How Cells Release Chemical Energy

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How Cells Release Chemical Energy

• Chapter 7

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Impacts, IssuesWhen Mitochondria Spin Their
Wheels

• Mitochondria are the organelles responsible for
  releasing the energy stored in foods
• In Lufts syndrome, the mitochondria are active
  in oxygen consumption, but with little ATP
  formation to show for it
• In Friedreichs ataxia, too much iron in the
  mitochondria causes an accumulation of free
  radicals that attack valuable molecules of life

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The Impact

• Proper, or improper, functioning of mitochondria
  is the difference between health and disease

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Section 7.1

• Overview of
• Energy-Releasing Pathways

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Producing the Universal Currency of Life

• All energy-releasing pathways
• require characteristic starting materials
• yield predictable products and by-products
• produce ATP

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ATP Is Universal Energy Source

• Photosynthesizers get energy from the sun
• Animals get energy second- or third-hand from
  plants or other organisms
• Regardless, the energy is converted to the
  chemical bond energy of ATP

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Making ATP

• Plants make ATP during photosynthesis
• Cells of all organisms make ATP by breaking down
  carbohydrates, fats, and protein

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Main Types of Energy-Releasing Pathways

• Aerobic pathways
• Evolved later
• Require oxygen
• Start with glycolysis in cytoplasm
• Completed in mitochondria

• Anaerobic pathways
• Evolved first
• Dont require oxygen
• Start with glycolysis in cytoplasm
• Completed in cytoplasm

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Energy-Releasing Pathways
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Overview of Aerobic Respiration

• C6H1206 6O2 6CO2 6H20
• glucose oxygen carbon dioxide
  water

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Overview of Aerobic Respiration
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Main Pathways Start with Glycolysis

• Glycolysis occurs in cytoplasm
• Reactions are catalyzed by enzymes
• Glucose 2 Pyruvate
• (six carbons) (three carbons)

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p.106a
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Three Series of Reactions Are Required for
Aerobic Respiration

• Glycolysis is the breakdown of glucose to
  pyruvate
• Small amounts of ATP are generated

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Three Series of Reactions Are Required for
Aerobic Respiration

• The Krebs cycle degrades pyruvate to CO2 and
  water
• NAD and FAD accept H ions and electrons to be
  carried to the electron transfer chain
• Small amounts of ATP are generated

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Three Series of Reactions Are Required for
Aerobic Respiration

• Electron transfer phosphorylation processes the
  H ions and electrons to generate lots of ATP
• Oxygen is the final electron acceptor

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The Role of Coenzymes

• NAD and FAD accept electrons and hydrogen from
  intermediates during the first two stages
• When reduced, they are NADH and FADH2
• In the third stage, these coenzymes deliver the
  electrons and hydrogen to the transfer chain

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Overview of Aerobic Respiration
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Section 7.2

• The First Stage Glycolysis

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Glucose

• A simple sugar
• (C6H12O6)
• Atoms held together by covalent bonds

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Glycolysis Occurs in Two Stages

• Energy-requiring steps
• ATP energy activates glucose and its six-carbon
  derivatives

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Glycolysis Occurs in Two Stages

• Energy-releasing steps
• The products of the first part are split into
  3-carbon pyruvate molecules
• ATP and NADH form

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Energy-Requiring Steps
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Energy-Releasing Steps
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Glycolysis in a Nutshell

• Glucose is first phosphorylated in
  energy-requiring steps, then split to form two
  molecules of PGAL
• Enzymes remove H and electrons from PGAL to
  change NAD to NADH (which is used later in
  electron transfer
• PGAL is converted eventually to pyruvate
• By substrate-level phosphorylation, four ATP are
  produced

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Substrate-level What?

• Substrate-level phosphorylation means that there
  is a direct transfer of a phosphate group from
  the substrate of a reaction to some other
  molecule in this case, ADP
• Substrate is a reactant in a reaction the
  substance being acted upon, for example, by an
  enzyme

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Net Energy Yield from Glycolysis

• Energy requiring steps
• 2 ATP invested
• Energy releasing steps
• 2 NADH formed
• 4 ATP formed
• Net yield is 2 ATP and 2 NADH

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Section 7.3

• Second Stage of Aerobic Respiration

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Second-Stage Reactions

• Occur in the mitochondria
• Pyruvate is broken down to carbon dioxide
• More ATP is formed
• More coenzymes are reduced

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Fig. 7-5b, p.112
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• Second Stage of Aerobic Respiration

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Two Parts of Second Stage

• Preparatory reactions
• Pyruvate is oxidized into two-carbon acetyl units
  and carbon dioxide
• NAD is reduced
• Krebs cycle
• The acetyl units are oxidized to carbon dioxide
• NAD and FAD are reduced

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Preparatory Reactions

• pyruvate coenzyme A NAD
• acetyl-CoA NADH CO2
• One of the carbons from pyruvate is released in
  CO2
• Two carbons are attached to coenzyme A and
  continue on to the Krebs cycle

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What Is Acetyl-CoA?

