[Intro] If you’ve ever taken an Intro to Biology class,
you might have noticed that there are some pretty big but pretty basic things about your
own cells that never really get explained. Or at least not explained well. Like, what
makes your cells divide when they copy themselves, and how does stuff get carried from one part
of cell to another, and who’s doing all the carrying?
The answer to all of these questions and more is a feat of biological engineering in which
a tiny peg-legged pirate walks microscopic planks to carry cargo all around your cells.
Sorta. The pirates are two-legged molecules called
motor proteins. Their job is to carry cellular material wherever it’s needed, whether it’s
food or signaling molecules or genetic information. And to get around, they use your cell’s internal
highway system, known as the microtubule cytoskeleton. This network of tiny tubes is what gives each
of your cells its unique structure. It’s made up of a protein called tubulin, and depending
on how it’s arranged, it can form a cylindrical stomach cell or a spiky nerve cell or a squashed
cell of connective tissue. But day-to-day, second-by-second, its most
important role is serving as a kind of catwalk for your motor proteins. A particularly delightful
kind of motor protein is kinesin, which looks kind of like a little buccaneer swaggering
around with a giant beach ball head. Typical kinesins have a head-like region up top, to
hold their cargo, followed by a coiled middle region and two feet that literally walk along
the microtubule. In order to move, each foot uses chemical
energy in the form of power-packed molecules floating around in the cell called ATP. When
one foot grabs an ATP molecule that’s passing by, the foot changes shape and swings around
the central coil, flinging itself forward. The other foot remains stuck to the microtubule,
so it won’t fall off, and then it gets a zap of energy from another ATP molecule and takes
a step, and the whole motor protein lurches forward.
A typical kinesin motor can travel this way at the rate of about one micrometer per second.
That’s about 0.000002 miles per hour. But those motors don’t have to move very fast,
because cells aren’t very big, right? Well, most of them aren’t. But the single nerve
cell that runs the length of your leg can be up to a meter long. This is the nerve cell
that lets you move your leg, and it’s the job of the kinesins to constantly supply the
very end of that cell with neurotransmitters, or messenger chemicals, so it can communicate
with your muscles. A motor protein moving at an average pace
would take 11 and a half days to make this meter long journey from the cell’s nucleus,
where the cell’s chemicals are made, all the way to the ends. But these kinesins can do
it in as little as two or three days. Thanks to them, your legs are always ready to run
when you need ’em. And not only can motor proteins run along
microtubules, they can also, like, jog in place as if they were on tiny treadmills,
and this is how your cells divide: by essentially running in place, these proteins cause the
whole microtubule to roll along beneath them, which helps wrench apart the splitting cells.
This is also how the chromosomes in the center of the newly dividing cell get pulled toward
either end. Without these motor proteins running in place, your genetic material wouldn’t be
distributed correctly to the trillions of new cells that you make every day.
So even though your textbook or your teacher probably didn’t tell you, now you know that
most of your cell’s important functions happen because of tiny swaggering bobble-headed pirates.
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