Download Comparison With Photosynthesis

Comparison With
Photosynthesis
Photosynthesis
Respiration
Where?
In chlorophyll-bearing
cells
In all cells
When?
In the p
presence of light
g
All the time
Input
Carbon dioxide and
water
Reduced carbon
compounds and oxygen
Output
Reduced
R
d
d carbon
b
compounds, oxygen,
and water
Carbon dioxide and water
Energy Sources
Light
Chemical Bonds
Energy Result
Energy Stored
Energy Released
Chemical Reaction
Reduction of carbon
compounds
Oxidation of carbon
compounds
Energy Carrier(s)
NADP
NAD and FAD
Cellular Respiration:
A O
An
Overview
i
Cellular respiration consists of a series
of pathways by which carbohydrate
and other molecules are oxidized for
the purpose of retrieving the energy
stored in photosynthesis and to obtain
carbon skeletons used in the growth
and
d maintenance
i
off the
h cell.
ll
Comparison of Photosynthesis
and
d Respiration
R
i i
• Photosynthesis:
6 CO2 + 6 H2O
→
C6H12O6 + 6 O2
light plant
light,
• Respiration:
R
i ti
C6H12O6 + 6 O2 + 6 H2O ↔ 6 CO2 + 12 H2O
(Hexose)
Perspective from a Chemical
S d i
Standpoint
• Respiration
p
represents
p
the oxidation of a 12carbon molecule sucrose and reduction of 12
molecules of CO2.
C12H22O11 + 13 H2O → 12 CO2+ 48 H+ + 48 e12 O2 + 48 H+ + 48 e- → 24 H2O
• Net Reaction
C12H22O11 + 12 O2 → 12 CO2 + 11 H2O
60 ADP + 60 Pi → 60 ATP + 60 H2O
Respiration
• Anabolism: formation of large
g molecules from small
molecules; requires an input of energy.
• Catabolism: degradation or breakdown of large
molecules to small molecules; this process often
releases energy.
– Respiration is the major catabolic process that
gy in all cells.
releases energy
– Respiration involves oxidative breakdown of
sugars to CO2 and H2O.
O
Respiration
•
Respiration is a multi-step
multi step process in
which carbon is oxidized through a
series of reactions, which can be
divided into four stages:
1. Glycolysis (Embden-Meyerhoff-Parnas)
2. Tricarboxylic Acid Cycle (TCA or
Krebs)
3. Oxidative pentose phosphate pathway
4 Electron Transport Chain (ETC)
4.
Respiration
• Glucose is commonly
y cited as the main
substrate in respiration; however, the
reduced carbon may come from many
sources such as:
•
•
•
•
•
•
•
Starch
Sucrose
Fructose
Other sugars
Lipids
Organic acids
Proteins
Overview of Respiration
Respiratory Metabolism
Glucose
O2
NAD+
TCA
Cycle
Respiratory
Chain
Pyruvic acid
H2O
NADH
CO2
C t l
Cytosol
Mit h d i
Mitochondria
Adapted from Intro. Plant Physiology, 2nd edition;
(Noggle and Fritz, 1983.)
Reactions of Plant Glycolysis
and Fermentation
Respiration
• Location of glycolysis: carried out by a group of
soluble
l bl enzymes located
l
t d in
i the
th cytosol
t
l and
d in
i
plastids.
• Chemically what happens to sucrose in glycolysis?
– It undergoes a limited amount of oxidation.
– Produces 4 molecules of pyruvate (3
(3-C
C
compound).
– A little ATP.
– Produces stored reducing power in the form of a
reduced pyridine nucleotide, NADH.
Functions of Glycolysis
1.
Converts one hexose molecule into two molecules
off pyruvic
i acid,
id and
d partial
ti l oxidation
id ti off hexose
h
occurs.
2.
Produces a limited amount of ATP.
3.
Forms molecules that can be removed from the
pathway to synthesize several other constituents
of which the plant is made.
4.
