“Macromolecules. It’s kind of an oxymoron, isn’t it?” Zach had opened up the textbook as he settled into the shotgun seat of Aunt Jeannie’s old VW bug. Em had brought it down east with her along with the driver’s license. It mostly sat in the parking lot, the exception being for out of town trips like this one to nearby farming community.
“Scale. Everything is a matter of scale. They’re called macromolecules because they’re big and often pretty complex, and they’re made up of component pieces. Of course all the component pieces are made of of their own parts, probably infinitum, so you’re right, it’s another one of those silly language things. Since the component pieces offer a particular scale of interest to biologists, and the bigger organic molecules they’re built into usually have really obvious functions, I guess they become macros. Who knows? Once you get much past the scale of these big molecules, you’ve got a cell, right?”
“I guess. Those levels of organizational hierarchy that were introduced in the first chapter seem pretty organic. I mean, cells are made of molecules and molecules are made of atoms. When you’ve got a bunch of cells that are all the same doing something, you’ve got a tissue. And a bunch of tissues make an organ, and then you’ve got organ systems, like the digestive system, and organisms, whole living things. Those things aren’t exactly arbitrary.” Zach was pretty masculine; he liked structure, a lot.
“Yeah; I guess.” She shifted gears. “I’ll have to think about that. I mean, once you get past organism then you have populations and communities and ecosystems, and those demarcations are really, really arbitrary. I mean, I can call all the white pines on campus a population or all the white pines in New England a population, or all of the white pines in the world a population. Communities are even more random. One cubic meter of soil right here might have a totally different microbial community from one over there, but it would all be one community if we decided to look at a ten cubic meters. Same kind of things with patches of forest.”
‘But we were talking about macromolecules.” He went back to the book. “Or at least I think that’s where we are. Explain this functional group thing to me.”
“Well, all organic molecules have carbon hydrogen bonds, so a carbon skeleton.” She pointed at the picture in the book. “It’s these different functional groups that give them their unique characteristics. So, like, this OH group is really polar, so it’s going to make a molecule more water soluble. This amino group acts like a base, so it hooks up with protons, this carboxyl group like an acid, so it tends to give them up. Phosphate groups tend to form high energy bonds, and methyl groups tend to be pretty non-reactive and hydrophobic. They don’t react with water much, at all. This double CO bond thingie kind of varies depending on where it is, but it tends to make that particular part of the molecule a good place for bonding. When you look at the difference between the straight chain sugar and the ring form, and can see what I mean.” She found she liked practicing explaining it, it really helped her understand it herself.
“Ah, I see. And so these different kinds of monomers, the little subunits, are made up carbon chains with different functional groups on them, so they tend to have different kinds of functions.” He got it.
“Yes. And so monosaccharides, simple sugars, have this basic structure, with the carbons and the carbonyl and hydroxyl groups. Just Carbon, Hydrogen, and Oxygen. And the simple sugars are what cells like to use as an energy source.” Em also knew a bit about nutrition, another big issue at the farm. “And sucrose, which is made up of two monosaccharides, is that stuff that you like to put in your coffee. It’s really not the best form of carbs to be putting into your body.”
“I don’t get it. If simple sugars are what my cells want, why shouldn’t I just eat them?” He did like his cookies, and yes, lots of sugar in his coffee.
She smiled, a really deep smile, so deep it picked his head up from the book, caused it to turn and look at her.
“What?” he asked her.
“Think of metabolism as a grand dance of life, an erotic quest, something like trying to elucidate what turns a woman on.” She smiled again, loved making up fun little analogies as she went along. “Evolution might have produced a way that we might just put on biomass more efficiently, or extract energy from our food directly, or even just absorbing the sun’s radiation. But no. Instead, we have the grand dance.
“A potato is made up of starch. That’s basically just really long strings of glucose, one of the most common monosaccharides, all strung together by those polymerization reactions. Those reactions remove water, they take a H off of one glucose and an OH off the other, and they get bonded together. See the picture?” She was a very attentive driver, and he was a very attentive student, so they did well together.
“And starch is how plants store energy short term. Cool. And this other molecule, glycogen, is how animals store it. They’re really similar molecules. Cool.”
“Yeah. And trees also store starch in their roots. When spring rolls around, they start breaking it down, via those hydrolysis reactions, back into simple sugars. That’s why we can collect maple syrup up where I live.” She smiled, thinking about the pancakes and bacon she’d make for breakfast the next day. “And when we eat the starch from the potato, first our body breaks down the starch into simple sugars, and it uses some for energy, and it uses some to make glycogen. Or fat.”
