Power, Sex, Suicide Mitochondria and the meaning of life.pdf by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 0192804812
Published: 2008-02-18T19:00:04+00:00

182 Size and the Ramp of Ascending Complexity endurance raises the mitochondrial power of muscle cells, leading to a faster maximum metabolic rate; but what about the second part? Why is there a link between maximal and resting metabolic rate? The reason is not clear, insofar as none of the possible explanations has been proved. Even so, there is a good intuitive reason to expect a connection. I mentioned that lizards may often take several hours to recover from exhaustion, even after a few minutes of vigorous exertion. Such a slow recovery is less dependent on muscles than on organs, such as the liver and kidneys, which process the metabolic waste and other breakdown products of vigorous exercise. The rate at which these organs operate depends on their own metabolic power, which in turn depends on their mitochondrial power—the more mitochondria, the faster the recovery.

Presumably the advantages of endurance also apply to recovery time: given the eightfold rise in aerobic power of mammalian muscles, if there were no compensating changes in organ function it would take a whole day, rather than a few hours to recover from exercise.

Unlike muscles, organs are not faced with a dilemma of space allocation—

while the density of mitochondria doesn’t change with size in muscles, it does in the organs. As animals get bigger, the power laws that we discussed in the last chapter mean that their organs become more sparsely populated with mitochondria. This is an opportunity in waiting. For the organs of a large animal to gain power, the tissue architecture doesn’t need to be restructured as it does in muscle: it can simply be repopulated with mitochondria. This opportunity seems to have given rise to endothermy. In their classic comparative studies, Hulbert and Else showed that the organs of mammals contain five times as many mitochondria as an equivalent lizard, but in all other respects the mitochondria are no different. For example, the efficiency of their respiratory enzymes is exactly the same. In other words, for every hard-won increment in muscle power, it’s relatively simple to counterbalance the new power by filling up the half-empty organs with a few more mitochondria, so as to ensure speedy recovery from aerobic exertion. The important point is that the function of organs like the liver is coupled to muscle demand, and not with the need to keep warm.

Proton leak

But there is a diabolical catch. We have seen that the muscles contribute little to the resting metabolic rate: the danger of oxygen toxicity means that blood is diverted away from the muscles and into the organs, where there are relatively few mitochondria to cause damage. So what might have happened in the first mammals? They had extra mitochondria in their organs to compensate for their higher aerobic capacity, but nowhere to divert the blood, which had to pass through either the organs or the muscles.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.