How Cells Harvest Chemical Energy

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Chapter 6

• How Cells Harvest Chemical Energy

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Cellular Respiration

• In cellular respiration, an organism obtains O2
  from the environment and releases CO2 as a waste
  product
• Mitochondria in cells use the CO2 in cellular
  respiration to break down the sugar and other
  organic molecules
• The energy retrieved from the sugar is used to
  make ATP

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Cellular Respiration

• Harvesting energy is the fundamental function of
  cellular respiration
• Summary equation for respiration is

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Cellular Respiration

• Glucose and other organic molecules are burned
• Respiration does not harvest all of the energy in
  usable form
• 40, is usable as ATP, most of the rest is lost
  as heat

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How cells get energy from organic fuels

• As glucose is dismantled, cellular respiration
  gathers the electron energy released when bonds
  break and are reformed
• A series of reactions reduce the glucose to six
  molecules of CO2 water and energy is released
  and stored as ATP

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How cells get energy from organic fuels

• Glucose looses a hydrogen as it changes into CO2
  and O2 gains hydrogens as it is converted to
  water
• Couples an
• Energy releasing (exergonic) reaction- breaking
  the bonds of the glucose molecule
• Energy storing (endergonic) reaction- storing the
  energy as ATP

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Redox Reactions

• The movement of electrons from one molecule to
  another is an oxidation-reduction (redox)
  reaction
• The loss of electrons from one substance is
  called oxidation
• The addition of electrons to another substance is
  called reduction
• Oxidation and reduction are always coupled
  together
• The electron only leaves when it contacts another
  molecule to which the electron is more strongly
  attracted

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Redox Reactions

• Glucose loses electrons (in H atoms), while O2
  gains electrons (in H atoms)

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1. Shuttling of electrons to NAD

• NAD with the help of a dehydrogenase enzyme
  remove the electrons from the hydrogen and
  shuttle electrons by reducing the NAD to NADH

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2. The Electron Transport Chain

• NADH gives up electron and becomes NAD
• The electron is shuttled to several different
  electron carriers (redox)
• Each electron in the chain has a greater affinity
  for the electron than the one before it, so the
  electron goes downhill
• Each time a redox reaction happens a little
  energy is released that the cell uses to make ATP
• Eventually the electrons fall to O2 and the waste
  is water (H2O)

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2. The Electron Transport Chain
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How ATP is Generated

• Phosphorylation
• Adding a phosphate group to ADP
• ADP synthetases are embedded in the membrane make
  ADP
• ADP synthetase uses the H gradient to drive the
  reaction

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How ATP is Generated

• ATP can also be made by transferring phosphate
  groups from organic molecules to ADP
• This process is called substrate-level
  phosphorylation
• Happens because the ATP bond is more stable

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Overview of Respiration

• Cellular respiration oxidizes sugar and produces
  ATP in three main stages
• Glycolysis occurs in the cytoplasm
• The Krebs cycle and the electron transport chain
  occur in the mitochondria
• Glycolysis and Krebs Cycle are exergonic
  (energy-releasing)
• Electron Transport Chain is endergonic (energy
  using

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Overview of Respiration
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Glycolysis

• Splitting of sugar, starts with 1 molecule of
  glucose (6C) and ends up with 2 molecules of
  pyruvic acid (3C)
• There are 9 intermediate steps between glucose
  and pyruvic acid
• Produces 2 molecules of ATP and 2 molecules of
  NADH
• Can use ATP immediately, but to use NADH it must
  pass down the electron transport chain

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Glycolysis

• Overall reaction of glycolysis

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Glycolysis
PREPARATORYPHASE(energy investment)
ENERGY PAYOFF PHASE
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Glycolysis

• Net energy gain of glycolysis is
• 2 ATP molecules
• 2 NADH molecules
• Energy gain in glycolysis accounts for only 16
  of the amount of energy a cell can get from
  glucose

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Pyruvic Acid

• Pyruvic acid is transformed into Acetyl-CoA in
  preparation for entering the Krebs cycle

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Krebs Cycle

• The Krebs cycle is a series of reactions in which
  enzymes strip away electrons and H from each
  acetyl group

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Krebs Cycle
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Krebs Cycle

• Each turn of the cycle produces
• 1 ATP
• 3 NADH molecules
• 1 FADH molecule (another hydrogen carrier)
• Every glucose molecule produces 2 pyruvic acid
  molecules

X2
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The Electron Transport Chain and Chemiosmosis

• The electrons from NADH and FADH2 travel down the
  electron transport chain to oxygen
• Energy released by the electrons is used to pump
  H into the space between the mitochondrial
  membranes
• In chemiosmosis, the H ions diffuse back through
  the inner membrane through ATP synthase
  complexes, which capture the energy to make ATP

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Chemiosmosis in the Mitochondrion
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Some Poisons Interrupt Cellular Respiration
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Some Poisons Interrupt Cellular Respiration

• They essentially starve the cell for energy in
  three ways
• Block electron transport chain
• Inhibits ATP synthetase from making ATP
• Makes the membrane wall leaky to H ions

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Each molecule of glucose yields many molecules of
ATP

• For each glucose molecule that enters cellular
  respiration, chemiosmosis produces up to 38 ATP
  molecules

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Anaerobic Respiration

• Cellular respiration without oxygen
• Survive on the 2 molecules of ATP produced by
  glycolysis
• But they need a way of replenishing NAD
• They do this in two ways
• Alcoholic Fermentation
• Lactic Acid Fermentation

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Alcoholic Fermentation

• In alcoholic fermentation, pyruvic acid is
  converted to CO2 and ethanol
• This recycles NAD to keep glycolysis working

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Lactic Acid Fermentation

• In lactic acid fermentation, pyruvic acid is
  converted to lactic acid
• As in alcoholic fermentation, NAD is recycled
• Lactic acid fermentation is used to make cheese
  and yogurt

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Cell use many types of molecules as fuel

• Polysaccharides can be hydrolyzed to
  monosaccharides and then converted to glucose for
  glycolysis
• Proteins can be digested to amino acids, which
  are chemically altered and then used in the Krebs
  cycle
• Fats are broken up and fed into glycolysis and
  the Krebs cycle

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Pathways of Molecular Breakdown
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Food Provides Raw Materials for Biosynthesis

• In addition to energy, cells need raw materials
  for growth and repair
• Some are obtained directly from food
• Others are made from intermediates in glycolysis
  and the Krebs cycle
• Molecules that are not present in food
• Biosynthesis consumes ATP

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Biosynthesis of Macromolecules