METABOLISM

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METABOLISM

• FUELING CELL GROWTH

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METABOLISM

• Metabolism encompasses two general processes
• Catabolism
• Processes harvesting energy released during the
  breakdown of various compounds
• Harvested energy generally used to synthesize
  ATP
• Anabolism
• a.k.a., Biosynthesis
• Processes using stored energy to synthesize and
  assemble subunits of macromolecules
• Catabolism is coupled to anabolism

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METABOLIC PATHWAYS

• Ordered sequence of steps resulting in an
  end-product
• Reactants ? intermediate 1 ? ? end product
• Pathway may be linear, branched, or cyclic
• Each reaction typically catalyzed by a specific
  enzyme

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NEED FOR ENERGY

• The environment within a cell is highly
  organized and separate from the external
  environment
• Maintaining this ordered environment costs
  energy
• Many processes within a cell require energy
• The requirement for energy is a unifying feature
  of life
• Many organisms extract energy from food via
  aerobic cellular respiration

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HARVESTING ENERGY

• Your car harvests energy from food (gasoline)
• Your furnace harvests energy from food (methane)
• Your cells harvest energy from food
• In each of these cases, O2 is required
• Food O2 ? energy CO2 H2O

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HARVESTING ENERGY

• Methane and glucose possess a large amount of
  energy in their chemical bonds
• Reduced forms of carbon
• CO2 possesses little useful energy in its
  chemical bonds
• Oxidized form of carbon
• Energy is released when reduced molecules are
  burned (oxidized)
• Converted from a reduced form to a more
  oxidized form

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OXIDATION REDUCTION

• Oxidation is required to harvest large amounts of
  energy from glucose (or methane, gasoline, etc.)
• Oxidation involves the loss of electrons
• Whenever one molecule is oxidized, another is
  reduced
• e.g., O2 is reduced to H2O (½O2
  2H 2e- ? H2O)
• e.g., NAD is reduced to NADH (NAD H
  2e- ? NADH)

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HARVESTING ENERGY

• Your car, your furnace, and your cells burn
  (oxidize) food to release energy
• Much of this energy is converted into a useful
  form
• Your car transforms much of this energy into
  movement
• Your furnace transforms much of this energy into
  heat
• Your cells transform much of this energy into ATP
• Food O2 ? energy CO2 H2O

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ATP

• The oxidation of food molecules converts
  harvested energy to ATP (adenosine triphosphate)
• High energy molecule
• Energy currency of cell
• Spent (hydrolyzed into ADP) to fuel many
  processes

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ATP CYCLE

• ATP and ADP are readily interconverted
• ATP ? ADP releases energy
• ADP ? ATP requires energy

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ATP CYCLE

• Energy-releasing reactions store energy as ATP
• ADP ? ATP
• Energy-requiring reactions receive energy from
  ATP
• ATP ? ADP

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NADH

• The reduced coenzyme NADH is also produced during
  cellular respiration
• Nicotinamide adenine dinucleotide
• High energy molecule
• Can be spent to make more ATP later

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NADH

• NAD and NADH are readily interconverted
• NAD H 2e- ? NADH
• As glucose is oxidized, NAD is reduced to NADH

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STAGES OF RESPIRATION

• Aerobic cellular respiration can be divided into
  three main stages
• Glycolysis
• Krebs Cycle
• Electron Transport Pathway

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GLYCOLYSIS

• Occurs within eukaryotic cytoplasm
• Multi-step metabolic pathway
• Partial oxidation of glucose
• Products
• 2 ATP
• 2 NADH
• 2 pyruvate

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ALTERNATE PATHWAYS

• Other ways to convert glucose to pyruvate
• Entner-Doudoroff Pathway
• Utilized by some bacteria archaea
• Instead of or in addition to glycolysis
• Uses different enzymes
• Generates reducing power as NADPH
• Yields slightly less ATP
• Pentose Phosphate Pathway
• Operates in conjunction with other
  glucose-degrading pathways
• Primary role is production of compounds used in
  biosynthesis
• Generates reducing power as NADPH
• Can produce a single pyruvate molecule

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ALTERNATE PATHWAYS

• The pentose phosphate pathway removes carbons
  from glucose one at a time
• C6 ? C5 ? C4 ? C3 (pyruvate)
• The pentose sugar (C5) is of particular
  importance as a precursor metabolite
• What critically important biomolecule do you
  think is generally made from this pentose sugar?

