IV. MICROBIAL GENETICS AND MICROBIAL METABOLISM

II. BACTERIAL GROWTH AND MICROBIAL METABOLISM

D. Cellular Respiration

1. Aerobic Respiration

d. The Theoretical Maximum ATP Yield for Aerobic Respiration

Learning Objectives for this Section

Aerobic respiration (def) is the aerobic catabolism of nutrients to carbon dioxide, water, and energy, and involves an electron transport system (def) in which molecular oxygen is the final electron acceptor. Most eukaryotes and prokaryotes use aerobic respiration to obtain energy from glucose. The overall reaction is:

C6H12O6 + 6O2 yields 6CO2 + 6H2O + energy (as ATP)

Note that glucose (C6H12O6 ) is oxidized to produce carbon dioxide (CO2) and oxygen (O2) is reduced to produce water (H2O).

Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid (Krebs) cycle, and an electron transport chain and chemiosmosis. We will now look at the theoretical maximum ATP yield for aerobic respiration.

The Theoretical Maximum ATP Yield for Aerobic Respiration

Determining the exact yield of ATP for aerobic respiration is difficult for a number of reasons. In addition to generating ATP by oxidative phosphorylation in prokaryotic cells, proton motive force is also used for functions such as transporting materials across membranes and rotating flagella. Also, some bacteria use different carriers in their electron transport chain than others and the carriers may vary in the number of protons they transport across the membrane. Furthermore, 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 and this requires energy. As a result, between 1 and 2 ATP are generated from these NADH.

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, 3 ATP will be generated; for each electron pair transferred by FADH2, 2 ATP will be generated. Keep in mind, however, that less ATP may actually be generated.

As seen above, one molecule of glucose oxidized by aerobic respiration in prokaryotes yields the following:

Glycolysis

2 net ATP from substrate-level phosphorylation
2 NADH yields 6 ATP (assuming 3 ATP per NADH) by oxidative phosphorylation

Transition Reaction

2 NADH yields 6 ATP (assuming 3 ATP per NADH) by oxidative phosphorylation

Citric Acid Cycle

2 ATP from substrate-level phosphorylation
6 NADH yields 18 ATP (assuming 3 ATP per NADH) by oxidative phosphorylation
2 FADH2 yields 4 ATP (assuming 2 ATP per FADH2) by oxidative phosphorylation

Total Theoretical Maximum Number of ATP Generated per Glucose in Prokaryotes

38 ATP: 4 from substrate-level phosphorylation; 34 from oxidative phosphorylation.

In eukaryotic cells, the theoretical maximum yield of ATP generated per glucose is 36 to 38, 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.

Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.

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Last updated: Feb., 2020
Please send comments and inquiries to Dr. Gary Kaiser