graphic

Bigger animals have lower metabolic rates (B).

SAVAGE/NAS

Cells use two basic strategies to adapt to the size of the organism in which they reside, say Van Savage (Harvard Medical School, Boston, MA), Geoffrey West (Santa Fe Institute, Santa Fe, NM), and colleagues. Depending on how often they divide, comparable cells in a mouse and an elephant differ in either metabolic rate or cell volume, but usually not both.

The need for such adaptation stems from simple geometry. As body volume increases, surface area increases more slowly. So an elephant radiates and loses less energy per gram than a mouse and thus requires less replacement energy per gram. Differences in organism shape and capillary density alter the exact numbers but not the principle.

Thus, what Savage and others call the “cell is a cell is a cell” theory cannot hold. With energy consumed per unit volume decreasing with increasing animal size, average cell volume and average cellular metabolic rate cannot both remain constant.

There are at least two possible solutions. Under theory one, average cell volume stays constant but each cell in the larger organism consumes less energy. Theory two keeps energy consumption per cell constant but the cells in the larger organism are larger so that there are fewer of them per unit volume.

Digging through the literature, the researchers found that rapidly dividing cell types were a close fit to theory one. Slower metabolism in these cells in larger organisms may explain why these animals accumulate damage and age more slowly.

Cells such as neurons and adipocytes, however, divide infrequently and must maintain their structural integrity using a constant energy supply. Their variation fit theory two.

The findings reflect the extent to which organisms also affect cells, says Savage. “For a cell type to exist in an organism it has to adapt to an organism,” he says. He plans to study the phenomenon in yeast that can be manipulated to grow at different sizes and metabolic rates.

Reference:

Savage, V.M., et al.
2007
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Proc. Natl. Acad. Sci. USA.
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