• A two-carbon acetyl group linked to coenzyme
  A CH3
• CO
• S
• Coenzyme A

Acetyl group
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• Second Stage of Aerobic Respiration

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CoA
KREBS CYCLE
acetyl-CoA
CoA
oxaloacetate
citrate
isocitrate
a-ketoglutarate
Stepped Art
Fig. 7-7a, p.113
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The Krebs Cycle

• Overall Products
• Coenzyme A
• 2 CO2
• 3 NADH
• FADH2
• ATP

• Overall Reactants
• Acetyl-CoA
• 3 NAD
• FAD
• ADP and Pi

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Results of the Second Stage

• All of the carbon molecules in pyruvate end up in
  carbon dioxide
• Coenzymes are reduced (they pick up electrons and
  hydrogen)
• One molecule of ATP is formed
• Four-carbon oxaloacetate is regenerated

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Two pyruvates cross the inner mitochondrial
membrane.
outer mitochondrial compartment
inner mitochondrial compartment
NADH
2
NADH
6
Krebs Cycle
Eight NADH, two FADH 2, and two ATP are the
payoff from the complete break-down of two
pyruvates in the second-stage reactions.
FADH2
2
ATP
2
The six carbon atoms from two pyruvates diffuse
out of the mitochondrion, then out of the cell,
in six CO
6 CO2
Fig. 7-6, p.112
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Coenzyme Reductions during First Two Stages

• Glycolysis 2 NADH
• Preparatory 2 NADH
• reactions
• Krebs cycle 2 FADH2 6 NADH
• Total 2 FADH2 10 NADH

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Section 7.4

• Third Stage of Aerobic Respiration The Big
  Energy Payoff

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Electron Transfer Phosphorylation

• Occurs in the mitochondria
• Coenzymes deliver electrons to electron transfer
  chains
• Electron transfer sets up H ion gradients
• Flow of H down gradients powers ATP formation

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Electron Transfer Phosphorylation

• Electron transfer chains are embedded in inner
  mitochondrial compartment

• NADH and FADH2 give up electrons that they picked
  up in earlier stages to electron transfer chain
• Electrons are transferred through the chain
• The final electron acceptor is oxygen

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Creating an H Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT
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ATP Formation
ATP
INNER COMPARTMENT
ADPPi
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Summary of Transfers
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Importance of Oxygen

• Electron transfer phosphorylation requires the
  presence of oxygen
• Oxygen withdraws spent electrons from the
  electron transfer chain, then combines with H to
  form water

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Summary of Energy Harvest(per molecule of
glucose)

• Glycolysis
• 2 ATP formed by substrate-level phosphorylation
• Krebs cycle and preparatory reactions
• 2 ATP formed by substrate-level phosphorylation
• Electron transfer phosphorylation
• 32 ATP formed

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Energy Harvest from Coenzyme Reductions

• What are the sources of electrons used to
  generate the 32 ATP in the final stage?
• 4 ATP - generated using electrons released during
  glycolysis and carried by NADH
• 28 ATP - generated using electrons formed during
  second-stage reactions and carried by NADH and
  FADH2

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Energy Harvest Varies

• NADH formed in cytoplasm cannot enter
  mitochondrion
• It delivers electrons to mitochondrial membrane
• Membrane proteins shuttle electrons to NAD or
  FAD inside mitochondrion
• Electrons given to FAD yield less ATP than those
  given to NAD

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Energy Harvest Varies

• Liver, kidney, heart cells
• Electrons from first-stage reactions are
  delivered to NAD in mitochondria
• Total energy harvest is 38 ATP
• Skeletal muscle and brain cells
• Electrons from first-stage reactions are
  delivered to FAD in mitochondria
• Total energy harvest is 36 ATP