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TRANSITION STEP

• The pyruvate produced in glycolysis (etc.)
• Enters the mitochondria
• Is converted into acetyl CoA
• Enters the Krebs Cycle

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KREBS CYCLE

• Occurs within mitochondrial matrix
• Multi-step metabolic pathway
• Remnants of glucose completely oxidized
• Products
• 2 ATP
• 6 NADH
• 2 FADH2
• 4 CO2

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GLYCOLYSIS ? KREBS

• Several high-energy molecules are produced during
  glycolysis and the Krebs cycle
• 4 ATP
• 10 NADH
• 2 FADH2
• Most of the energy harvested from glucose is in
  the form of reduced coenzymes
• However, only ATP is readily usable to perform
  cellular work
• The Electron Transport Pathway oxidizes NADH and
  FADH2 to produce more ATP

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ELECTRON TRANSPORT PATHWAY

• Occurs within the inner mitochondrial membrane
• Electrons are removed from NADH and shuttled
  through a series of electron acceptors
• Energy is removed from the electrons with each
  transfer
• This energy is used to make ATP
• NADH ? 3 ATP
• FADH2 ? 2 ATP
• O2 is the terminal electron acceptor
• ½O2 2H 2e- ? H2O
• Anaerobic respiration utilizes a molecule other
  than O2 as the terminal electron acceptor
• e.g., NO3-, SO42-, CO2, etc.

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CHEMIOSMOSIS

• As electrons are shuttled from one acceptor to
  the next, protons are transported across the
  membrane
• Electrochemical (proton) gradient produced
• Protons re-enter the matrix via a proton channel
  associated with an ATP synthase complex
• As protons move down their concentration
  gradient, ATP is produced

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PROKARYOTES

• Do prokaryotes have energy needs?
• Do some of them perform aerobic respiration, even
  though they have no mitochondria?
• How?

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ENERGY YIELD

• 4 ATP
• 10 NADH ? 30 ATP
• 2 FADH2 ? 4 ATP
• 38 ATP total

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THEORETICAL YIELD

• Theoretical yield of 38 ATP not generally reached
  because
• Intermediates in central pathways siphoned off
  as precursor metabolites for biosynthesis
• Electrons of NADH generated in cytosol often
  shuttled into mitochondria as FADH
• Each NADH typically yields slightly less than 3
  ATP

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BURNING OTHER STUFF

• Glucose can be oxidized to yield ATP
• Other biomolecules can also be oxidized to yield
  ATP
• These molecules are converted to either glucose
  or to an intermediate in the catabolism of glucose

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O2 REQUIREMENT

• 38 ATP produced per glucose molecule
• 4 ATP from substrate level phosphorylation
• 34 ATP from oxidative phosphorylation
• Produced via ETP
• Requires adequate supply of oxygen
• Under conditions of insufficient oxygen, ATP
  yields can be severely reduced

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What happens when O2 is unavailable?

• Some cells cannot obtain energy when deprived of
  O2
• e.g., human heart cells
• Obligate aerobes
• Some cells normally perform aerobic respiration,
  but can still obtain energy when O2 is lacking
• e.g., skeletal muscle cells, S. cerevisiae
  (yeast), E. coli
• Facultative anaerobes
• Others do not use O2 to obtain energy
• e.g., Clostridium botulinum, an obligate
  anaerobe
• e.g., Streptococcus pyogenes, an aerotolerant
  anaerobe

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FACULTATIVE ANAEROBES

• In the absence of O2, aerobic respiration is
  impossible
• O2 is the terminal electron acceptor of the ETP
• Without O2, the ETP does not function
• The Krebs Cycle produces mainly NADH
• Without O2, NADH is useless
• Without O2, the Krebs Cycle does not function
• Some anaerobes normally undergo anaerobic
  respiration, where a molecule other than O2
  serves as the terminal electron acceptor. For
  these organisms, the Krebs Cycle and ETP continue
  to function in the absence of O2.

