The human digestive system: digestive enzymes and bile

Why does a plain cracker start to taste sweet if you keep chewing it, without swallowing, for a minute or two? Nobody added sugar, and nothing was cooked. It did not taste sweet when it went in. The tempting answer is that sugar was hiding in the recipe all along. But then it would have tasted sweet straight away. Something in your mouth is changing the cracker while you chew. The sweet taste is the first stop on a route that runs from mouth to small intestine.

Your digestive system is an organ system: a group of organs working together on one job. That job is turning big, insoluble food molecules into small, soluble ones. It matters because a molecule has to be small and soluble before it can be absorbed into the blood. A starch molecule is far too big. Enzymes do the breaking down. Each one is specific, fitting only one kind of substrate, so each food group needs its own enzyme. Every enzyme gets two facts, kept apart: the organ where it is made, and the place where it works. They are often different. Now predict: bread is mostly starch. What must happen to it before it can be absorbed? It must be broken down into small, soluble sugars, by an enzyme whose active site fits starch.

Amylase acts on starch, a carbohydrate, to make simple sugars. Amylase is made in three places: the salivary glands, the small intestine and the pancreas. It works in the mouth and in the small intestine. As a word equation: starch plus water gives simple sugars, with the enzyme amylase named over the arrow. That is the sweet cracker. Amylase from your salivary glands acts on the starch in the cracker to make sugars, right there in your mouth. Swallowed, the chewed cracker moves down to the stomach, which is strongly acidic. What happens to the amylase from your saliva when it reaches that acid, and why? It is denatured. The acid changes the shape of its active site, so starch no longer fits, and the carbohydrate work waits for the small intestine.

Protease acts on proteins to make amino acids. Protease is made in the stomach, the small intestine and the pancreas. It works in the stomach and in the small intestine. The stomach is the acid one, and the protease made there works best in acid. That is why protein digestion starts in the stomach. Now a why: would the protease in the stomach do anything to starch? No. Starch is not complementary to its active site, so it cannot fit. Each enzyme needs its own substrate. The large intestine is different. No digestive enzyme is released there; its main job is taking water back from what is left. One examiner's report, on a Foundation paper, puts a number on where students place protein digestion. Less than forty-five per cent of students correctly identified the stomach as the site where protein was digested in the human digestive system. A significant number of students selected the large intestine, indicating a misconception about enzyme action in the digestive system. The way to keep it straight is to tie protein to acid. Protease works in the acid of the stomach, and no enzyme is released in the large intestine.

Lipase acts on lipids to make glycerol and fatty acids. Lipase is made in the small intestine and the pancreas, and it works in the small intestine. A student writes that lipase breaks lipids down into glycerol. Is that complete? No. A lipid gives two products, glycerol and fatty acids, and an answer needs both. The liver is not on that list. The liver does not make lipase; it makes bile. One examiner's report, on a Higher paper, says this. The common misconception seen frequently year on year is that the liver produces lipase. Give the liver one label, and only one, as a habit: it makes bile. Lipase comes from the small intestine and the pancreas.

Bile is not an enzyme. It is made in the liver and stored in the gall bladder. Bile has two jobs. The first is to neutralise stomach acid. Where does that happen: in the stomach itself, or after the acid has left it? After it has left. Bile is released into the small intestine, where the enzymes work best in alkaline conditions. In the stomach, it would stop the protease that needs acid. A second report, on a Higher paper, records students who thought that bile neutralises the acid within the stomach, rather than in the small intestine. They put the job in the wrong organ. The fix is to follow the order: acid is made in the stomach, leaves it, and meets bile in the small intestine. The second job is to emulsify fat: to break one big blob into many small droplets. Why does breaking fat into droplets make lipase work faster? More surface area, so a faster rate. Lipase can only work at the surface of fat, and the droplets have far more surface between them than the fat had in one piece. The fat is not digested yet. Bile breaks fat up into droplets; lipase breaks it down into glycerol and fatty acids. Four statements about bile. The first: bile is an enzyme that digests fat. The second: bile neutralises the acid in the stomach. The third: bile is made in the gall bladder. The last: bile breaks fat into small droplets so lipase has more surface area to work on. Which of the four is right, and what one change fixes each of the others? Only the last is right. The first: bile is not an enzyme; it emulsifies fat. The second: change stomach to small intestine. The third: change made to stored.

A cheese sandwich carries all three food groups: starch in the bread, protein and fat in the cheese. In the mouth, amylase from the salivary glands acts on the starch in the bread to make simple sugars. In the stomach, protease made in the stomach acts on the protein in the cheese to make amino acids. Then the mixture leaves the stomach and arrives in the small intestine. Fill in the small intestine: each enzyme, where it was made, and what it makes. Three enzymes work there, all made in the small intestine and the pancreas. Amylase acts on starch to make simple sugars. Protease, from those same two organs, acts on proteins to make amino acids. Lipase acts on lipids to make glycerol and fatty acids. Now one more box: at which stop is bile released, and what does it change? The small intestine. Bile neutralises the acid from the stomach and emulsifies the fat from the cheese, so lipase has more surface to work on. A different case: a damaged liver that cannot keep up. Fat is digested, but at a crawl. Bile is in short supply: why is fat digested so slowly? Name both its jobs. With little bile, the acid from the stomach is not neutralised, so the enzymes in the small intestine work slowly. And the fat is not emulsified, so it stays in big blobs with little surface for lipase. Both slow lipase down. The lipase itself is still made, because the liver never made it. Now the bread in that same meal: is its digestion hit as hard, and why? Much less. Bile does nothing to starch itself, so amylase still acts on it. At most the extra acid slows amylase a little, while fat is hit twice.

Three questions to check from memory, starting with the cracker. The cracker turned sweet. Which enzyme did that, and where was it made? Amylase, made in the salivary glands. It acts on starch to make simple sugars. Now a new case: the pancreas stops releasing enzymes. Can the small intestine still digest? Yes, more slowly. The small intestine makes amylase, protease and lipase of its own. One more: someone has their gall bladder removed. Is bile still made, and why? Yes. Bile is made in the liver; the gall bladder only stores it.

This one's done if you can fill in the made-in and works-in columns blind; tap the thumb under it and it leaves your list. If the columns still swap places, copy the table onto a blank sheet from memory, then check it; doing that once is often enough.

Next in the chapter: Building and breaking down carbohydrates, proteins and lipids. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh three, oh four.

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