Aerobic Respiration Yields about 60 Molecules of ATP per Molecule of Sucrose

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The complete oxidation of a sucrose molecule leads to the net formation of

• 8 molecules of ATP by substrate-level phosphoryla-tion (4 during glycolysis and 4 in the citric acid cycle)

• 4 molecules of NADH in the cytosol

• 16 molecules of NADH plus 4 molecules of FADH2 (via succinate dehydrogenase) in the mitochondrial matrix

On the basis of theoretical ADP:O values (see Table 11.1), a total of approximately 52 molecules of ATP will be generated

Uptake of pyruvate in exchange for a hydroxy! ion is mediated by the pyruvate transporter.

CYTOSOL pH 7.5

Inner membrane Intermembrane space Outer membrane

Pore

CYTOSOL pH 7.5

Pore

The apH drives the electroneutral uptake of Pi through the phosphate transporter.

Free energy released by the dissipation of the proton gradient is coupled to the synthesis of ATP4-from ADP3- and Pi via the many FoFr ATP synthase complexes that span the inner membrane

The membrane potential component (ae) of the proton gradient drives the electrogenic exchange of ADP from the cytosol for ATP from the mitochondrion via the adenine nucleotide transporter.

The tricarboxylic acid citrate is exchanged for a dicarboxylic acid such as malate or succinate.

Uncouplers (and the uncoupling protein) permit the rapid movement of protons across the inner membrane, preventing buildup of the electrochemical proton gradient and reducing the rate of ATP synthesis but not the rate of electron transfer.

Uptake of dicarboxylic acids such as malate or succinate in exchange for a phosphate ion is mediated by the dicarb-oxylate transporter.

FIGURE 11.10 Transmembrane transport in plant mitochondria. An electrochemical proton gradient (A/~H+) consisting of a membrane potential (AE, -200mV, negative inside) and a ApH (alkaline inside) is established across the inner mito-chondrial membrane during electron transport as outlined in the text. Specific metabolites are moved across the inner membrane by specialized proteins, called transporters or carriers (After Douce 1985).

per sucrose by oxidative phosphorylation. The result is a total of about 60 ATPs synthesized per sucrose (Table 11.2).

Using 50 kJ mol-1 (12 kcal mol-1) as the actual free energy of formation of ATP in vivo, we find that about 3010 kJ mol-1 (720 kcal mol-1) of free energy is conserved in the form of ATP per mole of sucrose oxidized during aerobic respiration. This amount represents about 52% of the standard free energy available from the complete oxidation of sucrose; the rest is lost as heat. This is a vast improvement over the conversion of only 4% of the energy available in sucrose to ATP that is associated with fermentative metabolism.

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