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FACULTATIVE ANAEROBES

• In the absence of O2, aerobic respiration is
  impossible
• Glycolysis still occurs
• Net ATP production 2 ATP
• 2 is significantly less than thirty-something
• NAD is converted to NADH
• NADH is not useful to the cell
• The absence of NAD is detrimental to the cell
• NADH must be converted back to NAD
• Fermentation

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FERMENTATION

• NADH is produced during glycolysis
• Energy in NADH cannot be used
• NADH must be oxidized to replenish NAD
• No payoff
• NADH is oxidized to NAD
• Pyruvate is reduced to _______
• (Different substances in different organisms)
• Human muscle pyruvate ? lactic acid
• Yeast pyruvate ? ethanol CO2
• Other cells ? many other molecules
• Total energy yield of fermentation is the 2 ATP
  generated in glycolysis

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FERMENTATION

• Skeletal muscles normally undergo aerobic
  respiration
• During strenuous exercise, O2 may be rapidly
  depleted
• Fermentation can continue to provide energy
• Pyruvate ? lactic acid
• Lactic acid builds up
• Buildup causes muscle fatigue pain
• Lactic acid ultimately removed

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FERMENTATION

• Saccharomyces cerevisiae (yeast) normally
  undergoes aerobic respiration
• O2 is not always available
• Fermentation can continue to provide energy
• Pyruvate ? ethanol CO2
• Ethanol ultimately toxic

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FERMENTATION

• Many other organisms also undergo fermentation
• Some are facultative anaerobes
• Some are obligate fermenters
• Pyruvate is converted into a host of different
  molecules by a host of different organisms
• Many of these molecules are commercially
  important

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HOW MUCH ENERGY?

• Aerobic and anaerobic respiration are both
  reasonably efficient
• Fermentation is far less efficient

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PHOTOSYNTHESIS

• Cellular respiration involves the oxidation of
  glucose and other high-energy (reduced)
  molecules
• These molecules are ultimately produced by
  photosynthesis
• Photosynthesis nourishes virtually the entire
  world, either directly or indirectly

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PHOTOSYNTHESIS

• Who does photosynthesis?
• Though plants can photosynthesize, microorganisms
  are responsible for the majority of the
  photosynthesis occurring on the planet

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CHLOROPLASTS

• Chloroplasts are the site of photosynthesis
• Remember, photosynthetic bacteria dont have
  chloroplasts, they essentially are chloroplasts
• Chloroplasts contain the pigment chlorophyll
• Pigments absorb light
• Multiple similar forms of chlorophyll exist
• The light absorbed is ultimately used to reduce
  CO2 to glucose

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PHOTOSYNTHESIS

• Plants can perform photosynthesis
• Can plants perform aerobic cellular respiration?

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PHOTOSYNTHESIS

• Photosynthesis consists of two processes
• Light reactions
• a.k.a., Light-dependent reactions
• Dark reactions
• a.k.a., Light-independent reactions
• a.k.a., Calvin cycle
• Both the light reactions and the dark reactions
  are dependent on light, either directly or
  indirectly
• Neither the light reactions nor the dark
  reactions can occur in the absence of light

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PHOTOSYNTHESIS

• Photosynthesis consists of two processes
• Light reactions
• a.k.a., Light-dependent reactions
• Dark reactions
• a.k.a., Light-independent reactions
• a.k.a., Calvin cycle
• Both the light reactions and the dark reactions
  are dependent on light, either directly or
  indirectly
• Neither the light reactions nor the dark
  reactions can occur in the absence of light

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LIGHT REACTIONS

• Sunlight is absorbed by chlorophyll pigments
• Light energy is used to produce high-energy
  molecules
• ATP
• NADPH
• O2 is produced as a waste product
• H2O ? ½ O2 2H 2e-

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CALVIN CYCLE

• The high energy molecules produced during the
  light reactions are used to reduce CO2 to glucose
• This glucose can be converted to other compounds
  (amino acids, etc.)
• This glucose can be oxidized to yield energy

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ANOXYGENIC PHOTOSYNTHESIS

• The general formula for photosynthesis is
• 6CO2 12H2X energy ? glucose 12X 6H2O
• The X is generally oxygen (O)
• Electrons are removed from H2O to form O2
• Purple sulfur bacteria cannot remove electrons
  from H2O
• H2 and H2S are used instead of H2O
• What is the overall formula in each of these
  cases?