Macromolecules

Becka says I should write something science-like so my page isn’t blank.
I thought I would relate one of my Bio prof’s pet peeves. It has to do with Macromolecules ( not to be confused with Macromedia ). Macromolecules are large molecules (usually protein) – they are measured in a unit called “daltons” or “kilodaltons” (kDa). One dalton is a measure of the mass of a molecule, and actually it is the same as the molecular mass. But I think biochemists use the term Dalton just so they can confuse people…. or because a dalton is usually measured using a centrifuge or electrophoresis, instead of with more exacting means such as mass spectrometry.
So the impressive thing about macromolecules is just how large they can be. For example a small molecule like water has a molecular mass of 18. Keratin, a major constituent of skin weighs in at 67,000 (67 kDa). Yeah, that’s big. But that’s nothing compared to polymers; polymers are repeating chains of molecules. DNA is a polymer. Lots of structural proteins are polymers, too. For example, collagen, microtubules, and intermediate filaments. In their polymer form, these things weigh in a millions and millions of Daltons. Actually, taken to the extreme, a car tire is actually one giant molecule — a polymer or polymers. How many Daltons is a car tire? Well, it is on the order of 6.02 times 10²³…
Diffusion across a membrane is greatly affected by the size of a molecule. Small molecules can fit through the small gaps in the membrane whereas large ones can’t. That’s why you can lose water through your skin on a hot dry day without ever sweating. That’s also how your lungs can take oxygen out of the air you breathe. Or maybe you’ve noticed that if you go swimming in fresh water after a while you really need to pee even though you haven’t been drinking anything; the water diffused into your body. But you don’t need to worry about most bad things getting into your skin when you swim. They’re usually too big to diffuse through your skin.
Anyhow, my Bio Prof’s peeve was for the hand lotion commercials that talked about how their product revitalized skin. They would have a diagram showing a cross section of the skin, with arrows showing the deep penetration of the collagen in the lotion. The problem is that collagen is a gigantic macromolecule; it cannot diffuse into your skin because it is too large. Instead it just sits on the surface and is all slimy or maybe after a while it gets crusty. Probably the only thing in lotion that does penetrate the skin is the water and oils; so some vaseline or shortening is probably just as effective.