One of the biggest challenges in biology today is to explain the structure of cristae, the inner membranes of mitochondria. An explanation in this case is a set of principles to predict what form the cristae will take after basic metabolic manipulations of the environment the mitochondria are in. These principles will therefore be a description of the true function of mitochondria, something that has so far only been scarcely imagined.
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Source: Phys.org