Produces two molecules of pyruvate that can be
oxidized in mitochondria to yield relatively large
produced in
amounts of ATP, much more than is p
glycolysis.
Functions of Glycolysis
1. Glycolysis
y y
converts one hexose molecule
into two molecules of pyruvic acid, and
partial oxidation of hexose occurs.
–
No O2 is used and no CO2 is released.
–
Conversion
C
i off each
h hexose
h
results
lt in
i the
th
reduction of two molecules of NAD+ to NADH
(+2H+).
–
These NADH can subsequently be oxidized by
O2 in the mitochondrion such that NAD+ is
regenerated and two molecules of ATP are
formed.
Functions of Glycolysis
– Some of these NADH do not enter mitochondria
and
d are used
d in
i the
th cytosol
t
l to
t drive
d i various
i
anabolic, reductive processes.
– Example: nitrate reductase
reductase, an enzyme that
transfers two electrons from NADH or, in a few
species, from NADPH.
NO
O3- + NADH + H+ → NO
O2- + NAD+ + H2O
(Nitrate reductase)
• How many steps in glycolysis result in the formation
of NADH? Only one; during the oxidation of 3phosphoglyceradehyde to 1,3-bisphosphoglyceric
acid, NAD+ is reduced to NADH.
Functions of Glycolysis
y y
2 Glycolysis produces ATP
2.
ATP.
•
When glucose or fructose enters glycolysis,
each is phosphorylated by ATP in reactions
catalyzed by hexokinase or fructokinase.
•
Glucose-6-phosphate
Glucose
6 phosphate and fructose-6-phosphate
fructose 6 phosphate
are the products.
Functions of Glycolysis
– Two ATPs are formed from each triose phosphate
p
p
derived from the split of fructose-1,6bisphosphate, making a total of four ATPs for
each glucose or fructose respired
respired.
• A yield
of four ATPs minus the two (or one)
required to form fructose-1,6-bisphosphate
leaves a net yield of either two or three ATPs
for each hexose used in glycolysis (if the PPiPFK route is used, then three ATPs are
formed).
Functions of Glycolysis
•
What two routes exist by which
fructose-6-phosphate can be
converted to fructose-1,6bisphosphate?
1. ATP – dependent –
phosphofructokinase
p
p
(ATP-PFK)
(
)
2. PPi – dependent - PFK
Functions of Glycolysis
Fructose-6-Pi
PPi-dependent
phosphofructokinase
(PPi-PFK)
PPi
Pi
ATP
ATP-dependent
pphosphofructokinase
p f
(ATP-PFK)
ADP
Fructose-1,6-bisphosphate
,
p p
Functions of Glycolysis
ATP-phosphofructokinase
p
p
(ATP-PFK):
(
)
current evidence suggests that the ATPPFK route is involved in so-called
“maintenance respiration” by cells that are
not rapidly growing differentiating, or
adapting
p g to changing
g g environments (Black
(
et al., 1987)
•
•
Reaction occurs mainly in cells that are mature,
or nearly so, and that exist for some time in a
moderately constant environment.
Functions of Glycolysis
•
Pyrophosphate phosphofructokinase
(PPi-PFK): route that is much more
adaptive and can increase or
decrease in importance depending on
developmental processes and
environmental conditions
conditions.
Functions of Glycolysis
• Conversion of glucose or fructose to
fructose-1,6-bisphosphate requires:
– 2 ATPs if ATP-PFK route used.
– 1 ATP if PPi-PFK route used.
Fermentation
Fermentation
• Glycolysis
y y
can function well without O2, but
without O2 the TCA cycle and electron
transport chain cannot function.
• Why does this present a problem for the
continued operation of glycolysis? The
cell’s
cell
s supply of NAD+ is limited
limited, and once all
the NAD+ becomes tied up in the reduced
state (NADH), the glyceraldehyde-3phosphate dehydrogenase reaction cannot
take place.
– NADH and pyruvate begin to accumulate when O2
is limiting.
