The maximum yield of ATP per molecule of glucose in eukaryotic cells is 36 or 38 ATP, depending on cell type and conditions. In prokaryotic cells, the maximum yield is 38 ATP

Question

a. The maximum yield of ATP in eukaryotic heart muscle cells is 38, but in skeletal muscle cells the yield is only 36. What is the basis for this difference?
b. Why is the maximal ATP yield for prokaryotic cells 38 and not 36?
c. Why is the calculated ATP yield referred to as "Maximal Theoretical Yield" in eukaryotic and in prokaryotic cells?

What will be an ideal response?

Answer 1

Answer:
a. There is no transport protein in the inner mitochondrial membrane to move the two NADH molecules reduced during glycolysis through the inner membrane into the matrix where the electrons can be donated to ETC complex I. One of two shuttle mechanisms moves electrons across the inner membrane. In cells such as liver, kidney, and heart cells, the malate-aspartate shuttle moves electrons across the membrane and reduces NADH in the mitochondrial matrix. Matrix NADH electrons can be donated to ETC complex I driving the synthesis of 3 ATPs. In cells such as skeletal muscle, brain, and cells with low cytoplasmic NADH, the NADH electrons are transported by the glycerol phosphate electron shuttle, which reduces FADH2 in the matrix. Matrix FADH2 electrons are donated to ETC complex II and drive the synthesis of only 2 ATPs. The difference of 1 ATP for each of two cytosolic NADHs is the difference between 36 and 38 maximal ATPs.
b. Prokaryotes do not have inner membranes and therefore do not lose potential for producing 2 ATPs by glycolysis NADH shuttling electrons across a membrane.
c. In eukaryotic cell mitochondria, transport of molecules across inner membranes can decrease the proton motive force; the pyruvate carrier cotransports by a H+ and pyruvate into the matrix and the phosphate carrier couples the inward movement of phosphate with the outward movement of hydroxyl ions, which are neutralized by protons in the intermembrane space. Pmf decrease would decrease the actual ATP yield.
In prokaryotes, especially anaerobic bacteria producing ATP by fermentation, the pmf often is used for purposes other than ATP synthesis, such as transport of ions, substrates, intermediates, and metabolites across the cell membrane.