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Digestive System

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The digestive system controls the chemical and mechanical breaking down of food into molecules that can be used throughout the body. It includes organs such as the esophagus, stomach, and small and large intestines. There are two stages to the digestive system: mechanical and chemical. Mechanical digestion involves the physical breakdown of food while chemical digestion uses enzymes to break the chemical bonds holding polymers together.

The digestive system is one of the easier systems in Biology to understand because you use it everyday and you kind of pay attention to what you put in your mouth. Now the digestive system obviously from its name is involved in digesting food but what a lot students forget is that it also involved in the absorption of the food. Remember to digest means to break down the chunks of food you're sucking in through your mouth into the individual molecules that can then be absorbed.

You do this kind of digestion in two different ways. One is mechanical digestion which is what your teeth do when you chew and the mixing and churning by your stomach also helps break down the larger chunks of food into smaller and smaller pieces, why bother doing that? Well, it increases the amount of surface area that the enzymes have available to attack these large chunks of food and break down the individual molecules. So by chopping it up into smaller pieces, it allows you to digest food faster and better.

Now chemical digestion is generally done by enzymes, those proteins that speed up chemical reactions and they're taking the long chains of organic molecules like polysaccharides like starch for example and chopping those long molecules into the smaller ones that can more easily be absorbed.

Now, one of the first digestive enzymes involved and that's often focused on in bio classes is salivary amylase. It's the enzyme that's in saliva that breaks starch down into glucose which is why if you keep chewing a cracker, eventually you'll start to detect a sweet taste and that's because of the glucose is being broken off of the starch that makes a cracker.

In your stomach the enzyme that everybody focuses on is something called pepsin. What pepsin does is it starts the breakdown of long proteins into smaller chunks. Now pepsin is interesting that it's a very effective enzyme but it's actually created in the stomach cells in an inactive form called pepsinogen. Gen means to create. So pepsinogen is going to make pepsin but it only does that when you toss it into a bunch of hydrochloric acid. That's why other cells in the stomach pump out hydrochloric acid into your stomach and that lowers the ph and activates the pepsinogen into pepsin. Additionally, all that HCl, that hydrochloric acid helps to kill off bacteria and other things that might have been in your food.

Your small intestine is one of the major important sites of the digestive system because it has all the enzymes that attack everything that comes into the digestive system. Now, it makes some of those enzymes plus the pancreas pumps out a lot of enzymes as well as the insulin in glucagon that you may be aware of as hormones that regulate blood sugar. But the pancreas produces a lot of pancreatic juices, these are enzyme that go into the intestine and help break down the organic molecules.

Bile is produced by your liver, stored in your gall bladder. Now bile is not an enzyme but what it does do is it does this process called emulsification of fats and that takes big chunks of fats and breaks them into small little globules and that again makes it easier for the fat in digesting enzymes like lipase to get at and attack the globes of fat.

Ultimately absorption is now ready to happen. We've broken down the large chunks of food into small individual monomers and now the small intestine can do that absorption. And it has these folds in the walls of the small intestine that greatly increases the surface area of the intestine. And in fact those folds have microfolds called microvilli.

Last, the large intestine reabsorbs the water that's left over from the breakdown of food and anything that your body couldn't break down is then stored and ultimately formed into the faeces.

Now, let's take a quick look at this diagram here just to remind you all the parts that make up your digestive system, here's your mouth. as you can see it's actually a shared cavity with your nose which is why, if you're drinking some milk and one of your buddies tells a really funny joke, up the nose. You swallow, goes down the oesophagus, sometimes you'll hear about a movement called parastalsis. That's rythmic contractions of the muscles that line your oesophagus to force the food down into your stomach. Your stomach churns it up, mixes it up and then sends it into the small intestines. It goes through the long, small intestines ultimately into the large intestine and then eventually out.

We take a look. This shows those folds I mentioned earlier, of the small intestines. These are the villi and then if we zoomed in on one small little fold of this picture on the left, and we can see on the right, one of those things I've highlighted with a blue arrow, you can see the little microvilli. It's kind of like why you have a cloth towel to absorb all the water from your body when you get out of the shower, as opposed to using a sheet. A sheet may have more material but the towel has more surface area and has a faster absorption rate because of that higher surface area volume ratio. In fact, I read estimates of the surface area of the small intestine, which is only maybe 15 to 18 feet long. It has a surface area roughly equivalent to that of a tennis court. And that's why we have all these folds. And that's the digestive system.