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Respiration
Lecture overview
• What do plants do with the energy they get?
• Mitochondria
• Glycolysis (or oxidative pentose phosphate
shunt)
• Citric acid cycle
• Oxidative phosphorylation
• Fermentation
• Gluconeogenesis
• Respiration and stress
Net primary production (NPP)
•NPP = GPP – Plant Respiration
What controls respiration?
Rplant = Rgrowth + Rmaint +Rion
● Growth – building new biomass
● Maintenance– maintaining tissues
● Ion – transport across membranes is energetically expensive.
Depends on form of nutrients taken up,
(e.g., cost of reducing NO3- to NH4+), resource availability…
- all processes that require energy, ATP
What is maintenance respiration?
• Respiration associated with repair
–Proteins
–Membranes
–Other
• 85% is associated with maintaining proteins,
which turn over at a rate of about 6% per day
• Thus, there is a strong correlation between
protein content and respiration rate in nongrowing tissues
• Maintenance respiration is probably about
50% of total plant respiration
Respiration:
occurs in
mitochondria
From Rost et al., “Plant biology,” 2nd edn.
Plant respiration - chapter 11 of Taize
Simple definition: The oxidation of sugars to produce
usable energy (ATP), reductant (NADH), and carbon
“skeletons” for biosynthesis.
C12H22O11 + 12O2 --> 12 CO2 + 11 H2O
60 ADP + 60 Pi --> 60 ATP + 60 H2O
What are the major steps?
1) glycolysis
2) citric acid cycle
3) electron transport/oxidative phosphorylation
Often, starch is the
most important
substrate for
respiration, and it
moves from the
amyloplasts or
chloroplasts by
various
mechanisms:
- as glucose by a
glucose transporter
-as triose-P by the
triose-P/Pi antiporter
Steps of respiration
•
•
•
•
Glycolysis
Pentose phosphate pathway
Citric acid cycle
Oxidative phosphorylation
Glycolysis
- conversion of carbohydrates into pyruvate
producing some ATP and NADH in cytoplasm and
plastids
- can happen in presence or absence of O2
- if O2, then pyruvate converted to acetyl CoA and
into the citric acid cycle
- if no O2,then “fermentation” (see later) occurs,
pyruvate is reduced to lactic acid and/or ethanol
If aerobic - pyruvate from glycolysis is converted to acetyl
CoA, which enters the citric acid cycle
Citric Acid Cycle
• takes place in mitochondrial matrix,
except succinate dehydrogenase
• most NADH is produced
Electron transfer chain
• Inner mitochondrial membrane
• NADH made into ATP using an
“ATP synthase” – basically a
proton pump in reverse
• Called oxidative
phosphorylation
• Four major complexes involved
Mitochondrial electron transport
Note: not all carbon entering
respiratory pathway ends up
as carbon dioxide – many
important carbon skeletons are
made e.g. for making proteins,
lipids, DNA, cellulose etc.
Relationship of respiratory intermediates to
other plant biosynthetic pathways
Oxidative Pentose Phosphate
Pathway
• Alternative to glycolysis
• Also called the hexose monophosphate shunt
• Converts glucose to Triose-P – this enters the
last stages of glycolysis, carbon then enters
citric acid cycle as normal
• Only 5 – 20% respiration occurs this way
• But – makes useful intermediates needed for
making DNA, RNA and phenolics
• Appears important during plant recovery from
stress
Fermentation
• Without oxygen, citric acid cycle and oxidative
phosphorylation cannot work
• “Fermentation” metabolizes pyruvate to give
some ATP, CO2 and ethanol or lactic acid
• Only 4% as efficient as the oxidative
phosphorylation, and ethanol and lactic acid
may be toxic
• Note the “Pasteur effect” (absent in flooding
tolerant plants)
Oilseeds are able to convert stored oil
to carbohydrate
• Many seeds store a significant portion of
photoassimilate as oil, not carbohydrate
• This oil is mobilized as an energy source
upon germination
– e.g., canola (45% oil by dry weight versus
maize 5%)
• Oil – not water soluble, not transportable
• Most plants convert oil droplets (triglycerides) 
sucrose to mobilize its energy
• Animals cannot interconvert lipids and
carbohydrates!
• Again, this gives plants metabolic flexibility in
allocating carbon between lipids and carbohydrates
– Seeds can be smaller because lipids store more energy
per gram!
