harvesting energy cellular respiration & fermentation
TRANSCRIPT
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Harvesting Energy
CELLULAR RESPIRATION & FERMENTATION
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Photosynthesis and respiration provide the energy needed for life
This energy ultimately comes from the sun
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RESPIRATION
Harvesting of energy from food molecules
Performed at the cellular level
This energy can then be stored for later use
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RESPIRATIONRespiration is a catabolic process: large molecules are broken down and the energy released from bonds is used for:
maintenancegrowth (anabolic process)reproduction
The energy released is transformed into ATP
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Summary Equation for Aerobic Respiration
C6H12O6 + 6O2 6CO2 + 6H2O glucose oxygen carbon water
dioxide
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What’s happening?
C6H12O6 + 6O2 6CO2 + 6H2O glucose oxygen carbon water
dioxide
Glucose is losing electrons - oxidation
Oxygen is gaining electrons - reduction
Energy released
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This doesn’t happen at once
Much energy lost as heat
Energy conserved if smaller reactions take place
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STAGES OF RESPIRATION• Aerobic cellular respiration can be divided into three (or
four) main stages:#1 Glycolysis - cytoplasm
#1.5 Transition step
cytoplasm mitochondria
#2 Krebs Cycle - inner
compartment (matrix)
#3 Electron Transport
Pathway - Inner
membrane
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GLYCOLYSIS• Occurs within eukaryotic cytoplasm• Multi-step metabolic pathway• Partial oxidation of glucose (6
carbon)• No oxygen required• Products:
– 2 ATP (net)– 2 NADH– 2 pyruvate
(3 carbon)
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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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TRANSITION STEP• The pyruvate produced in glycolysis (etc.)
– Enters the mitochondria– Is converted into acetyl CoA (2 carbon)– Enters the Krebs Cycle
• Products:– 2 NADH
– 2 CO2 formed
– 2 acetly CoA
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KREBS CYCLE• a.k.a., Citric Acid 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 and 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
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Generation of ATPChemiosmosis
Electrons attract H+ and pull them through transport proteins to outer-compartment of mitochondria
H+ then diffuse back through ATP synthase channels making ATP andwater
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ENERGY YIELD• 4 ATP (glucose, krebs)• 10 NADH 30 ATP
• 2 FADH2 4 ATP (electron transport)
• 38 ATP total
• This total yield depends on different things
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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 FADH2
– 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– 34 ATP from 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– 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 if energy is not extracted– 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