2008-10-13-pruny-fingerWhy is it that your fingers get all wrinkly when you’re in the bath too long?

It’s a pretty simple little answer.  You know how a spongue gets bigger when it gets wet.  The outer layer of our skin is like that too — it soaks up a bunch of water and gets swollen.  But it can’t just get big and puffy because it’s firmly fastened to the layer of skin underneath.  But that extra surface area has to go somewhere, so it buckles up into folds, and wrinkles.  This happens after a long time in the bath because the skin oils (sebum), which usually protect your skin, eventually washes away, letting the water in to your skin.

As a commenter on the Wonderquest site put it,

My high school biology teacher explained it as: you have a size 3 finger and size 3 skin. After you have been in water, you still have a size 3 finger but now you have size 7 skin.

This is all in the epidermis, the outermost layer of our skin.  The stratum corneum is the part that is on the outside, and it’s got a bunch of dead keratin cells.  Keratin’s the stuff found in our hair and fingernails.  The dead keratin cells absorb water.  It happens mostly on our hands and feet because those parts of our body go through a lot of wear and tear, so they’ve got more dead keratin cells on them than, say, the sensitive and soft underside of our arm.

Great physiology and physics!

More on the Library of Congress Everyday Mysteries Site, and Wonderquest.


A reader to this blog posted this excellent question:

Hi. My kids heard about this “trick,” one which I assume has to do with muscles or musculo-skeletal mechanics (or crampinG) but for which I don’t really have a definite, or detailed explanation. If you extends your arms out, holding your fists side by side together as hard as possible for 30 seconds, when you try to pull them apart it feels as if they are stuck or magnetized together. What is a detailed explanation for that biological mechanics trick?

This is similar to when you stand in a doorway and push your arms against the door frame really hard for 30 seconds, and then your arms seem to float up on their own when you let go. However, this seemed really counterintuitive to me, because I would expect your perception to be the *opposite* of what your muscular effort was. For instance, in visual perception, if you stare at a blue dot, then you get a yellow afterimage because you saturate your blue receptors and so the opposite color comes out stronger. With muscles, I would expect that if I push my hands together, then when I let go it would feel as if they’re pulling away, because I’ve fatigued my “pushing” muscles. But that’s not the case.

So, I had to go to the experts. I got this great explanation back from Patrick, a biology teacher in California:

This happens because of how muscle cells actually cause contraction. There are proteins called myosin and actin arranged inside the cells with little “cross bridges” that reach out from the myosin filaments to grab and pull the actin filaments, sliding them over the myosin. They are arranged in multiple small clusters called sarcomeres. Each sarcomere is shortened by this pulling action, their collective shortening causing the “contraction” that we are familiar with. This action requires energy, but interestingly enough, the ATP energy molecule comes in at the end of a cross bridge’s action. The addition of the ATP molecule allows the cross bridge to release the actin so it can grab further along (kind of like a kid on the monkey bars). There’s other stuff involved (Calcium from the Sarcoplasmic Reticulum), but when you’ve been exerting your muscles as hard as you can for 30 seconds, you temporarily exhaust the ATP levels in those muscles. When your brain stop sending signals to those muscles, they stop actively contracting, but because of the lack of ATP, the myosin can’t release the Actin. When you try to separate your arms, using OTHER muscles, the first ones haven’t relaxed yet. Since you KNOW you aren’t contracting those first muscles (since you aren’t sending signals to do so), your brain interprets this as some external force, etc. Once those fatigued muscles have recovered their ATP, they’ll be fully relaxed.

This is the same effect in play that causes rigor mortis, the stiffening of a corpse as the individual cells run out of ATP and the myosin cross bridges start to get locked on to actin filaments.

thin_filamentIf you’re an educator (or anyone) and would like a poster that includes a great illustration of the actin/myosin, sarcomere, ATP, and ratcheting action as described by Patrick. The poster and classroom supplemental material are part of the Exploratorium’s digital library learning collection. You can download a PDF of the poster here.

You can download the Exploratorium Muscle Poster Activity Guide (including dissecting a muscle, and other muscle-related activities).

Hey guess what!  Science Teaching Tips was just highlighted in the Websights section of The Physics Teacher.   Woo hoo!

I’ve got a new episode of  the podcast posted — The drama of the immune system. This is one of the favorites of our group at the Teacher Institute, and teachers are always asking Tory to do this little bit of theater.  In this classroom activity, staff educator Tory Brady shows you how to make the immune system into a bit of drama.  This is especially good for K-8 students, to help them understand the roles that each of the main characters in the immune system (macrophages, white blood cells) play.  Heck, I found it helpful to make all that vocabulary into a little story.  Much more memorable.  Enjoy!