Fermentation
•
Thus, in the absence of O2, fermentation ((anaerobic
respiration) allows the regeneration of NAD+
needed for glycolysis.
•
When O2 is
Wh
i lacking,
l ki
plants
l t can further
f th metabolize
t b li
pyruvate by carrying out one or more forms of
fermentation metabolism:
–
Lactic acid fermentation (does occur in plants and
animals): the enzyme lactate dehydrogenase uses NADH
pyruvate to lactate,, thus regenerating
g
g NAD+.
to reduce py
–
In alcoholic fermentation the two enzymes pyruvate
decarboxylase and alcohol dehydrogenase act on
pyruvate producing ethanol and CO2; oxidizes NADH.
Pathways of Fermentation Leading
t the
to
th Reoxidation
R
id ti off NADH
Conditions Enhancing
F
Fermentative
i Metabolism
M b li
• Low (hypoxic) or zero (anoxic)
concentrations of ambient oxygen.
• Flooded or water logged soils best
example.
– Diffusion of O2 to roots sufficiently
reduced to cause root tissues to become
hypoxic.
Pentose Phosphate
Pathway
Reactions of the Oxidative Pentose
Ph
Phosphate
h t Pathway
P th
in
i Higher
Hi h Plants
Pl t
Pentose Phosphate Pathway
(PPP)
• Oxidative pentose pathway
• Hexose monophosphate shunt
• Phosphogluconate pathway
What is the major
j difference between the
Calvin cycle and the PPP?
– Calvin
C l i cycle
l – sugar phosphates
h
h t synthesized.
th i d
– PPP – sugar
g phosphates
p
p
degraded.
g
Pentose Phosphate Pathway
(PPP)
• Glycolysis and PPP are greatly
interwoven.
• What is the one important difference
between the PPP and glycolysis?
– PPP electron acceptor: NADP+
– Glycolysis electron acceptor: NAD+
Main Functions of the Oxidative
P
Pentose
Ph
Phosphate
h
Pathway
P h
1. Generates NADPH used as a reductant in
biosynthetic processes when NADPH is not
being generated by photosynthesis; e.g.
fatty
y acid and several isoprenoids.
p
–
Particularly important in non-photosynthetic
tissues,, e.g.
g in differentiating
g tissues,,
germinating seeds and during the hours of
darkness.
2 G
2.
Generates ribose-5-phosphate
ib
h
h
required
i d for
f
biosynthesis of nucleotides and nucleic
acids ((RNA and DNA).
)
Main Functions of the Oxidative
P
Pentose
Ph
Phosphate
h
Pathway
P h
3. Produces erythrose-4-phosphate
y
p
p
required
q
for synthesis of shikimic acid, the
precursor of aromatic rings.
•
Erythrose-4-phosphate + PEP → initial reaction
that produces plant phenolic compounds in the
shikimic acid pathway
pathway.
4. During the early stages of greening, before
leaf tissues become fully photoautotrophic,
the oxidative pentose phosphate pathway
is thought to be involved in generating
Calvin cycle intermediates.
Oxidative Pentose Phosphate Pathway:
Hi h Plants
Higher
Pl t
• NADPH is g
generated in the first two reactions
of the pathway.
• The resulting ribulose 5-phosphate is
converted to the glycolytic intermediates
fructose 6 phosphate and glyceraldyhyde 3phosphate
p
p
through
g a series of metabolic
interconversions.
• The first two reactions are essentially
i
irreversible,
ibl while
hil the
th interconversions
i t
i
between ribulose 5-phosphate and the
glycolytic
g
y y compounds
p
are freely
y reversible.
The Citric Acid Cycle
(Tricarboxylic Acid or Krebs
Cycle)
A Mitochondrial Matrix Process
Respiration
• Where are the TCA cycle and the
electron transport chain located?
