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PROFESSOR: So as I mentioned before, we're going to see two devices threaded throughout all of these tutorials. Take a look at this music box for our first case study. So I place it on the glass of the scanner, and it looks pretty good. But as I look carefully. Which I hope you will do when you try this sort of stuff with your work, a couple of things that I'm not thrilled with. First of all, we're seeing some writing on the barrel. Looking closer, it appears to be numbers and letters. That's not great. Our eyes go straight to letters and numbers, and they basically attract our eye and actually become a distraction from the image, as far as I'm concerned. So I'm not happy with that. I'm also not thrilled with the positioning of the turning mechanism. Let's call it a stem. Somehow it doesn't feel composed properly. So what I simply did, was I played it a bit so the barrel no longer shows the writing, and I turned the stem so that it land...

Just very quickly, I mentioned to you the other day that article about Romney making a policy statement about embryonic stem cell research and said here were sort of examples of issues we were thinking about in this course that would come into your ordinary life. Here's on today's Globe there's, ìKennedy rips Romney over stem cell policy. This debate is continuing. It's on the front page of today's Boston Globe. We have two people neither of whom probably have the background in biology, that you guys are going to have by the time you finish this course, having to grapple with these very serious scientific issues that have all kinds of implications. Here's another thing. This was in yesterday's newspaper. This was on, I think, the second page of the Boston Globe. It's about a curator over at Harvard at The Museum of Comparative Zoology that got all these different birds, some of whom are extinct and something. And now the curator, instead...

Julia just mentioned that a few of you had commented, when we were talking about the genetic code, that some of you thought the fact that it was degenerate, it had some redundancy in it, like multiple codons or threonine, that that was kind of cool, and some of you thought it was sort of a waste and would have maybe designed the thing differently. That's, you know, part of when you study biology you don't get to design it from first principles. You found out what happened during evolution and what got selected for. And once it gets selected for then that gets sort of fixed in nature. If there were four nucleotides then you could have one, two and three-letter words. And it's going to be a three-letter word to have at least 20 then you've got some degeneracy or redundancy, but that's not necessarily a bad thing. And, in fact, if you go into the evolution of the code more deeply, people are beginning to suspect it evolved from a simpler one. And there ac...

As I mentioned last Friday, there is one more topic that I want to cover. And that is the topic of diffusion. And so today, I'm going to talk about diffusion, which will be in the context of what diffusion depends on and how do we describe it. And in particular, there's two laws, Fick's first law and Fick's second law, that I want to talk about. And I want to talk about how this looks in the context of some of the chemistry that we've learned, some of the things that we've learned. So diffusion, this is kind of a standard picture people show for diffusion, although really, this is probably mostly convection, which is the movement. It's not diffusion. But diffusion kind of has taken on this broader meaning. And so what do we mean by diffusion? Well, diffusion comes from the Latin word diffundere. And that means to spread out. But in this class, we're going to start by thinking about it as a particular movement down a concentration gradient. ...