Now, this is a guess, because I wasn’t there, and we don’t have any clear records of this
period of history, but at some point, in the early history of life on earth, it’s very
likely nothing could move.
Cells existed where they were, they got sloshed around through physical processes, and survived
when they ended up, by chance, in some place that had the necessary chemicals for them
to continue their lives and reproduce.
If that didn’t happen they just died.
But since food is never evenly distributed in our world, and also sometimes you are food,
being able to move is fantastic, and so it has been selected for pretty intensely so
that, now, in the microcosmos, almost everything moves.
But before we get to how they do it, we have to confront a reality.
You need to forget everything you know about swimming.
Things do not work the same when you are tiny.
You and I are constantly moving through a fluid, it’s just hard to feel it, air.
It’s barely there, until you stick your hand out of a car window on the highway.
Then you feel that fluid.
But for a tiny grain of pollen, air is a thick fluid that can keep it airborne for days.
Now sink that tiny grain of pollen into far more viscous water, and it’s basically in
a sticky, glue.
Imagine being completely submerged in honey and you have a vague idea of what it’s like
swimming in the microcosmos.
Which is why, in that microscopic world, when something stops moving, it just stops.
You and I, if we push through the water, we’ll coast for a bit as our momentum carries us
forward, but for a single cell, the viscosity of the water overcomes that inertia instantly.
This can lead to organisms looking as if they are moving somewhat unnaturally to our eyes, which is
we think why we’ve had several people have ask us if our footage is sped up.
But, no, unless it says so on-screen, all of our clips are in real time.
These little folks can just move!
And they need to move...to search for food, to avoid predators, to get into or out of
sunlight, to move toward chemicals they need, or away from chemicals that poison them.
And the wild thing is, with all of the diversity of microscopic life single celled eukaryotes
basically all move around in three or maybe four different ways.
How organisms move is so important, and so obvious when they’re observed, that protozoans,
which is the general name for single-celled eukaryotes, are actually loosely classified
by their style of movement.
We’ve got ciliates, which move using cilia, flagellates, which move using flagella, and
amoeboids, which move using pseudopodia.
Oh...and then there’s Sporozoa…which we can’t show you a picture of, for reasons
that will become clear.
Sporozoa almost never move, and when they do it’s in a very weird limited way.
And the reason we can’t show you a picture of one is that they are, every one them, parasites.
Many of them live inside of, and cause disease in animals so we choose to, y’know, not
keep them around.
But, overall, among eukaryotes we’ve got cilia, flagella, and pseudopodia.
And we’re going to start with the shapeshifters of the Microcosmos, amoeba!
These cells can change their shape as they wish, allowing them to extend parts of themselves
into features called pseudopodia.
These extend in the direction they want to move, and then solidify as the cell moves
into the newly occupied space.
Pseudopodia is latin for, basically, false feet, and these extensions which are half
reaching arm, half cell body are how these crawlers move and hunt.
The secret here is that the cytoplasm of an amoeba can be easily changed from a fluid
state into a solid state and back again.
When an amoeba moves, liquid cytoplasm flows through the center, up to the tip of the pseudopod
and then gushes to the sides where it becomes more of a solid gel allowing the cell to lock
into its new location.
Predatory amoebae create these extensions in all directions to trap prey between them,
once the prey organism is surrounded by the pseudopodia the amoeba simply swallows it.
The two other main mechanisms of eukaryotic movement, cilia and flagella, are more common,
and they’re actually really similar, both functionally and structurally at the molecular
level.
Though they look like rods sticking through the surface, they’re actually extensions
of the cell membrane wrapped around rigid tubes called microtubules that are anchored
in place.
However, hydrodynamically they’re very different.
There are usually an easily countable number of flagella per cell whereas ciliated organisms
have huge numbers of cilia.
The word cilia actually comes from the latin word for eyelash, which makes sense as you
can sometimes see them ringing a cell as our eyelashes ring our eyes.
But, in those cases, the cilia actually often cover the entire cell, they’re just harder
to spot against the background of all the stuff on the inside.
Cilia move organisms by beating in waves over their surface.
A cilium, which is the singular of cilia because...latin...has only two possible positions.
During the effective stroke, the cilium sticks out perpendicular to the cell, and during
recovery it folds back towards the cell’s surface.
In most ciliates the cilia are arranged in rows and the cilia of a row don't move all
together at once; when some of the cilia are in the effective stroke some are in recovery
stroke, and in some ciliates you can see the "wave" pattern this creates.
These waves are basically grabbing onto the sticky viscous water, yanking the cell through.
One added complication here, some ciliates have dense bundles of cilia called cirri.
Cirri are used to "walk" on a solid substrate rather than pulling the organism through the
water.
Though it’s the same structure, it’s a bit of a different system of movement.
Now on to eukaryotic flagella.
The motion of most flagella is characterized by this long, wave-like beating pattern beginning
from the base of the flagella and moving out to the tip.
This is Phacus longicauda, we recorded this footage in phase contrast which makes the
transparent parts of the cell more visible so you can see the beating flagellum better.
Can you see it at the tip of the cell?
Phacus longicauda is a photosynthetic flagellate...it uses sunlight to produce sugar.
You can see the chloroplasts in the cell like green peas.
And the round transparent part at the middle of the cell is a starch-like carbohydrate
storage unit called paramylon.
Now, of course, as always, the deeper you look the more confusing and amazing things
get.
Diatoms are sometimes completely non-motile, but some species can also move by excreting
mucus through their cell, slowly propelling themselves over a substrate.
And finally, we said nothing in this entire video about the marvelous movements of prokaryotes
like bacteria.
And they have systems every bit, if not more ingenious than their more complex eukaryotic
friends.
But they’re also tiny...so tiny that these structures cannot be observed by microscopes
like the ones we use.
So we just have to be content knowing that they use structures that function similarly
to the structures eukaryotes use, but that are made of completely different stuff.
And all of these chemical structures, eukaryotic and prokaryotic, so complex and marvelous,
were selected for through billions of years of evolution.
And almost shockingly, just a few extremely effective systems of locomotion were uncovered.
The result...we can watch them, these little balls of chemicals working to get what they
need...they are each a little soup that wants...look at them wanting.
Thank you for coming on this journey with us as we explore the unseen world that surrounds
us.
If you want to see more from our master of microscopes, James, check out Jam and Germs
on Instagram.
And if you want more from us, we’ve got a few videos over at youtube.com/microcosmos
and we put out a new one every week
We’ve been doing a pretty good job of keeping on top of it.
So, thanks for liking what we do.