– Both are located within the confines of the
membrane-bound organelle known as the
mitochondrion.
mitochondrion
Reactions and Enzymes of the
Pl
Plant
Citric
Ci i A
Acid
id Cycle
C l
Conversion of Pyruvate to AcetylC Ab
CoA
by P
Pyruvic
i Dehydrogenase
D h d
• Acetyl-CoA
y
is the “fuel”
of the Krebs Cycle.
• The release of CO2 in
the Krebs Cycle
accounts for the
product CO2 in the
summary equation for
respiration but no O2 is
respiration,
absorbed during any
Krebs Cycle reaction.
Primary Functions of the Krebs
C l
Cycle
1.
Reduction of NAD+ and ubiquinone
q
to the electron
donors NADH and ubiquinol, which are
subsequently oxidized to yield ATP.
2.
Direct synthesis (substrate-level phosphorylation)
of a limited amount of ATP (one ATP for each
pyruvate oxidized).
oxidized)
3.
Forms carbon skeletons that can be used to
synthesize certain amino acids that, in turn, are
converted to large molecules.
Primary Functions of the Krebs
C l
Cycle
• The flavin FAD is usually identified as the
acceptor of electrons and H+ from succinic
acid, with FADH2 as the product.
• FAD and FADH2 are bound to succinic acid
dehydrogenase, but they represent transitory
intermediate compounds during the overall
reduction
d
i off membrane-soluble
b
l bl quinone
i
(ubiquinone) to ubiquinol.
Primary Functions of the Krebs
C l
Cycle
• None of the dehydrogenase
y
g
enzymes
y
of the
cycle use NADP+ as an electron acceptor;
NADP+ is usually nearly nondetectable in
plant mitochondria.
p
• The release of CO2 in the Krebs cycle
accounts
t ffor the
th product
d t CO2 in
i the
th summary
equation for respiration.
• No O2 is absorbed during any Krebs-cycle
reaction.
Respiration
• What p
purpose
p
does the TCA cycle
y
serve?
– Brings about the complete oxidation of pyruvate
to CO2.
– Generates a considerable amount of reducing
power (about 16 NADH + 4 FADH2 equivalents per
sucrose).
– These reactions,
reactions with one exception
exception, involve a
series of soluble enzymes located in the internal
aqueous compartment, or matrix, of the
mitochondrion.
mitochondrion
Glycolysis, The Pentose Phosphate
P th
Pathway
and
d the
th Citric
Cit i Acid
A id Cycle
C l
Glyoxylate Cycle
Electron Transport and ATP
Synthesis at the Inner
Mitochondrial Membrane
Mitochondrial Electron Transport
Ch i
Chain
Electron Transport ATP Synthesis at
th Inner
the
I
Mitochondrial
Mit h d i l Membrane
M b
• ATP is the energy
gy carrier used by
y cells to
drive living processes.
• The chemical energy conserved during the
citric acid cycle in the form of NADH and
FADH2 (redox equivalents with high-energy
electrons)
l t
) mustt be
b converted
t d to
t ATP to
t
perform useful work in the cell.
• This O2-dependent process occurs in the
inner mitochondrial membrane through
oxidative phosphorylation
phosphorylation.
Electron Transport Chain Catalyzes
a Flow
Fl
off El
Electrons
t
ffrom NADH to
t O2
• For each molecule of sucrose oxidized
through glycolysis and the citric acid cycle
pathways:
– 4 molecules of NADH are generated in the cytosol.
– 16 molecules
l
l off NADH plus
l
((mitochondria)
it h d i )
– 4 molecules of FADH2
• These reduced compounds must be
reoxidized to keep the respiratory process
functional.
Electron Transport Chain Catalyzes
a Flow
Fl
off El
Electrons
t
ffrom NADH to
t O2
• The electron transport chain catalyzes an electron
fl
flow
from
f
NADH ((and
d FADH2) to
t oxygen, the
th final
fi l
electron acceptor of the respiratory process.
