Carbohydrate Metabolism: Glycolysis

The respiratorypathways in the plant cytoplasminclude glycolysisand the pentosephosphate pathway.

Glycolysis andfermentationtake place in thecytoplasm

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 requiresATP.

Phosphofructokinases play an importantrole 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. Oxidationof 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).

Aerenchyma in O2-deficient maize root

O2-sufficient maize root

Aerenchyma in rice roots

Aerenchyma can form in stems, too.

The formation of aerenchyma involvesapoptosis 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 inaerobic conditions. I just read another definition, which is more quantitativeand 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 whenthe tissue is in N2. The QO2(air) is the rate of uptake of O2 when the tissueis 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 inaerobic conditions. I just read another definition, which is more quantitativeand 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 whenthe tissue is in N2. The QO2(air) is the rate of uptake of O2 when the tissueis 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 inaerobic conditions. I just read another definition, which is more quantitativeand 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 whenthe tissue is in N2. The QO2(air) is the rate of uptake of O2 when the tissueis 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:PentosePhosphatePathway

Products:

NADPH(needed to makefatty acids)

Ribose-5-P(nucleic acids,ascorbic acid)

Erythrose-4-P(shikimic acid:aromatic aminoacids)

CO2

P-pentoseisomerase

epimeraseTK

TKTA

Aromatic aminoacids

Using the PPPto make NADPH

membranes,storage lipids inseeds

Using the PPPto 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.