0610 · 7.4Chemical digestion

Why chemical digestion is needed

0:00A cheese sandwich is full of useful food. The bread contains starch, and the cheese contains protein and fat. But not one molecule of that starch, protein or fat can get into your blood as it is. The molecules are far too big. Before your body can use any of it, the food has to be taken apart, molecule by molecule, into pieces small enough to pass through the wall of your gut.

0:21That job is done by chemical digestion. By the end of this video you will be able to describe what chemical digestion is and why it is needed, state what amylase, protease and lipase do, and state where each one is secreted and where it acts. You will also be able to describe the two jobs of hydrochloric acid in the stomach. For the extended syllabus, you will follow starch all the way to glucose, compare the proteases pepsin and trypsin, and explain how bile neutralises stomach acid.

0:49You need a few ideas from earlier videos. From the enzymes video, remember that an enzyme is a protein that acts as a biological catalyst. Its active site has a shape that is complementary to its substrate. Each enzyme has an optimum pH, and if the pH moves too far from it, the enzyme is denatured. You also need the names of the parts of the digestive system: the mouth, the oesophagus, the stomach, the small intestine, which starts with the duodenum, and the pancreas, liver and gall bladder.

Chemical digestion and enzyme products

1:17Let us start with the definition. Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules. Learn those four words as two pairs: large and insoluble at the start, small and soluble at the end. Why does this matter? In the last video, physical digestion broke food into smaller pieces. That gives enzymes more surface to work on, but the molecules themselves are still exactly the same.

1:42A tiny crumb of bread is still made of huge starch molecules. Chemical digestion actually breaks the bonds inside those molecules. The role of chemical digestion is to produce small soluble molecules that can be absorbed. Absorbed means they pass through the wall of the small intestine and into the blood or lymph. Large molecules cannot fit through, and insoluble ones cannot be carried in solution. Small soluble molecules can.

2:07We will look at absorption itself in the next video. Three groups of enzymes do this work, one for each of the large food molecules. First, amylase breaks down starch to simple reducing sugars. Notice the exact wording: simple reducing sugars. These are the sugars that give a positive result when heated with Benedict's solution. Second, proteases break down protein to amino acids. Protease is the name for a whole group of enzymes that break down protein.

2:34Third, lipase breaks down fats and oils to fatty acids and glycerol. Be careful here. The products are fatty acids and glycerol, two different substances, and you need to name both of them. A useful pattern is in the names. Protease acts on protein. Lipase acts on lipids, which are fats and oils. And all three names end with the letters A, S and E, which tells you that each one is an enzyme.

Enzyme secretion, action and stomach acid

2:59Now, where does each enzyme come from, and where does it work? The place an enzyme is made and released is where it is secreted. The place where it meets its substrate is where it acts. These are often different places, so keep the two ideas separate. Amylase is secreted by the salivary glands into the mouth, and by the pancreas into the duodenum. So amylase acts in the mouth and in the small intestine.

3:23Protease is secreted by the stomach wall, and by the pancreas. The lining of the small intestine also makes some protease. So protease acts in the stomach and in the small intestine. Lipase is secreted by the pancreas, and the small intestine also makes some. Lipase acts in the small intestine. Notice that the pancreas makes all three. Pancreatic juice flows along a tube into the duodenum, carrying amylase, protease and lipase together.

3:51The pancreas does not digest any food itself. It secretes the enzymes, and they act in the duodenum. Next, the stomach. Its wall secretes gastric juice, and gastric juice contains hydrochloric acid. For your exam, the acid has exactly two functions. First, it kills harmful microorganisms in food, such as bacteria that you swallow with your meal. Second, it provides an acidic pH for optimum enzyme activity. The protease in the stomach works best at a pH of about 2, which is very acidic.

4:21Notice what the acid does not do. In this syllabus, hydrochloric acid does not digest the food itself. The acid kills microorganisms and gives the right pH. The digesting is done by the enzyme. This also explains something about amylase. Salivary amylase works best close to neutral. When the food is swallowed and reaches the stomach, the acid lowers the pH far below its optimum. The active site of the amylase changes shape, and the enzyme is denatured.

4:49So salivary amylase stops working in the stomach. Starch digestion only starts again in the duodenum, when amylase from the pancreas arrives.

Extended digestion: starch and proteases

4:58For the extended syllabus, you need more detail on starch. Digestion of starch happens in two steps. In the first step, amylase breaks down starch to maltose. Maltose is a sugar made of two glucose units joined together. It is one of those simple reducing sugars, but it is still too big to be absorbed easily. In the second step, an enzyme called maltase breaks down maltose to glucose. Glucose is small and soluble, so it can be absorbed.

5:25Where does maltase work? This is a detail examiners like. Maltase is found on the membranes of the epithelium lining the small intestine. The epithelium is the layer of cells lining the inside of the gut. So maltase is not floating freely in the gut contents. It is held on the cell membranes. Maltose molecules touch the surface of these cells, and they are broken down into glucose right where the glucose can then be absorbed.