• The role of the electron transport chain is to bring
about the oxidation of NADH (and FADH2) and, in the
process, utilize some of the free energy released to
generate an electrochemical proton gradient Δ ~ H+ ,
across the inner mitochondrial membrane.
• Th
The individual
i di id l electron
l t
transport
t
t proteins
t i are
organized into four multi-protein complexes, all of
which are localized in the inner mitochondiral
membrane.
membrane
Electron Transport Chain:
C
Complex
l I
• Complex
p
I (NADH
(
dehydrogenase)
y
g
) oxidizes
electrons from NADH generated in the
mitochondrial matrix during the TCA cycle.
– Electron carriers include FMN and several ironsulfur centers.
• Complex I then transfers these electrons to
ubiquinone.
• Four protons are pumped from the matrix to
the intermembrane space for every electron
pair passing through the complex.
Electron Transport Chain:
Ubi i
Ubiquinone
• Ubiquinone is a small lipid
lipid-soluble
soluble
electron and proton carrier within the
inner membrane.
• The ubiquinone (UQ) pool diffuses
freely within the inner membrane and
serves to transfer electrons from the
dehydrogenases to either complex III or
the alternative oxidase.
Electron Transport Chain:
C
Complex
l II (succinate
(
i t dehydrogenase)
d h d
)
• Oxidation of succinate in the TCA cycle
is catalyzed by this complex.
• The reducing equivalents are
transferred via the FADH2 and a small
group of iron-sulfur proteins into the
q
pool.
p
ubiquinone
• Complex II does not pump protons.
Electron Transport Chain:
C
Complex
l III (cytochrome
( t h
b 1 complex)
bc
l )
• Complex
p
III oxidizes reduced ubiquinone
q
(ubiquinol) and transfers the electrons via an
iron-sulfur center, two b-type cytochromes
((b565 and b560) and a membrane-bound
cytochrome c1 to cytochrome c.
• F
Four protons
t
per electron
l t
pair
i are pumped
d
by complex III.
• Cytochrome c is a small protein loosely
attached to (peripheral protein) that transfers
electrons from complex III to complex IV
IV.
Electron Transport Chain:
C
Complex
l IV (cytochrome
( t h
c oxidase)
id
)
• Complex IV contains two copper centers (CuA and
C B) and
Cu
d cytochrome
t h
a and
d a3.
• Complex
p
IV is the terminal oxidase and brings
g about
the four-electron reduction of O2 to two molecules of
H2O.
• Two protons are pumped per electron pair.
• Structurally and functionally
functionally, ubiquinone and the
cytochrome bc1 complex are very similar to
plastoquinone and the cytochrome b6f complex,
p
y in the p
photosynthetic
y
electron transport
p
respectively,
chain.
Unique Electron Transport
E
Enzymes
in
i Plant
Pl
Mitochondria
Mi h d i
• Two NAD(P)H dehydrogenases
– Ca2+-dependent
– Attach to the outer surface of the inner membrane
facing intermembrane space.
– Oxidize cytosolic NADH and NADPH.
– Electrons from these external dehydrogenases
enter the main electron transport chain at the
level of the ubiquinone pool.
Unique Electron Transport
E
Enzymes
in
i Plant
Pl
Mitochondria
Mi h d i
• Two p
pathways
y for oxidizing
g matrix NADH
– Electron flow through Complex I → sensitive to
inhibition by several compounds, including
rotenone and piericidin.
– In addition, plant mitochondira have a rotenoneresistant dehydrogenase for oxidation of NADH
derived from citric acid cycle substrate → this
pathway may be a bypass that is engaged when
complex I is overloaded, such as under
photorespiratory conditions
conditions.
Unique Electron Transport
E
Enzymes
in
i Plant
Pl
Mitochondria
Mi h d i
• An NADPH dehydrogenase
y
g
is p
present on the
matrix surface; little is known about this
enzyme.
• Most, if not all, plants have an “alternate”
respiratory pathway for reduction of oxygen
oxygen.
