Download RESPIRATION & PHOTOSYNTHESIS

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
RESPIRATION
Respiration is a catabolic process: large
molecules are broken down and the energy
released from bonds is used for:
maintenance
growth (anabolic process)
reproduction
The energy
released is
transformed
into ATP
Summary Equation for Aerobic
Respiration
C6H12O6 + 6O2
glucose
oxygen
6CO2 + 6H2O
carbon
dioxide
water
What’s happening?
Glucose is losing electrons - oxidation
C6H12O6 + 6O2
glucose
oxygen
Energy released
6CO2 + 6H2O
carbon
dioxide
water
Oxygen is gaining electrons - reduction
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 ATP
Chemiosmosis
Electrons attract
H+ and pull them
through transport
proteins to outercompartment of
mitochondria
H+ then diffuse
back through ATP
synthase channels
making ATP and
water
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