Detailed Accounting

Each NADH formed in the mitochondrion donates two electrons to the electron transport system at the first proton pump (fig. 5.10). The electrons are then passed to the second and third proton pumps, activating each of them in turn until the two electrons are ultimately passed to oxygen. The first and second pumps transport four protons each, and the third pump transports two protons, for a total of ten. Dividing ten protons by the four it takes to produce an ATP gives 2.5 ATP that are produced for every pair of electrons donated by an NADH. (There is no such thing as half an ATP; the decimal fraction simply indicates an average.)

Three molecules of NADH are formed with each Krebs cycle, and 1 NADH is also produced when pyruvate is converted into acetyl CoA (see fig. 5.6). Starting from one glucose, two Krebs cycles (producing 6 NADH) and two pyruvates converted to acetyl CoA (producing 2 NADH) yield 8 NADH. Multiplying by 2.5 ATP per NADH gives 20 ATP.

Electrons from FADH2 are donated later in the electron-transport system than those donated by NADH; consequently, these electrons activate only the second and third proton pumps. Since the first proton pump is bypassed, the electrons passed from FADH2 result in the pumping of only six protons (four by the second pump and two by the third pump). Since 1 ATP is

Table 5.1 ATP Yield per Glucose in Aerobic Respiration

Phases of Reduced Respiration ATP Made Directly Coenzymes

ATP Made by

Oxidative Phosphorylation*

Glucose to pyruvate (in cytoplasm)

Pyruvate to acetyl CoA (x 2 because one glucose yields 2 pyruvates) Krebs cycle (x 2 because one glucose yields 2 Krebs cycles)

2 ATP (net gain) None

1 ATP (x 2) = 2 ATP

2 NADH, but usually goes into mitochondria as 2 FADH2

3 NADH (x 2) 1 FADH2 (x 2)

2.5 ATP per NADH x 3 = 7.5 ATP x 2 = 15 ATP 1.5 ATP per FADH2 x 2 = 3 ATP

Subtotals

4 ATP

26 ATP

Grand Total

30 ATP

*Theoretical estimates of ATP production from oxidative phosphorylation are 2 ATP per FADH2 and 3 ATP per NADH. If these numbers are used, a total of 32 ATP will be calculated as arising from oxidative phosphorylation. This is increased to 34 ATP if the cytoplasmic NADH remains as NADH when it is shuttled into the mitochondrion. Adding these numbers to the ATP made directly gives a total of 38 ATP produced from a molecule of glucose. Estimates of the actual number of ATP obtained by the cell are lower because of the costs of transporting ATP out of the mitochondria.

*Theoretical estimates of ATP production from oxidative phosphorylation are 2 ATP per FADH2 and 3 ATP per NADH. If these numbers are used, a total of 32 ATP will be calculated as arising from oxidative phosphorylation. This is increased to 34 ATP if the cytoplasmic NADH remains as NADH when it is shuttled into the mitochondrion. Adding these numbers to the ATP made directly gives a total of 38 ATP produced from a molecule of glucose. Estimates of the actual number of ATP obtained by the cell are lower because of the costs of transporting ATP out of the mitochondria.

produced for every four protons pumped, electrons derived from FADH2 result in the formation of 6 + 4 = 1.5 ATP. Each Krebs cycle produces 1 FADH2 and we get two Krebs cycles from one glucose, so there are 2 FADH2 that give 2 x 1.5 ATP = 3 ATP.

The 23 ATP subtotal from oxidative phosphorylation we have at this point includes only the NADH and FADH2 produced in the mitochondrion. Remember that glycolysis, which occurs in the cytoplasm, also produces 2 NADH. These cytoplasmic NADH cannot directly enter the mitochondrion, but there is a process by which their electrons can be "shuttled" in. The net effect of the most common shuttle is that a molecule of NADH in the cytoplasm is translated into a molecule of FADH2 in the mitochondrion. The 2 NADH produced in glycolysis, therefore, usually become 2 FADH2 and yield 2 x 1.5 ATP = 3 ATP by oxidative phosphorylation. (An alternative pathway, where the cytoplasmic NADH is transformed into mitochondrial NADH and produces 2 x 2.5 ATP = 5 ATP, is less common; however, this is the dominant pathway in the liver and heart, which are metabolically highly active.)

We now have a total 26 ATP (or, less commonly, 28 ATP) produced by oxidative phosphorylation from glucose. We can add the 2 ATP made by direct (substrate-level) phosphorylation in glycolysis and the 2 ATP made directly by the two Krebs cycles to give a grand total of 30 ATP (or, less commonly, 32 ATP) produced by the aerobic respiration of glucose (table 5.1).

Glycogen

Glucose 1-phosphate

Glycogen

Glucose ^_____ Glucose 6-phosphate

Fructose 6-phosphate

Fructose 1,6-biphosphate

Fat

P-—

Glycerol r___* 3-Phosphoglyceraldehyde

Fatty acids

Pyruvic acid i

Acetyl CoA

Test Yourself Before You Continue

1. Compare the fate of pyruvic acid in aerobic respiration with its fate in anaerobic respiration.

2. Draw a simplified Krebs cycle using C2 for acetic acid, C4 for oxaloacetic acid, C5 for alpha-ketoglutaric acid, and C6 for citric acid. List the high-energy products that are produced at each turn of the Krebs cycle.

3. Using a diagram, show how electrons from NADH and FADH2 are transferred by the cytochromes. Represent the oxidized and reduced forms of the cytochromes with Fe3+ and Fe2+, respectively.

4. Explain how ATP molecules are produced in the process of oxidative phosphorylation.

5. Explain why a cell gets an average of 2.5 ATP from NADH in the mitochondrion and 1.5 ATP from FADH2.

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  • quarto
    Why nadh produce 3 atp n gives 2.5?
    8 years ago

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