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Section 7.5

• Fermentation Pathways

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Anaerobic Pathways

• Do not use oxygen
• Produce less ATP than aerobic pathways
• Two types of fermentation pathways
• Alcoholic fermentation
• Lactate fermentation

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Fermentation Pathways

• Begin with glycolysis
• Do not break glucose down completely to carbon
  dioxide and water
• Yield only the 2 ATP from glycolysis
• Steps that follow glycolysis serve only to
  regenerate NAD

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Alcoholic Fermentation
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Yeasts

• Single-celled fungi
• Carry out alcoholic fermentation
• Saccharomyces cerevisiae
• Bakers yeast
• Carbon dioxide makes bread dough rise
• Saccharomyces ellipsoideus
• Used to make beer and wine

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Lactate Fermentation
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Lactate Fermentation

• Carried out by certain bacteria
• Electron transfer chain is in bacterial plasma
  membrane
• Final electron acceptor is compound from
  environment (such as nitrate), not oxygen
• ATP yield is low

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Lactate Fermentation

• Lactobacillus and some other bacteria produce
  lactate
• This produces cheeses, yogurt, buttermilk and
  other dairy products
• Fermenters also are used to cure meats and in
  pickling
• Sauerkraut is an example
• Sour taste due to lactic acid (form of lactate)

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Slow-twitch v. Fast-twitch muscles

• Slow-twitch muscles make ATP only by aerobic
  respiration (no fermentation)
• Slow-twitch muscles are for light, steady,
  prolonged activity
• Slow-twitch muscles are red because they have
  lots of myoglobin, a pigment used to store oxygen
• They also have many mitochondria

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Fast-twitch Muscles

• These pale (lighter colored) muscles have few
  mitochondria and no myoglobin
• Fast-twitch muscles, which are used for immediate
  and intense energy demands, use lactate
  fermentation to produce ATP
• It works quickly, but not for long
• Chickens have fast-twitch breast muscles used for
  quick flights (white meat)
• Ducks fly long distances what color is their
  breast meat?

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Alcoholic Fermentation
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Lactate Fermentation
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Section 7.6

• Alternative Energy Sources
• in the Body

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The Fate of Glucose

• After eating, glucose is absorbed into the blood
• Insulin levels rise, causing greater uptake of
  glucose by cells
• Glycolysis will follow
• Excess glucose is converted into glycogen
• Glycogen is known as animal starch, and is the
  main storage polysaccharide in animals
• Stored in the muscles and the liver

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Between Meals

• When blood levels of glucose decline, pancreas
  releases glucagon, a hormone
• Glucagon stimulates liver cells to convert
  glycogen back to glucose and to release it to the
  blood
• Glycogen levels are adequate, but can be depleted
  in 12 hours
• (Muscle cells do not release their stored
  glycogen)

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Energy Reserves

• Glycogen makes up only about 1 percent of the
  bodys energy reserves
• Proteins make up 21 percent of energy reserves
• Fat makes up the bulk of reserves (78 percent)

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Energy from Fats

• Most stored fats are triglycerides in adipose
  tissue
• Triglycerides are three-tailed fats
• Triglycerides are broken down to glycerol and
  fatty acids
• Glycerol is converted to PGAL, an intermediate of
  glycolysis
• Fatty acids are broken down and converted to
  acetyl-CoA, which enters Krebs cycle

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Energy from Proteins

• Proteins are broken down to amino acids
• Amino acids are broken apart
• Amino group is removed, ammonia forms, is
  converted to urea and excreted
• Carbon backbones can enter the Krebs cycle or its
  preparatory reactions

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Reaction Sites
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Section 7.7

• Perspective on Life

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Evolution of Metabolic Pathways

• When life originated, atmosphere had little
  oxygen
• Earliest organisms used anaerobic pathways
• Later, cyclic pathway (simple form) of
  photosynthesis increased atmospheric oxygen
• Much more efficient cells arose that used oxygen
  as final acceptor in electron transfer

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Processes Are Linked

• Aerobic Respiration
• Reactants
• Sugar
• Oxygen
• Products
• Carbon dioxide
• Water

• Photosynthesis
• Reactants
• Carbon dioxide
• Water
• Products
• Sugar
• Oxygen

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Life Is System of Prolonging Order

• Powered by energy inputs from sun, life continues
  onward through reproduction
• Following instructions in DNA, energy and
  materials can be organized, generation after
  generation
• With death, molecules are released and may be
  cycled as raw material for next generation