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. Thanks for joining us for this SciShow Dose.

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