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Chapter 18: Problem 1

Energy harvest. What is the yield of ATP when each of the following substrates is completely oxidized to \(\mathrm{CO}_{2}\) by a mammalian cell homogenate? Assume that glycolysis, the citric acid cycle, and oxidative phosphorylation are fully active. (a) Pyruvate (b) Lactate (c) Fructose 1,6 -bisphosphate (d) Phosphoenolpyruvate (e) Galactose (f) Dihydroxyacetone phosphate

Short Answer

Expert verified
(a) 12.5 ATP, (b) 12.5 ATP, (c) 32 ATP, (d) 13.5 ATP, (e) 30-32 ATP, (f) 16 ATP.

Step by step solution

01

Understand the Full Oxidation of Carbohydrates

To determine the ATP yield from full oxidation of these substrates, we need to consider their complete breakdown through glycolysis, the citric acid cycle (also known as Krebs or TCA cycle), and oxidative phosphorylation. For simplicity, we generally consider the ATP yield for one molecule of glucose as a reference which gives approximately 30-32 ATP.
02

Pyruvate Oxidation ATP Yield

Each pyruvate molecule feeds into the citric acid cycle, where it generates approximately 12.5 ATP. This includes 1 ATP from substrate-level phosphorylation, 3 NADH (which yield about 7.5 ATP), and 1 FADH2 (which yields about 1.5 ATP).
03

Lactate Oxidation ATP Yield

Lactate is converted back to pyruvate by lactate dehydrogenase, so it has the same ATP yield as pyruvate, which is about 12.5 ATP per molecule of lactate.
04

Fructose 1,6-bisphosphate Oxidation ATP Yield

Fructose 1,6-bisphosphate splits directly into two molecules of glyceraldehyde 3-phosphate and then follows through the glycolysis pathway down to pyruvate. Each molecule will then enter the citric acid cycle. Therefore, it nets approximately 32 ATP.
05

Phosphoenolpyruvate Oxidation ATP Yield

Phosphoenolpyruvate converts to pyruvate and yields about 1 ATP in the conversion. Once this pyruvate is oxidized through the citric acid cycle, it yields an additional 12.5 ATP, totaling approximately 13.5 ATP.
06

Galactose Oxidation ATP Yield

Galactose is converted to glucose-6-phosphate, entering glycolysis and being catabolized similarly to glucose. Thus, complete oxidation yields approximately 30-32 ATP.
07

Dihydroxyacetone Phosphate Oxidation ATP Yield

Dihydroxyacetone phosphate converts to glyceraldehyde 3-phosphate, and follows the glycolysis pathway to pyruvate, and then to the citric acid cycle. This gives a net yield of about 16 ATP for each molecule.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Glycolysis
Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. This process takes place in the cytoplasm of cells and does not require oxygen, making it an anaerobic process. During glycolysis, one molecule of glucose, a six-carbon sugar, is converted into two molecules of pyruvate, a three-carbon compound.

Here's a summary of glycolysis:
  • It includes 10 enzyme-catalyzed reactions.
  • Net produce of 2 ATP molecules and 2 NADH molecules per glucose molecule.
  • Sets the stage for further energy extraction through the citric acid cycle.
Glycolysis is crucial because it breaks down glucose, which can be converted into pyruvate, feeding into the citric acid cycle and contributing to the complete oxidation of glucose.
Citric Acid Cycle
Also known as the Krebs cycle or TCA cycle, the citric acid cycle occurs in the mitochondria after glycolysis. Pyruvate generated from glycolysis is transported into the mitochondria, where it is converted to acetyl-CoA, which enters the cycle.
  • Each turn of the cycle produces 1 ATP, 3 NADH, and 1 FADH2.
  • It takes two turns of the cycle to completely oxidize one glucose molecule.
NADH and FADH2 are crucial as they carry electrons to the oxidative phosphorylation stage. The citric acid cycle completes the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins, transforming them into carbon dioxide and capturing energy in the form of reduced coenzymes.
Oxidative Phosphorylation
Oxidative phosphorylation is the final stage of cellular respiration occurring in the mitochondria. This process utilizes the electrons captured by NADH and FADH2 generated in both glycolysis and the citric acid cycle. These electrons pass through the electron transport chain, ultimately reducing oxygen to water in a process that generates ATP by chemiosmosis.
  • Consists of the electron transport chain and chemiosmosis.
  • The electron flow leads to the pumping of protons generating a proton gradient used by ATP synthase to produce ATP.
  • Typically yields about 26-28 ATP from one glucose molecule.
Oxidative phosphorylation is highly efficient, transforming the energy stored in NADH and FADH2 into a usable form, ATP, which powers various cellular functions.
Full Oxidation
Full oxidation refers to the entire metabolic process where glucose is completely broken down into carbon dioxide and water, and maximum energy in the form of ATP is extracted. It encompasses glycolysis, the citric acid cycle, and oxidative phosphorylation.
  • Begins with glycolysis in the cytoplasm.
  • Continues with the citric acid cycle and oxidative phosphorylation in the mitochondria.
  • Total ATP yield from one glucose molecule ranges between 30 to 32 ATP.
This complete oxidation ensures that energy is effectively extracted from biochemical substrates not only essential for primary energy metabolism but also for various biosynthetic pathways.
Biochemical Substrates
Biochemical substrates refer to the initial compounds that enter metabolic pathways to yield energy through cellular respiration. These include carbohydrates, fats, and proteins, but in the context of glucose metabolism, we focus on molecules like pyruvate, lactate, fructose 1,6-bisphosphate, and others seen in glycolysis and the citric acid cycle.
  • Each substrate has a specific role and energy yield in metabolic pathways.
  • Conversion to intermediates allows entry into the citric acid cycle.
  • For instance, pyruvate can enter directly into the citric acid cycle or undergo conversion from lactate.
Understanding how these substrates contribute to the full oxidation of glucose helps explain the variations in ATP yield from different starting compounds.

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Most popular questions from this chapter

Exaggerating the difference. Why must the ATP-ADP translocase (also called adenine nucleotide translocase or ANT) use \(\mathrm{Mg}^{2+}\) -free forms of ATP and ADP?

No exchange. Mice that are completely lacking ATP-ADP translocase \(\left(\mathrm{ANT}^{-} / \mathrm{ANT}^{-}\right)\) can be made by the knockout technique. Remarkably, these mice are viable but have the following pathological conditions: (1) high serum levels of lactate, alanine, and succinate; (2) little electron transport; and (3) a six- to eightfold increase in the level of mitochondrial \(\mathrm{H}_{2} \mathrm{O}_{2}\) compared with that in normal mice. Provide a possible biochemical explanation for each of these conditions.

An essential residue. The conduction of protons by the \(\mathrm{F}_{0}\) unit of ATP synthase is blocked by the modification of a single side chain by dicyclohexylcarbodiimide. What are the most likely targets of action of this reagent? How might you use sitespecific mutagenesis to determine whether this residue is essential for proton conduction?

Coupled processes. If actively respiring mitochondria are exposed to an inhibitor of ATP synthase, the electron-transport chain ceases to operate. Why?

Opposites attract. An arginine residue (Arg 210 ) in the a subunit of ATP synthase is near the aspartate residue (Asp \(61)\) in the matrix-side proton channel. How might Arg 210 assist proton flow?
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