Question

1.

a) Advantages of supercomplexes in electron transfer. There is increasing evidence that mitochondrial complexes I, II, III, and IV are included in larger supercomplexes. What could be the advantage of having all four complexes in a super complex?

b) Why is the cellular concentration of ATP kept stable? Why can't the cell simply be storing ATP?

c) Why is it physiologically advantageous that the heart muscle has more mitochondria than liver cells?

Answer 1

(a)
In the mitochondrial inner membrane the respiratory enzymes associate to form supramolecular assemblies known as supercomplexes.
mitochondrial respiratory chain has four major respiratory complexes, complexes I, II, III and IV. They catalyse the step-wise transfer of electrons from NADH to O2 .
They are: NADH–ubiquinone oxidoreductase (commonly known as complex I), succinate–ubiquinone oxidoreductase (complex II), ubiquinol–cytochrome c oxidoreductase (cytochrome bc1 complex or complex III), cytochrome c–O2 oxidoreductase (complex IV).
The oxidation of NADH by O2 releases a lot of energy, which is trapped by the complexes. This energy is used in transporting protons across the inner mitochondrial membrane. These protons flow back across the membrane through the ATP synthase rotor, turning it to generate ATP.

The advantages of super complexes are: substrate-channeling, catalytic enhancement, sequestration of reactive intermediates , stabilization of protein complexes, increasing the capacity of the inner mitochondrial membrane for protein insertion, and generating mitochondrial cristae morphology.

(b)
ATP usually reaches high concentrations within cells, in the millimolar range. Nonetheless, because of the high rate of ATP-dependent processes, together with its low stability in water, ATP content could quickly be depleted if it were not immediately replenished by glycolysis and oxidative phosphorylation. Hence, ATP cannot be stored easily within cells, and the storage of carbon sources for ATP production (such as triglycerides or glycogen) is the best choice for energy maintenance.

(c)
In heart muscle cells about 40% of the cytoplasmic space is taken up by mitochondria. In liver cells the figure is about 20-25% with 1000 to 2000 mitochondria per cell. Thus, heart muscle cells have more mitochondria than liver cells. It is because of the maintenance of the heart beat in the heart cell requires large amount of energy throughout the life.
Hence, heart muscle cells have mitochondria than liver cells.