How Cells Release Chemical Energy

1
How Cells Release Chemical Energy

• Chapter 7

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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

3
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

4
Making ATP

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

5
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 water
• dioxide

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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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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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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
• Energy-releasing steps
• The products of the first part are split into
  three-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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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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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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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
• Coenzyme A

Acetyl group
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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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Coenzyme Reductions during First Two Stages

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

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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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Second Stage of Aerobic Respiration
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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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Efficiency of Aerobic Respiration

• 686 kcal of energy are released
• 7.5 kcal are conserved in each ATP
• When 36 ATP form, 270 kcal (36 X 7.5) are
  captured in ATP
• Efficiency is 270 / 686 X 100 39 percent
• Most energy is lost as heat

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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

• 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
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Carbohydrate Breakdown and Storage

• Glucose is absorbed into blood
• Pancreas releases insulin
• Insulin stimulates glucose uptake by cells
• Cells convert glucose to glucose-6-phosphate
• This traps glucose in cytoplasm where it can be
  used for glycolysis

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Making Glycogen

• If glucose intake is high, ATP-making machinery
  goes into high gear
• When ATP levels rise high enough,
  glucose-6-phosphate is diverted into glycogen
  synthesis (mainly in liver and muscle)
• Glycogen is the main storage polysaccharide in
  animals

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Using Glycogen

• When blood levels of glucose decline, pancreas
  releases glucagon
• Glucagon stimulates liver cells to convert
  glycogen back to glucose and to release it to the
  blood
• (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
• 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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Evolution of Metabolic Pathways

• When life originated, atmosphere had little
  oxygen
• Earliest organisms used anaerobic pathways
• Later, noncyclic pathway of photosynthesis
  increased atmospheric oxygen
• 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