“And so a hydrolysis reaction is basically just the opposite of the polymerization reaction, it just puts water back in where it was taken out before. And the same thing happens in the human liver when we need sugars, we break down glycogen. Cool. So how does it all work?” It was getting more interesting all the time.
“Enzymes. We have quite a ways to go before we get to enzymes. They’re proteins.” She made a turn. “So those storage carbs, starch and glycogen, they’re pretty simple. They’re easy to break down, mostly used for short term storage. The structural carbs, like cellulose and chitin, they’re a lot tougher. We can’t digest cellulose. There’s lots of cellulose in turds.” She smiled at him, loved talking dirty.
“Cellulose is the stuff in fiber, right?” His mother always nagged him about eating fiber.
“”Fiber’s got even tougher stuff in it along with the cellulose, lignin. But all plant cells have cellulose cell walls, so all plant foods have lots of cellulose in them, unless they’ve been processed to death.” Another turn. “And chitin, it’s tough too. Fungi have chitin in their cell walls. And lobster shells have it, too.” If you look at the pictures, though, you can see, that there’s extra hydrogen bonding that occurs because of the arrangement of the monomers, so you get a structure that’s more of a matrix that a string, for both of these.”
“Yup, that’s really pretty simple, once you look at it. That would be a lot harder to break down.” Zach like good imagery along with good text, particularly with this kind of material. The pictures in lots of books just tended to add mass, probably to justify their weighty price tags.
“What’s this one, peptidoglycan? God, the words you biologists use are pretty awful.” He was glad he wouldn’t have to test on this stuff, not being enrolled in the class. He’d hate to try to have to remember all this vocabulary. Science had a language all its own.
“Bacterial cell walls; the monomers are more complex. I imagine there’s all sorts of variation, especially among the ancient ones.” She was getting restless, ready to be out in the sun working.
“So we probably don’t eat a lot of that.” He was, too.
“We probably contain a lot of that already. Remember those trillions and trillions and trillions of bacterial cells we have in bodies?” They’d arrived.
“Oh yeah. Man, I’m ready for some bacon.” The CSA had a cafe associated with it to help offset costs of producing food organically. “Let’s talk about fat.”
“Lipids. They’re kind of interesting, don’t really have any monomer subunits. They’re also pretty diverse. The only thing they all have in common is hydrophobic regions that don’t react with water.” They were heading toward the barn, where they’d pick up some tools and find out what needed to be done. Em hoped they’d be pulling weeds or harvesting; she’d mucked out a pig pen last month and hauled the slop to the compost pile.
“But there are still those dehydration and hydrolysis reactions, right? I seem to remember that bit about triglycerides being made of glycerol and fatty acids and whether the fatty acid tails were bent or straight and so why some lipids, like that wonderful olive oil, are liquid at room temperature, while others, like that lovely bacon fat, are solids.” He had paid attention to the stuff he could relate to in school, which wasn’t much of it. Food, he cared about, quite a lot.
“Yes, but they aren’t broken down as readily as the carbs. That’s why they’re good for long term energy storage. All those CH bonds represent really dense stored energy.” They headed into the barn.
“I still don’t really get that.”
“We’ll get there.”
They shoveled manure together.
“It’s the hydrophobic regions that are the thing.” Em was shoveling right, Zach left. Every few minutes they’d switch sides, practicing the balanced yoga of shit slinging. “So while oils and fats are great for energy storage, they’re also really good for water proofing. Same with waxes. That outermost layer on a leaf is waxy, kind of waterproof. Same with the oils in our skin.”
“I thought the cell membrane thing was the coolest, that phosphate head and fatty acid tail thing and the phospholipids in the water? I mean, I can imagine evolution happening, a cell being born just like that in the middle of the primordial soup!” He accidentally slug a piece of pig poop right onto Em’s cheek as he turned to switch places before finishing his toss.
She gave him the look.
“Does this look like some kind of skin conditioning oil to you?” She was actually smiling as she pointed to the stinky poo that graced her visage.
“Probably lots of cellulose in there, right?” He was working hard not to laugh, trying to decide if she was going to swing her shovel at him. “And billions and billions of bacteria, too.” He couldn’t resist a little Carl Sagan, it might be his only hope.
She laughed out loud.
“Let’s go eat, the wheelbarrow is full and so are the pigs, this is a never ending job and we’ve put plenty of time in.” She was pulling her gloves off, stretching her back in that way that now totally turned him on.
“You should probably wash your face first.” He’d try to keep his mind clean.
He had plenty of time to study with Emily.
Linda Brooke Stabler, Ph.D.