– This pathway involves the so-called
so called alternative
oxidase; unlike cytochrome c oxidase, it is
insensitive to inhibition by cyanide, azide, or
carbon monoxide.
Transporters Exchange
S b
Substrates
and
d Products
P d
• Although ATP is synthesized in the
mitochondrial matrix, most of it is used
outside the mitochondrion.
– An efficient mechanism for moving ATP in
and out of the organelle is needed.
needed
• Adenylate transport involves the
ADP/ATP (adenine
( d i nucleotide)
l tid )
transporter, another inner-membrane
transporter.
p
Transmembrane Transport in
Pl
Plant
Mitochondria
Mi h d i
Electron Transport Chain
Chemiosmotic Theory
• As electron pairs pass down the ETC, H+
ions are pumped across the inner
mitochondrial membrane from the matrix
to the intermembrane space.
• The electrochemical gradient of H+ ions
across the inner membrane constitutes
the driving
g force for the ATP-ase ((ATP
synthase) catalyzed phosphorylation of
ADP.
Chemiosmotic Theory
• This ‘driving
driving force’
force has been termed
‘proton motive force’ and is composed
of two elements:
– pH difference or gradient across the inner
membrane of about 1.5
1 5 pH units (7.0
(7 0 in
intermembrane space and 8.5 in matrix).
– A
An electrical
l t i l potential
t ti l difference
diff
or
gradient of about 0.15 V (H+ ions on outer
surface of inner membrane and OH- on
the inner surface of the membrane).
Chemiosmotic
Theory
Five Complexes of Integral
Membrane Proteins in Inner
Mitochondrial Membrane
Complex
• *I
• II
• *III
• *IV
• V
Name
NADH dehydrogenase
Succinate dehydrogenase
Cytochrome c reductase
(complex cytochrome bc1)
Cytochrome c oxidase
ATP synthase
* Sit
Sites att which
hi h protons
t
are pumped
d
Electron Pathways in Plants
ATP / Pi Transporter in
Mitochondria
• The one-for-one exchange of mitochondrial ATP and
cytosolic ADP across the inner membrane is driven
by the membrane potential
potential.
• Pi is returned to the matrix in exchange for OH-.
Alternate Electron Pathways
in Plants
Alternate Electron Pathways
in Plants
CYANIDE-RESISTANT
RESPIRATION
• The way the plant controls the flow of electrons
through the two electron transport chains is not
clear but there is evidence to indicate that the
intensity of the electron flux through the
cytochrome system of the conventional chain is a
regulating factor.
– It
Its activity
ti it is
i highest
hi h t in
i cells
ll rich
i h in
i sugars (as
(
after rapid photosynthesis) when glycolysis
and the Krebs cycle occur unusually rapidly,
because then the normal electron-transport
pathway cannot handle all the electrons
provided to it.
CYANIDE-RESISTANT
CYANIDE
RESISTANT
RESPIRATION
– Several experts have concluded that the
alternative pathway operates largely as
an overflow mechanism to remove
electrons
l t
when
h the
th cytochrome
t h
pathway becomes saturated by rapid
glycolysis and Krebs-cycle activities
activities.
CYANIDE-RESISTANT
RESPIRATION
• Physiological
y
g
significance?
g
• The physiological significance of cyanideresistant respiration is not clear.
– Goodwin and Mercer (1981) indicate that it is
doubtful that its function is to allow plants to
survive in the presence of cyanide even
though many plants release HCN from
cyanogenic
i glucosides
l
id when
h they
th are injured.
i j
d
Why? Because those plant tissues with the
y
respiration
p
have
most active cyanide-resistant
no cyanogenic glucosides.
CYANIDE-RESISTANT
RESPIRATION
–C
Cyanide-resistant
id
i t t respiration
i ti is
i thought
th
ht to
t be
b
responsible for the climacteric in fruits (i.e.,
the marked increase in respiration
p
during
g and
just prior to ripening). The climacteric is
induced by ethylene and it is thought that
ethylene brings this about by stimulating
cyanide-resistant respiration.
