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
I tried another science analogy one. Personally I like my amino acid and fats ones better but I don't know. We'll see.