8.7: Energy yield by complete oxidation of glucose

Last updated
Save as PDF

Page ID: 279711

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

Learning Objectives

Determine the amount of ATP produced by the oxidation of glucose in the presence and absence of oxygen.

Determining the exact yield of ATP for aerobic respiration is difficult for a number of reasons. First of all, the number of ATP generated per reduced NADH or FADH₂ is not always a whole number. For every pair of electrons transported to the electron transport chain by a molecule of NADH, between 2 and 3 ATP are generated. For each pair of electrons transferred by FADH₂, between 1 and 2 ATP are generated. In eukaryotic cells, unlike prokaryotes, NADH generated in the cytoplasm during glycolysis must be transported across the mitochondrial membrane before it can transfer electrons to the electron transport chain. Muscle and brain cells use a transport mechanism that converts the NADH in the cytoplasm into FADH2. In the liver, kidneys, and heart cells, a different transport mechanism is used, and NADH in the cytoplasm is converted into NADH in the mitochondria. As a result, different numbers of ATP molecules are generated from cytoplasmatic NADH in each tissue.

For simplicity, however, we will look at the theoretical maximum yield of ATP per glucose molecule oxidized by aerobic respiration. We will assume that for each pair of electrons transferred to the electron transport chain by NADH, 2.5 ATP will be generated; for each electron pair transferred by FADH₂, 1.5 ATP will be generated. Keep in mind, however, that less ATP may actually be generated.

In eukaryotic cells, the theoretical maximum yield of ATP generated per glucose is 30 to 32, depending on how the 2 NADH generated in the cytoplasm during glycolysis enter the mitochondria and whether the resulting yield is 2 or 3 ATP per NADH.

Table \(\PageIndex{1}\): Maximum Yield of ATP from the Complete Oxidation of 1 Mol of Glucose
Reaction	Comments	Yield of ATP (moles)
glucose → glucose 6-phosphate	consumes 1 mol ATP	−1
fructose 6-phosphate → fructose 1,6-bisphosphate	consumes 1 mol ATP	−1
glyceraldehyde 3-phosphate → BPG	produces 2 mol of cytoplasmic NADH
BPG → 3-phosphoglycerate	produces 2 mol ATP	+2
phosphoenolpyruvate → pyruvate	produces 2 mol ATP	+2
pyruvate → acetyl-CoA + CO₂	produces 2 mol NADH
isocitrate → α-ketoglutarate + CO₂	produces 2 mol NADH
α-ketoglutarate → succinyl-CoA + CO₂	produces 2 mol NADH
succinyl-CoA → succinate	produces 2 mol GTP	+2
succinate → fumarate	produces 2 mol FADH₂
malate → oxaloacetate	produces 2 mol NADH
2 cytoplasmic NADH from glycolysis	yields 2.5 or 1.5 mol ATP per NADH (depending on tissue)	+3 to +5
2 NADH from the oxidation of pyruvate	yields 2.5 mol ATP per NADH	+5
2 FADH₂ from the citric acid cycle	yields 1.5 ATP per FADH₂	+3
6 NADH from the citric acid cycle	yields 2.5 ATP per NADH	+15
Net yield of ATP:		+30 to +32

ATP Yield from Glycolysis and Oxidative Phosphorylation

When glucose is chemically "burned" as a fuel to produce carbon dioxide (CO₂) and water (H₂O), the energy released from this oxidation process is 670 kcal/mol of glucose:

C₆H₁₂O₆+ 6 O₂→ 6CO₂+ 6 H₂O ΔH = -670 kcal/mol

The net energy yield from anaerobic glucose metabolism can readily be calculated in moles of ATP. In the initial phosphorylation of glucose (step 1), 1 mol of ATP is expended, along with another in the phosphorylation of fructose 6-phosphate (step 3). In step 7, 2 mol of BPG (recall that 2 mol of 1,3-BPG are formed for each mole of glucose) are converted to 2 mol of 3-phosphoglycerate, and 2 mol of ATP are produced. In step 10, 2 mol of pyruvate and 2 mol of ATP are formed per mole of glucose. For every mole of glucose degraded, 2 mol of ATP are initially consumed and 4 mol of ATP are ultimately produced. The net production of ATP is thus 2 mol for each mole of glucose converted to lactate or ethanol. If 7.4 kcal of energy is conserved per mole of ATP produced, the energy conserved in the anaerobic catabolism of glucose to two molecules of lactate (or ethanol) is as follows:

2× [7.4kcal / 670kcal] ×100 = 2.2 %

Thus anaerobic cells extract only a very small fraction of the total energy of the glucose molecule by glycolysis. Contrast this result with the amount of energy obtained when glucose is completely oxidized to carbon dioxide and water through glycolysis, the citric acid cycle, the electron transport chain, and oxidative phosphorylation. Note that a variable amount of ATP is synthesized, depending on the tissue, from the NADH formed in the cytoplasm during glycolysis. This is because NADH is not transported into the inner mitochondrial membrane where the enzymes for the electron transport chain are located. Instead, brain and muscle cells use a transport mechanism that passes electrons from the cytoplasmic NADH through the membrane to flavin adenine dinucleotide (FAD) molecules inside the mitochondria, forming reduced flavin adenine dinucleotide (FADH2), which then feeds the electrons into the electron transport chain. This route lowers the yield of ATP to 1.5 molecules of ATP, rather than the usual 2.5 molecules. A more efficient transport system is found in liver, heart, and kidney cells where the formation of one cytoplasmic NADH molecule results in the formation of one mitochondrial NADH molecule, which leads to the formation of 2.5 molecules of ATP. The total amount of energy conserved in the aerobic catabolism of glucose in the liver is as follows:

32× [7.4kcal / 670kcal] × 100 = 35 %

Conservation of 35% of the total energy released compares favorably with the efficiency of any machine. In comparison, automobiles are only about 20%–25% efficient in using the energy released by the combustion of gasoline. As indicated earlier, the 58% of released energy that is not conserved enters the surroundings (that is, the cell) as heat that helps to maintain body temperature. If we are exercising strenuously and our metabolism speeds up to provide the energy needed for muscle contraction, more heat is produced. We begin to perspire to dissipate some of that heat. As the perspiration evaporates, the excess heat is carried away from the body by the departing water vapor.

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS). Theoretical ATP Yield. LibreTexts content adapted under CC BY license.

Ball at all. Stage II of Carbohydrate Catabolism. The Basics_of GOB Chemistry. LibreTexts adapted under CC BY-NC-SA 3.0 license.