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If you’ve taken a biology class, at some point you likely had to sit down to a picture of a cell.
It was vaguely oval shaped and filled with circles and squiggly lines. And your task
was to look at those circles and lines and assign them labels like “vesicle” or “nucleus.”
And this exercise has its benefits. But at the same time, the picture it paints of a cell is—by
necessity—static and uniform. The reality is that cells may be full of analogous parts,
but their lives and physical appearance can appear vastly different when compared to each other.
The bodies of multicellular organisms like us are built on that principle, with a
diversity of cells dedicated to different tasks. But we can see it even more starkly in the lives
of single-celled eukaryotes. These are organisms who have managed to make an entire way of life
out of a single cell, taking on many different forms to do so. Even as the
inner parts remain approximately the same, the final results can be wildly different,
whether you’re looking at the flexible form of an amoeba or the determined shape of a diatom.
Here on Journey to the Microcosmos, we often focus on the organisms—on
their fascinating physiology and lives as they navigate the world around them. But
what about the even smaller world inside of them? If you think of a cell as its own little world, a
little society built on chemistry and biology, one of its most important tasks is establishing its
boundaries—separating and protecting the inside of the cell while still leaving it just open enough
to bring in essential nutrients from the outside. The essential component here is the plasma
membrane, a double layer of fatty acids that surrounds the cell and allows some things to pass
while keeping the cell protected. But for different protists, the cell
membrane may not be quite enough to establish the kind of boundaries it needs. Ciliates like
these pelagotrix encase their plasma membrane within a larger structure called the pellicle,
which has structures to define the organism’s shape and anchor their many hairy cilia.
Other organisms supplement their plasma membrane with another exterior wall, like this colony of
green algae called Pediastrum. Each of these cells is encased in a protective cell wall.
What comes next after the boundaries have been set is, well, everything. It is one
big soup of life, organelles and nutrients swimming in the watery broth of the cytoplasm.
And just like a good soup, there is a delicate balance to the cytoplasm. When you’re living in
a watery environment with a thin membrane separating you from the outside world,
water can seep through. A little bit of that water might be fine, but too much can be dangerous.
We’ve even seen microbes burst open and die when they’ve been pushed past their watery limits.
But microbes are not passive in the face of this danger. If you look at the edge of this Frontonia
vernalis, you can see its solution at work. There are canals all pointed towards the same
clear, circular region. That area is not just some blank space. It is a contractile vacuole,
swelling up with water until it is full. And when it is full, you can see it contract
and push the water out of the cell through tiny pores, leaving the world within balanced.
And as we travel further inside this world, there is a flurry of activity, not all of which is
visible to us. Much of that activity is powered by the mitochondria, though unfortunately they
are a bit too small for us to easily see. But we think that the thin rods inside this diatom
might be the famous powerhouses of the cell. The chemicals that mitochondria use to generate
power come from food. And of course, microbes have found many creative ways to obtain food.
Some organisms make their own, turning light into sugar through clever chemistry and chloroplasts.
And even though chloroplasts can come in different shapes and sizes,
they’re usually easy enough to identify through their most obvious trait: their vibrant green
color, derived from the light-absorbing chlorophyll that powers the chloroplast.
Except…of course biology loves an exception. There are examples of other colors in the
chloroplast realm, like in red algae that have other pigments that stain their chloroplasts red.
Whatever the color, these are organisms whose inner worlds can sustain themselves,
as long as they get enough light. But other organisms have to go out and seek their sustenance
by finding other organisms to eat. And if that sounds like a bit much to you, I will remind you,
you are one of them. So, humans and other organisms like us have to find a way to
introduce that food into their bodies. In some organisms, that gate is called
the cytostome. And in this glaucoma, the cytostome takes the form of an oval,
flapping as the organism turns around and assesses a possible meal.
But swallowing food is only one step in the process. After it enters the organism’s body, the
food gets wrapped up in its own little membrane like a present—a present stuffed with digestive
enzymes that will tear its contents apart. And just as with water, the cell doesn’t
necessarily want to keep all of this digested food around for the rest of its life. So what doesn’t
get used gets pooped out through the cytoproct, and if the mitochondria is the powerhouse of the
cell, the cytoproct is the butthole of the cell. And then, of course, there are other specialized
parts that help different organisms manage the specific challenges of their lifestyle. Like
take this Loxodes, which prefers to live in areas of water that are lower in oxygen—areas
that are, to put it simply, down. Down the water column, into the depths.
But to get there, the Loxodes needs to figure out just where “down” is. So the organism relies on
its Muller vesicles, a simple structure, it’s an organelle that allows it to detect gravity.
The vesicle holds a single mineral granule attached to a cilium. And as gravity pulls on the
mineral, the mineral pulls on the cilium, pointing the Loxodes downward like an inner compass.
Now if we take a step back and survey these little worlds inside of organisms,
it is remarkable to realize that these organisms have built themselves, assembled and sustained
by their own reactions. They have gathered and made their resources, and from it comes
all of these different parts that just seem to work. There are so many bits of chemistry and
probability that have come together to make each of these tiny remarkable lives happen.
And at the center of so much of that assembly is the nucleus, those round organelles that hold
the DNA that describes the organism’s existence. Nuclei are universal among eukaryotes, though of
course ciliates in all of their contrarian glory have also managed to make their own habits out
of familiar entities. Ciliates are not content to just have one type of nucleus, no they have two.
The larger nucleus is called the macronucleus, and it is responsible for the day-to-day genetic needs
of the organism. Whereas the smaller nuclei are the micronuclei, and their primary role is to pass
on genetic information during sexual reproduction. It feels right to end with the nucleus,
the part of the cell that ensures not just the life of this organism itself, but also of its
progeny. It is a map that is passed on and shared, promising the creation of new worlds to come.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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