From Capra (2007):
When we walk out into nature, living systems are what we see first. First, every living organism, from the smallest bacterium to all the varieties of plants and animals (including humans), is a living system. Next, the parts of living systems are themselves living systems. A leaf is a living system. Every cell in our bodies is a living system. Finally, communities of organisms, including both ecosystems and human social systems sun as families, schools, and other human communities, are living systems.