Biology: Egestion

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When digestion is done and absorption is complete, we have some stuff that's left. The bottom line is, whether you're a paramecium or a human, and anything in between, if you are going to ingest materials, then there's going to be some stuff that just does not seem to be able to be digested. One of the primary examples of that is cellulose. We can't digest cellulose. You can eat all the paper you want, you're getting no nutrition out of it. Now when we get down to the large intestine, though, it's role is not merely to compact this stuff and produce feces, even though that is indeed the end product, if you will, of digestion. But what we are going to see here is a process - and that, by the way, is called "egestion."
A lot of people think, "Oh, fecal material is excretion." No, there's a definite difference. Excretion is literally getting rid of something that has been produced by a metabolic process and has been produced by that product. For example, carbon dioxide is produced by the breakdown of glucose, and therefore that is a new product that you've made, and therefore you excrete it. Egestion, as we'll see, is just getting rid of leftovers; it's compacting that material and getting it out of the body to make room for more material to come down later.
But there's more to it than this. Also in the large intestine, we absorb water. So in the large intestine, there is going to be water absorption, and with that water will be ion absorption. Although we believe some of the ions and some of the water absorption does occur up in the small intestine, anything that's quite so polar as to be dissolved in water - like ions - are going to probably be further absorbed with water. Evidence of that is, people who get severe diarrhea often have ion deficiencies, have mineral deficiencies because of the diarrhea. So ions are important absorptive products that occur in the large intestine.
So that being said then, what else happens in the large intestine? Well, you know, there's a very interesting place - there are a lot of organisms that live in your large intestine. Maybe the word "organisms" isn't such a good one. There's a lot of cells that are not you that live in your large intestine. See you have this entire group of organisms squirming around down there - right now billions of them - called bacteria. We have what is called an intestinal flora, and they're harmless bacteria, generally speaking - mostly E. coli, and I know E. coli gets a bum rap - "Oh, there's E. coli on my hamburger, I'm going to die." Well, it depends on what strain of E. coli it is, and it depends where that E. coli came from.
See, here's one of the dilemmas: If you find E. coli in your drinking water, it could mean that there is human fecal contamination in that drinking water - which, besides being gross, humans also carry germs in their feces that, if they have a disease, they may pass on, like cholera. Cholera - when we talk about natural selection - I've got a couple of minutes here - cholera is an incredible organism. Most parasites don't want to kill their hosts, do they? Why? They've got a great place to live. Cholera's strategy is completely different than that. They give you diarrhea to death; they diarrhea you to death. But here's what happens: Because you lose the water absorptive capacities of your large intestine, every time you have a diarrhea attack from cholera - which is all day long - you're giving the cholera a chance to spread its genes, because in that diarrhea is billions upon billions of cholera bacteria, and they're being emptied into water systems - rivers, lakes, and that is why cholera is such a dangerous organism to be spread.
So when we find E. coli in our water supply - and the reason we chlorinate our waters or boil our water is to - in case not that there's E. coli in there - although there are some harmful E. colis - but to keep things like cholera out of our drinking water. But that's not what I want to talk to you about. I want to talk to you about the intestinal flora, because they do other things. These guys are actually mutualistic symbionts. Symbionts - remember close mutual relations - close relationship of some kind? A parasite is a symbiont. But these are mutualistic symbionts, which means that they have a mutual benefit. You give them a great place to live, and all the food they can eat - if you want to call that food - and what do they do for you? It turns out that they make vitamins. We have come up with - we have evolved this mutualistic relationship with them where some of their byproducts we use as coenzymes. Yes, we've evolved the ability to use their wastes, if you will, as coenzymes, such as vitamin K and biotin, which is one of the B vitamins. So that's why if you take antibiotics for a long period of time you can get deficiency of these enzymes, which is why you've got to be careful to make sure you take in - not these enzymes - these vitamins.
So what else do these bacteria do? Well, they eat a lot of the material you take in, and there are some things you taken in that you can't digest that they do. One very interesting example of that is, there's this polysaccharide - there is some real strong polysaccharide in beans. Yes, and we can't digest that polysaccharide, but they can; and one of the byproducts that these bacteria do, or that they produce, is methane gas, and hydrogen sulfide. So if you can imagine that's one of the reasons why some of the carbohydrates in beans are known to produce extra amounts of methane and hydrogen sulfide in some of our large intestines and release it in an explosive manner.
Well, any, last but not least, what about organisms that eat a lot of plants? How do they do that? How does a cow graze? Well, cows are a group of organisms called ruminants, and this is amazing. They have a multitiered stomach - and I don't want to give you a whole zoology course in cow biology, or cattle biology, or ruminant biology - but here's what they do: They actually have a special compartment for their stomach that they first swallow their food into. And in that stomach compartment called the rumen, there are bacteria. So they have their bacteria early in their digestive system rather than at the end, if you will. And what they do is the bacteria breaks down, and this particular bacteria has an enzyme called cellulase; and the ruminates actually will let these things break down the cellulose. Then they do something called "chewing their cud" they go [swallowing sound], and they kind of pull up the digested food, and then swallow it back down; but they send it to another section of their stomach where now the cellulose has been broken down. How cool is that?
I've got one more cool story for you. One time one of my students came to me and said, "You know, I have a rabbit, and I think we should put it to sleep." And I said, "Why?" She says, "Because it's really `out there.' It eats its own feces." I said, "Oh, that's coprophagy! Don't kill your rabbit; it's doing what it's supposed to do!" See, some organisms don't have these rumens, and they're herbivores. So when they, for example, have their - oh, I'm not even going to try to draw a rabbit - but when they put their food, their herbivorous food, through their large intestine, and the bacteria digest it, the bacteria leaves some of that cellulose broken down; so out come these little green pellets. But you know what? When you go out walking in the woods, do you find little green pellets? No, I don't think you do. I think you find brown pellets. Why? Because a rabbit, and any kind of coprophagous organism will take the green pellets and re-eat them - get the nutrients out - and then, when they come out the second time, they're brown pellets. And that, my friends, is the straight poop on egestion.
Animal Systems and Homeostasis
The Digestive System
Egestion Page [1 of 2]