There's a mansion on a hill
I've seen it numerous times
But,
I've never been inside

It's said to belong to an old woman
Who is very selective
in who enters her domain

Either you're an insignificant servant
And you slip inside
Through a back door

A tiny molecule diffusing
from high to low concentration

Or, you're a personal servant
Then, you gain special access
Still, through the back door

Water molecule
Diffusing through osmosis

After that are ordinary guests,
aided by the butler
through the front door

Facilitated diffusion
Molecules carried or channeled

And finally,
the VIP's  
Welcomed by a great procession
Through a special VIP door
People,
invited by the madam
with great effort

Active transport
From low to high concentration
Requiring added energy

But despite this selectivity
of who can and cannot enter
That old mansion on the hill
And the jobs it provides
Is essential to the livelihood
Of the people in this town

Just like the cell membrane to our bodies