线粒体：小圆形至棒状的细胞器，见于细胞质内，裹双层膜，层间有隙，内层深入细胞器内形成一系列突起的嵴。线粒体可氧化食物产能，以建立离子梯度和合成三磷酸腺甙(ATP)，线粒体含三羧酸循环、脂肪酸循环、呼吸链所需的酶类，亦具RNA、DNA，并能独自复制和提供密码以合成某些蛋白质
The origin of mitochondria. It has been suggested that these organelles originated as bacteria that were
engulfed by an ancestral eukaryotic cell and survived inside it, living in symbiosis with their host.