Harvesting Energy CELLULAR RESPIRATION & FERMENTATION.
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Transcript of Harvesting Energy CELLULAR RESPIRATION & FERMENTATION.
Harvesting Energy
CELLULAR RESPIRATION & FERMENTATION
Photosynthesis and respiration provide the energy needed for life
This energy ultimately comes from the sun
RESPIRATION
Harvesting of energy from food molecules
Performed at the cellular level
This energy can then be stored for later use
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
Summary Equation for Aerobic Respiration
C6H12O6 + 6O2 6CO2 + 6H2O glucose oxygen carbon water
dioxide
What’s happening?
C6H12O6 + 6O2 6CO2 + 6H2O glucose oxygen carbon water
dioxide
Glucose is losing electrons - oxidation
Oxygen is gaining electrons - reduction
Energy released
This doesn’t happen at once
Much energy lost as heat
Energy conserved if smaller reactions take place
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
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)
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
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
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
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
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
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
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
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
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
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
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”
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”
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
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
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
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