Mobilizing the energy in stored oil involves
the glyoxylate cycle and gluconeogenesis
• Triglyceride conversion to sucrose
involves 3 organelles + cytosol
• Fatty acids are removed from triglyceride
by lipase
• FA imported into glyoxysome –
specialized plant organelle
• Cleaved at every 2nd C to generate
acetyl CoA via ß-oxidation
• Glyoxylate cycle take home messages:
– Borrowing oxaloacetate from the
mitrochondrion allows citrate synthesis
from fatty acids
– It’s a cycle! Regeneration of OAA in mt
keeps acetyl CoA incorporation high
– The products of the cycle enter
gluconeogenesis to generate sucrose in
the __________
– Glycerol from triglyceride also enters
gluconeogenesis for sucrose biosynthesis
– NADH enters oxidative phosphorylation
Figure 7.13
a/k/a
_____________
Left - typical late spring waterlogging of poorly
drained field of peas (Pisum sativum) in
Cambridgeshire, UK. Right – close-up of the injury
sustained by leaves of a pea plant after several days
soil waterlogging.
Aerenchyma formation in flooded roots
Flooding
induced
aerenchyma
In a desperate attempt to get ATP, glycolysis is
stimulated (the “Pasteur effect”) resulting in pyruvate
accumulation, and the accumulation of the by-products
of anaerobic metabolism
Trial
illustrating the
superior
tolerance to 10
days complete
submergence
of a line of rice
(FR13A)
derived from
an old Indian
farmer variety
(left line of
green plants)
compared with
two other lines
of lowland rice
(right).
Example of specialized heavy equipment, often employing laserguided gradient sensing, to install subsurface plastic drainage
pipes in arable farmland.
Mitochondria are major sites of ROS
production following stress
Following stress,
components of
the mitochondrial
electron transfer
chain become
damaged (e.g.
ATP synthase).
H2O2 generation
in mitochondria
is proportion to
the pmf
Skulachev, 1998
Respiration in Plants
►
Energy coupled
►
“Uncoupled” – ΔΨ is dissipated
►
“Non-coupled” – electrons do not form
ΔΨ – original electron transfer chain is
modified, or other respiratory enzymes
are involved
Uncoupled or non-coupled respiration can
reduce ROS formation following stress
The alternative oxidase (AOX) is mainly responsible
for non-coupled respiration in plants
Feature of the AOX
►
Thermogenic
►
CN insensitive, SHAM inhibits
►
AOX1 (stress induced); 2a,b; 3
►
ROS stimulate expression
►
Over-expression reduces ROS formation
►
Anti-sense AOX increases ROS formation
►
Amount of protein not correlated to
engagement in respiration
Fungi also contain rotenone insensitive external and
internal NAD(P)H dehydrogenases (“class 2”) – lower
efficiency alternatives to complex 1 – also thermogenic
Organization of alternative internal and external
NADH dehydrogenases
Uncoupling proteins (UCPs) are responsible for
“uncoupled” respiration in plants
Energy dissipation by the uncoupling
proteins
Transmembrane arrangements of UCPs
Features of UCPs
►
Thermogenic
►
Several forms exist
►
Occur in plants and fungi
►
40% homology with mammalian UCP
►
Proton carrier or FFA carriers
►
FFA stimulate activity
►
Inhibition (purine nucleotides) increases
ROS formation, adding FFA decreases
ROS formation
More reasons to heat up!
Dead-horse arum (Helicodiceros muscivorus)
Real thermoregulation, very rare in plants!
Desiccation for 2.5 h followed by rehydration increases
heat production in the dark for more than 4 h in Peltigera
Oilseeds are able to convert stored oil
to carbohydrate
• Many seeds store a significant portion of
photoassimilate as oil, not carbohydrate
• This oil is mobilized as an energy source
upon germination
– e.g., canola (45% oil by dry weight versus
maize 5%)
• Oil – not water soluble, not transportable
• Most plants convert oil droplets (triglycerides) 
sucrose to mobilize its energy
• Animals cannot interconvert lipids and
carbohydrates!
• Again, this gives plants metabolic flexibility in
allocating carbon between lipids and carbohydrates
– Seeds can be smaller because lipids store more energy
per gram!
Mobilizing the energy in stored oil involves
the glyoxylate cycle and gluconeogenesis
• Triglyceride conversion to sucrose
involves 3 organelles + cytosol
• Fatty acids are removed from triglyceride
by lipase
• FA imported into glyoxysome –
specialized plant organelle
• Cleaved at every 2nd C to generate
acetyl CoA via ß-oxidation
• Glyoxylate cycle take home messages:
– Borrowing oxaloacetate from the
mitrochondrion allows citrate synthesis
from fatty acids
– It’s a cycle! Regeneration of OAA in mt
keeps acetyl CoA incorporation high
– The products of the cycle enter
gluconeogenesis to generate sucrose in
the __________
– Glycerol from triglyceride also enters
gluconeogenesis for sucrose biosynthesis
– NADH enters oxidative phosphorylation
Figure 7.13
a/k/a
_____________