5:49For the extended syllabus, you also need to know two proteases by name. The first is pepsin. Pepsin is secreted by the stomach wall, and it breaks down protein in the acidic conditions of the stomach. Its optimum pH is about 2, which is exactly what the hydrochloric acid provides. The second is trypsin. Trypsin is secreted by the pancreas, and it breaks down protein in the alkaline conditions of the small intestine.

6:15Trypsin works best in slightly alkaline conditions. Why do we need two different proteases? Because the pH is so different in the two places. Pepsin is suited to strong acid, and trypsin is suited to alkaline conditions. Each one has an active site that keeps its shape at its own optimum pH. If you moved pepsin into the alkaline small intestine, it would stop working.

Bile and common digestion mistakes

6:38That raises a problem. Food leaving the stomach is mixed with gastric juice, so it is strongly acidic. The enzymes of the small intestine need alkaline conditions. So how does the pH change? This is where bile comes in. Bile is made in the liver and stored in the gall bladder. After a meal, it flows down the bile duct into the duodenum. In the last video, you saw that bile emulsifies fats.

7:01Here we need its second job. For the extended syllabus, you need to know that bile is an alkaline mixture. It neutralises the acidic mixture of food and gastric juices entering the duodenum from the stomach. This provides a suitable pH for enzyme action in the small intestine. Be very careful with one thing. Bile is not an enzyme. It contains no enzymes, and it does not break any bonds in food molecules.

7:25It changes the pH, and it emulsifies fats, but the actual digesting is done by the enzymes. Let us collect the common mistakes. The first is saying that lipase breaks fat down into fatty acids only. You must name both products, fatty acids and glycerol. The second is mixing up where an enzyme is secreted with where it acts. The pancreas secretes amylase, protease and lipase, but they act in the duodenum, not in the pancreas.

7:53The third is calling bile an enzyme, or saying that bile digests fat. Bile is an alkaline mixture that neutralises acid and emulsifies fat. It does not break down any molecules chemically. The fourth is saying that the acid in the stomach kills the enzyme amylase. Enzymes are not alive, so they cannot be killed. The acid denatures amylase. The acid does kill microorganisms, because those are living. And the fifth is vague wording.

8:20Say small soluble molecules, not just small molecules. Say simple reducing sugars, or maltose for the extended syllabus, not just sugar.

Exam practice and recap

8:29Let us try an exam-style question. Part one asks you to complete a table for amylase, protease and lipase, showing where each is secreted, where it acts, its substrate and its product. For amylase: it is secreted by the salivary glands and the pancreas, it acts in the mouth and the small intestine, its substrate is starch, and its product is simple reducing sugars, or maltose. For protease: it is secreted by the stomach and the pancreas, it acts in the stomach and the small intestine, its substrate is protein, and its product is amino acids.

9:04For lipase: it is secreted by the pancreas, it acts in the small intestine, its substrate is fats and oils, and its products are fatty acids and glycerol. Part two is worth four marks. It asks you to explain why pepsin stops working when it reaches the duodenum. Think it through step by step. Pepsin works in the stomach, so its optimum pH is acidic. In the duodenum, bile is added.

9:29Bile is alkaline, so it neutralises the acid, and the pH rises. That pH is far from the optimum of pepsin. So a model answer is this. Pepsin has an acidic optimum pH, to suit the stomach. In the duodenum, alkaline bile neutralises the acid, so the pH rises well above the optimum of pepsin. The shape of the active site of pepsin changes, so protein no longer fits, and pepsin is denatured.

9:54Each of those points can earn a mark. Pepsin has an acidic optimum. Bile neutralises the acid, so the pH in the duodenum rises. The active site changes shape, so protein is no longer complementary and cannot fit, and fewer enzyme-substrate complexes form. And pepsin is denatured. Notice that the answer uses bile as the cause, links it to pH, and then uses the language from the enzymes video, active site and denatured.

10:21That chain of reasoning is what examiners are looking for. Let us recap. Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules, and its role is to produce small soluble molecules that can be absorbed. Amylase breaks down starch to simple reducing sugars. Proteases break down protein to amino acids. Lipase breaks down fats and oils to fatty acids and glycerol. Amylase is secreted by the salivary glands and pancreas, and acts in the mouth and small intestine.

10:52Protease is secreted by the stomach and pancreas, and acts in the stomach and small intestine. Lipase is secreted by the pancreas, and acts in the small intestine. Hydrochloric acid kills harmful microorganisms and provides an acidic pH for optimum enzyme activity. For the extended syllabus, amylase breaks down starch to maltose, and maltase, on the membranes of the epithelium lining the small intestine, breaks down maltose to glucose. Pepsin breaks down protein in the acidic stomach, and trypsin in the alkaline small intestine.

11:23Bile, which is alkaline but is not an enzyme, neutralises the acidic mixture arriving from the stomach, giving a suitable pH for enzyme action. In the next video, we will see how these small soluble molecules are absorbed.