can also be called _____.
• During anaerobic respiration what type of
  molecule becomes the electron accepter for NADH?
• What pathway is used by yeast in bread making?
• What pathway can be used by some muscle fibers?
• What other molecules can be used for energy?
• What are alternate energy sources generally
  broken down to?

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Summary

• Aerobic respiration
• Glycolysis
• Krebs
• Electron transfer chain
• Anaerobic respiration/fermentation
• Ethanol
• Lactic acid
• Alternate energy sources