Download Carbohydrate Metabolism: Glycolysis

Carbohydrate Metabolism: Glycolysis
The respiratory
pathways in the
plant cytoplasm
include glycolysis
and the pentose
phosphate pathway.
Glycolysis and
fermentation
take place in the
cytoplasm
The initial materials can come directly from the
chloroplast, from stored starch in an amyloplast,
or from imported sucrose.
The activation of fructose and glucose requires
ATP.
Phosphofructokinases play an important
role in controlling the rate of glycolysis
(inhibited by PEP, stimulated by Pi).
The reaction requires ATP or PPi.
One glucose or fructose yields two glyceraldehyde-3-P. Oxidation
of G3P uses 2 NAD+, yielding 2 NADH and 2 1,3-diPGA. The
next reaction yields 2 ATP.
Products of this section are 2 PEP or, with HCO3- addition, 2 OAA.
PEP is converted to pyruvate, forming 2 ATP, or to OAA.
Pyruvate or OAA can be reduced, regenerating NAD+,
which in the absence of O2 allows continued glycolysis.
Roots depend on respiration for ATP.
Flooding expels O2 from the soil spaces.
Anaerobiosis kills roots in ~ 24 hr through
accumulation of ethanol.
Temporary solution:
Storage of lactate or malate in vacuole.
Intolerant roots (pea, broad bean, Sitka spruce)
ADH higher in flood
Malic enzyme present
EtOH accumulates (in spruce, up to 12X normal)
Tolerant roots (canary grass, pine)
ADH lower in flood
No malic enzyme
Malate accumulates
EtOH limited to 3X normal
Malic enzyme
Roots depend on respiration for ATP.
Flooding expels O2 from the soil spaces.
Anaerobiosis kills roots in ~ 24 hr through
accumulation of ethanol.
Temporary solution:
Storage of lactate or malate in vacuole.
Longer-term solution:
Secretion of ethanol (aquatics)
Aerenchyma (constitutive or induced).
O2-sufficient maize root
Aerenchyma in O2-deficient maize root
Aerenchyma in rice roots
Aerenchyma can form in stems, too.
The formation of aerenchyma involves
apoptosis of anaerobically stressed
cells.
One definition of the “Pasteur Effect” is “a faster uptake of glucose by the
reactions of glycolysis under anaerobic conditions relative to the rate in
aerobic conditions. I just read another definition, which is more quantitative
and experimental: “The Pasteur Effect occurs when the QCO2(N2)/QO2(air)
is greater than 0.33.” The QCO2(N2) is the rate of evolution of CO2 when
the tissue is in N2. The QO2(air) is the rate of uptake of O2 when the tissue
is in air.
Are these two definitions the same? Under what conditions?
Explain why uptake of glucose might be faster under anaerobic conditions.
One definition of the “Pasteur Effect” is “a faster uptake of glucose by the
reactions of glycolysis under anaerobic conditions relative to the rate in
aerobic conditions. I just read another definition, which is more quantitative
and experimental: “The Pasteur Effect occurs when the QCO2(N2)/QO2(air)
is greater than 0.33.” The QCO2(N2) is the rate of evolution of CO2 when
the tissue is in N2. The QO2(air) is the rate of uptake of O2 when the tissue
is in air.
Are these two definitions the same? Under what conditions?
Respiration of 1 mol glucose/min in air will take up 6 mol O2/min.
Fermentation of 1 mol glucose/min in N2 will give 2 mol CO2/min.
The ratio is 0.33, assuming the ethanol fermentation pathway.
Explain why uptake of glucose might be faster under anaerobic conditions.
One definition of the “Pasteur Effect” is “a faster uptake of glucose by the
reactions of glycolysis under anaerobic conditions relative to the rate in
aerobic conditions. I just read another definition, which is more quantitative
and experimental: “The Pasteur Effect occurs when the QCO2(N2)/QO2(air)
is greater than 0.33.” The QCO2(N2) is the rate of evolution of CO2 when
the tissue is in N2. The QO2(air) is the rate of uptake of O2 when the tissue
is in air.
Are these two definitions the same? Under what conditions?
Respiration of 1 mol glucose/min in air will take up 6 mol O2/min.
Fermentation of 1 mol glucose/min in N2 will give 2 mol CO2/min.
The ratio is 0.33, assuming the ethanolic fermentation pathway.
Explain why uptake of glucose might be faster under anaerobic conditions.
The production of ATP is less efficient, relative to glucose used.
With unbalanced hydrolysis of ATP, [Pi] rises, stimulating PFK, a control
point. Also increased [Pi] and decreased [ATP] stimulate PEP pyruvate,
lowering [PEP], which also stimulates PFK.
PPP:
Pentose
Phosphate
Pathway
Products:
NADPH
(needed to make
fatty acids)
Ribose-5-P
(nucleic acids,
ascorbic acid)
Erythrose-4-P
(shikimic acid:
aromatic amino
acids)
CO2
Aromatic amino
acids
P-pentose
isomerase
epimerase
TK
TA
TK
Using the PPP
to make NADPH
membranes,
storage lipids in
seeds
Using the PPP
to make ribose-P
DNA, RNA
Summary
•Sugars and triose-P from photosynthesis or from import can be
metabolized to yield ATP and other compounds.
•Anaerobic conditions yield ethanol, lactate, or malate.
Net
production of ATP depends on the starting material and the
final product.
•The Pentose Phosphate Pathway produces NADPH, ribose-P,
and erythrose-P as useful compounds.