Motor Protein moving stuff about in the cell (Illustration from The Art of the Cell by John Liebler)

Dear Readers, what, you may ask, is this tiny little orange guy doing? He looks rather like Atlas carrying the world on his shoulders. Well, this week I have mostly been doing Cell Biology for my Open University degree, and in particular we’ve been studying the ways in which ‘stuff’ is moved about inside the cell. I had never really thought about what goes on in our cells, but of course each is a tiny world which is constantly demanding molecules from outside the cell, transporting stuff out into our bloodstream and moving all kinds of things around within the cell itself.

You might think (as I did) that they inside of the cell is just a mass of cytoplasm with things floating about in it, and indeed for the simplest organisms, such as bacteria, that’s pretty much what happens. However, in more complex cells (such as ours) there is a whole matrix of filaments and microtubules known as the cytoskeleton that helps the cell keep its shape, and provides ‘roads’ for the transport of different materials.

Enter the motor proteins. By a complex series of chemical reactions (you might remember Adenosine Triphosphate from your GCSE Biology? Well, here it is again), the proteins literally ‘walk’ along the cytoskeleton, carrying their load. Sometimes they’re carrying a vesicle (like the one in the illustration above), which is a kind of balloon full of whatever the cell needs or wants to get rid of. Sometimes, it’s an organelle like a mitochondria that needs to be somewhere else. Each motor protein only moves in one direction, so there are different kinds according to whether they’re coming or going.

Motor proteins are also responsible for the contraction of our muscles, including our gut during peristalsis, and our hearts. They only stop when their supply of ATP runs out, because part of the process of contraction is knowing when to stop, and ATP helps the motor proteins to relax. In the absence of ATP they just continue to contract, and this, dear Readers, is why rigor mortis occurs after death.

When scientists watch mitochondria moving, they seem to do it in little jumps as a result of each of the ‘steps’ made by the motor protein – this is known as saltatory movement, from the Latin saltare – to jump.

Several scientists have made animations of the process. While it’s clear that this is not an exact vision of what happens in a cell, I think it’s close enough. It looks like a cross between Blade Runner and a tired old cartoon character. And to think that this is happening in my cells as I sit here typing away, contracting my finger muscles and using up ATP, feels truly astonishing.

Have a look here.