This is caused by metabolic reactions within the body.

这是由体内代谢反应引起的。

A cell's mitochondria are its power packs, extracting energy from glucose and transferring it to a molecule called ATP, which then drives metabolic reactions.

细胞的线粒体是它的能量包，从葡萄糖中提取能量并将其转移到称为 ATP 的分子， 然后驱动代谢反应。

Of course, at this point, O2 is poisonous to most organisms, but over time, cells evolve to tolerate it, and because metabolic reactions using O2 were more efficient, cells could grow larger.

当然，在这一点上，氧气对大多数生物体都是有毒的，但随着时间的推移，细胞进化出了对氧气的耐受能力，而且由于使用氧气的代谢反应更有效，细胞可以生长得更大。