Because cells on their own are alive,

因为细胞可以独立生存
they can do the things that we associate with life,

可以像一个生命体那样
they replicate, they reproduce,

复制繁殖
they metabolize, they can move, they can grow,

代谢运动生长
and they can use nutrients and make wastes,

能利用营养物质并制造废物
and they work cooperatively in order to perform some function.

它们分工协作以实现某项机能
This is a picture that I showed you before,

这张图我给你们看过
but basically tells you something that you already know,

简单的说明了一些常识
that humans are made up of organ systems,

即人体由器官系统构成
organs and organ systems, those organs are composed of tissues.

器官和器官系统其中器官由组织组成
What I didn't mention before was the four major types of tissues

但我之前没讲过
which make up the organs of our body:

构成器官的四种主要组织类型