Several amazing photos of slime mold. Gorgeous!

Hemitrichia calyculata

Hemitrichia calyculata

Slime and mold are two words guaranteed to send a shiver down many a spine. However, plasmodial slime molds, fungus – like organisms with about eight hundred and fifty species worldwide – possess a strange beauty that you might not expect. Come and take a look at a few, thanks to some exquisite macro photography. You may never look at slime mold in the same light again.

via Swans on Tea

This blog has MOVED!  Please click here to read the whole post.

In this modern world, it gets tougher and tougher to figure out if someone is a Jim or a Jane. Whatever happened to the easy era of codpieces and corsets? Without those to fall back on, here’s a bit of physics you can use to figure it out in a pinch.

click here to read the whole post.

Kyle Flood from Victoria, British Columbia, Canada Hey all,

I just posted a new episode to my Science Teaching Tips podcast… a bunch of fun activities having to do with taste, and debunking some common myths about taste. Check it out — Episode 41. It’s a matter of taste.
This activity is from the Exploratorium’s Human Body Explorations: http://explo.stores.yahoo.net/humbodex.html

The Nose Knows activity: www.exploratorium.edu/ti/human_body/nose.html

More activities by Karen Kalumuck: http://philo.exploratorium.edu/karenk

Image from Kyle Flood from Victoria, British Columbia, Canada

Yesterday’s post in Engineering Life talks about the questions that are raised by genetic engineering, and whether we ought to be more worried than we are. I wanted to take the chance to point you to WNYC Radio Lab’s (So-called) Life episode, which talks about just this — what is life, what counts as natural? Brilliant radio. Listen to it!

Engineering Life’s blogger Carl Zimmer writes:

Imagine that mad scientists defied nature and violated the barriers between species. They injected human DNA into non-human creatures, altering their genomes into chimeras–unnatural fusions of man and beast. The goal of the scientists was to enslave these creatures, to exploit their cellular machinery for human gain. The creatures began to produce human proteins, so many of them that they become sick, in some cases even dying. The scientists harvest the proteins, and then, breaching the sacred barrier between species yet again, people injected the unnatural molecules into their own bodies.

This may sound like a futuristic nightmare, the kind that we will only experience if we neglect our moral compass and let science go berserk. But it is actually happening right now. Today millions of people with diabetes will inject themselves with insulin that was produced by E. coli.

The fact that no one is disturbed by this state of affairs says a lot.

But thirty years ago, the public rebelled against the same idea — of sticking genes into E. Coli so that it would produce human insulin. The project was condemned by many activitists. And yet…

We suffered no epidemic of diabetic comas, no cancer viruses spread by E. coli from host to host. None of the dire warnings about engineered E. coli, in fact, came to pass. It appears that the safeguards put in place were good enough, and that engineered E. coli could not compete with its wild cousins. Scientists continued to engineer E. coli, and today it can make all manner of substances, from blood-thinners to jet fuel

Today we’re capable of much more than this with genetic engineering, including the engineering of chimeras (beings created by mixing cells that originated from two different beings).

It’s not quite clear to me where the Engineering Life blogger (Carl Zimmer) stands on the issue, as he finishes with a less cautionary, more relativistic stance:

But it’s also important to bear in mind how easy it is to be terrified by a science-fiction caricature of what’s really going on in synthetic biology labs. We have a profound distrust of what seems unnatural, such as crossing species boundaries. Yet a casual glance at E. coli’s genome demonstrates that nature has been inserting foreign genes into it by the hundreds for millions of years. Our own genome is not immune from these violations. We carry the remains of thousands of viruses in our DNA, and most people on Earth may even carry genes inherited from another species of human–Neanderthals. We may be disgusted by the thought of violating species boundaries because of deeply ingrained instincts. But that disgust is an unreliable guide to the realities of biology, whether that biology is in E. coli or in ourselves.

I’m not really sure where I stand on the issue, to be honest. I don’t have an emotional reaction to mixing species — the “disgust” response outlined above. But I am cautious about introducing new creatures to our world, as the law of unintended consequences often seems to hold. But I feel somewhat powerless to make a stand one way or the other. I was not in favor of putting GMO corn out in the cornfields, and now look where we are — GMO corn has spread all over North America on the wind. Did we used to have more control over these new innovations, or has the public always felt unable to enter the debate about what is done in their world?

[Crossposted on Engineering Life]

[Picture: “The Young Family,” by Patrician Piccini (2002-3). Wikipedia]