In 1977, the physicist Edward Purcell
calculated that if you pusha bacteria and then let go,
it will stop in abouta millionth of a second.
In that time, it will have traveled less
than the width of a single atom.
The same holds true for a spermand many other microbes.
It all has to do with being really small.
Microscopic creatures inhabita world alien to us,
where making it through an inch of water is an incredible endeavor.
But why does size matterso much for a swimmer?
What makes the world of a spermso fundamentally different
from that of a sperm whale?
To find out, we need to dive into the physics of fluids.
Here’s a way to think about it.
Imagine you are swimming in a pool.
It’s you and a whole bunchof water molecules.
Water molecules outnumber youa thousand trillion trillion to one. So,
pushing past themwith your gigantic body is easy,
but if you were really small,
say you were about the sizeof a water molecule,
all of a sudden, it’s like you’re swimming
in a pool of people.
Rather than simply swishing byall the teeny, tiny molecules,
now every single water molecule is
like another person you have to push past
to get anywhere.
In 1883, the physicist Osborne Reynolds
figured out that there isone simple number
that can predict how a fluid will behave.
It’s called the Reynolds number,
and it depends on simple properties
like the size of the swimmer,
its speed, the density of the fluid,
and the stickiness,or the viscosity, of the fluid.
What this means is that creaturesof very different sizes
inhabit vastly different worlds.
For example, because of its huge size,
a sperm whale inhabits the large Reynolds number world.
If it flaps its tail once,
it can coast ahead for an incredible distance. Meanwhile,
sperm livein a low Reynolds number world.
If a sperm were to stop flapping its tail,
it wouldn’t even coast past a single atom.
To imagine what it wouldfeel like to be a sperm,
you need to bring yourself downto its Reynolds number.
Picture yourself in a tub of molasseswith your arms moving
about as slow as the minutehand of a clock,
and you’d have a pretty good idea
of what a sperm is up against. So,
how do microbesmanage to get anywhere? Well,
many don’t bother swimming at all.
They just let the food drift to them.
This is somewhat like a lazy cow that waits
for the grass under its mouth to grow back.
But many microbes do swim,
and this is where those incredible adaptations come in.
One trick they can use is to deform the shape of their paddle.
By cleverly flexing their paddle
to create more drag on the power stroke
than on the recovery stroke,
single-celled organisms like paramecia
manage to inch their waythrough the crowd of water molecules.
But there’s an even moreingenious solution
arrived at by bacteria and sperm.
Instead of waggingtheir paddles back and forth,
they wind them like a cork screw.
Just as a cork screw
on a wine bottle converts winding motion into forward motion,
these tiny creaturesspin their helical tails
to push themselves forward in a world where water feels as thick as cork.
Other strategies are even stranger.
Some bacteria take Batman’s approach.
They use grappling hooksto pull themselves along.
They can even use this grappling hook
like a sling shot and fling themselves forward.
Others use chemical engineering.
H. pylori lives onlyin the slimy, acidic mucus
inside our stomachs.
It releases a chemicalthat thins out the surrounding mucus,
allowing it to glide through slime.
Maybe it’s no surprise that these guys are also responsible for stomach ulcers. So,
when you look really closely
at our bodies and the world around us,
you can see all sorts
of tiny creatures finding clever ways to get around in a sticky situation.
Without these adaptations,bacteria would never find their hosts,
and sperms would nevermake it to their eggs,
which means you would neverget stomach ulcers,
but you would also never be bornin the first place.