– Certain germinating seeds exhibit cyanideresistant respiration during the early stages of
water imbibition.
imbibition
CYANIDE-RESISTANT RESPIRATION
– The best understood role of cyanide-resistant
respiration is, however, in generating heat in
thermogenic tissues:
• Thermogenicity is seen in the flowers of
inflorescences of certain p
plants such as water
lily, Victoria, and the arum lilies, Arum
maculatum.
• In skunk cabbage (Symplocarpus foetidus),
foetidus) the
inflorescence is a spadix covered with tiny
hermaphroditic flowers which produce heat and a
foul
ou smell.
s e The
e heat
eat is
sp
produced
oduced in tthese
ese
inflorescences to volatize the odiferous
compounds formed in them. The latter are
frequently amines or indoles and serve to attract
pollinating
lli ti insects.
i
t
CYANIDE-RESISTANT
RESPIRATION
• Energy production of cyanide-insensitive respiration?
– Since the P/O ratio of cyanide-resistant respiration is 1 in
contrast to 3 for conventional respiration, the cells have to
oxidize three times as much fuel to get the required amount of
ATP.
– Assuming that oxidation is via NADH, this will produce about
4.5 times as much heat (i.e., energy that is not conserved as
ATP) than normal.
Arum italicum
Skunk cabbage
Air temperature
15 C
5C
Inflorescence
51 C
30 C
CYANIDE-RESISTANT
RESPIRATION
Does Respiration Reduce
Crop Yields?
• Plant respiration can consume an
appreciable amount of carbon fixed
each day during photosynthesis over
and above losses due to
photorespiration
• To what extent can changes in a plant
plant’s
s
respiratory metabolism affect crop
yields?
Does Respiration Reduce
Crop Yields?
• Two components of respiration:
– Growth respiration - involves the processing of
reduced carbon to bring
g about the growth
g
of
new plant matter
– Maintenance respiration - component of
respiration needed to keep existing, mature
cells in a viable state
• Utilization of energy by maintenance
respiration is not well understood
• Estimates indicate that it can represent
more than 50% of the total respiratory flux
Whole-Plant Respiration
• Many factors affect the respiration rates of
a plant
– Species
– Growth habit of the plant
– Type and age of specific organ
– Environmental variables
• External oxygen concentration
• Temperature
• Plant water status
Whole-Plant Respiration
• Even though plants generally have low
respiration rates, the contribution of
respiration to the overall carbon economy
of the plant can be substantial
– Several herbaceous species surveyed
indicated that 30-60% of the daily gain in
photosynthetic
p
y
carbon was lost to dark
respiration
• These values tended to decrease with age
g
• Young trees lose roughly a third of their
daily photosynthate to respiration
Whole-Plant Respiration
• Loss can double in older trees as the
ratio of photosynthetic to
nonphotosynthetic tissue decreases.
• In tropical
p
areas,, 70-80% of the daily
y
photosynthetic gain can be lost to
respiration owing to the high nighttime
respiration
i i rates associated
i d with
ih
elevated night temperatures.
Whole-Plant Respiration
• Empirical relations between plant respiration
rates and crop yield
– Yield increases of 10-20% were correlated with
a 20% decrease in the leaf respiration
p
rate of
the forage crop perennial ryegrass (Lolium
perenne) (Wilson and Jones, 1982)
– Similar correlations have been found for other
plants, including corn and tall fescue
(Lambers, 1985)
• Appears that a potential for increasing crop
yields through reduction of respiration rates
exists
Whole-Plant Respiration
• What about the cyanide-resistant
alternative pathway?
– Cyanide-resistant pathway has the
potential
t ti l for
f utilizing
tili i considerable
id
bl
amounts of the cell’s reduced carbon to
no apparent useful end
– Estimates of the alternative pathway in
wheat roots alone suggest a loss of
carbon equivalent to 6% of the final
grain yield