许多药用植物的疗效来自于它们所含的特定生物碱。

The healing effects of many medicinal plants come from specific ___alkaloids they contain.

咖啡因是一种生物碱，能帮助植物抵御昆虫的侵害。

Caffeine is an ___alkaloid that helps plants defend against insects.

奎宁是一种从金鸡纳树皮中提取的重要生物碱，用于治疗疟疾。

Quinine is an important ___alkaloid extracted from cinchona bark, used to treat malaria.