0610 · 7.3Physical digestion

Physical digestion and surface area

0:00Drop a whole sugar cube into a glass of water, and it takes a long time to dissolve. Crush the same cube into fine grains first, and it dissolves much faster. It is still exactly the same sugar. You have not changed it chemically. You have just broken it into smaller pieces. Your body uses this same trick on every meal you eat, and that is what this video is about.

0:20By the end of this video you will be able to describe physical digestion and explain why it helps enzymes. You will be able to identify the four types of human teeth, describe their structure and their functions, and describe the role of the stomach. And for the extended syllabus, you will be able to outline how bile emulsifies fats. You need two ideas from earlier videos. First, digestion is the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed.

0:46It uses both physical and chemical processes. Second, enzymes are biological catalysts. Enzyme molecules can only reach the surface of a piece of food, so that is the only part they can act on. There are two kinds of digestion. Chemical digestion uses enzymes to break the bonds in large molecules, turning them into different, smaller molecules. Starch, for example, is broken down into simple sugars. That is the topic of the next video.

1:11Physical digestion is different. The syllabus describes it as the breakdown of food into smaller pieces without chemical change to the food molecules. The key words are smaller pieces and no chemical change. When you chew a piece of bread, you end up with lots of small pieces of bread. The starch molecules inside them are exactly the same as they were before. Nothing new has been made. So why bother?

1:32Because physical digestion increases the surface area of food for the action of enzymes in chemical digestion. Think of that big cube of food. Enzymes can only work on its outside. The food in the middle is hidden. Now cut the cube into smaller pieces. The total amount of food is the same, but many new surfaces are exposed. So enzymes can work on much more food at the same time, and chemical digestion happens faster.

1:55So physical digestion does not replace chemical digestion. It prepares food for it. Your body does physical digestion in three main ways. Teeth chew food in the mouth. The stomach churns it. And, for the extended syllabus, bile breaks up fats in the small intestine. Let us take them one at a time.

Types and structure of teeth

2:13Let us start with the teeth. Humans have four types of teeth: incisors, canines, premolars and molars. You need to be able to identify each type in diagrams and photographs, and the easiest way is by shape and position. Incisors are at the very front of the mouth. They are thin and flat, with a sharp straight edge, like a chisel. On each side of the incisors is a canine.

2:34Canines are pointed, with a single sharp tip. Behind the canines are the premolars, and right at the back are the molars. Premolars and molars are wide, with a broad, uneven top surface covered in bumps called cusps. Molars are the largest of all. In each half of each jaw, an adult usually has two incisors, one canine, two premolars and three molars. That makes 32 teeth in total, counting the wisdom teeth, which are the last molars at the very back.

3:01You do not need to memorise this, but it helps you recognise the pattern. Now, the functions. The shape of each tooth matches its job. Incisors have a sharp, straight edge, so they bite and cut food. When you bite into an apple, it is your incisors that slice off a piece. Canines are pointed, so they grip and tear food, such as a piece of tough bread crust. Premolars and molars have broad surfaces with cusps.

3:26When the upper and lower teeth meet and your jaw moves from side to side, the food between them is ground and crushed into small pieces. This is where most of the chewing happens. Next, the structure of a tooth. Here is a molar, cut down the middle. The part you can see above the gum is called the crown. The part hidden below the gum is the root. Teeth are embedded in the bone of the jaw and in the gums.

3:46The root sits inside a socket in the jaw bone, and the gum surrounds the tooth where it leaves the bone. This holds the tooth firmly when you chew. The outer layer of the crown is enamel. Enamel is the hardest substance in the human body. It forms a tough surface for biting and grinding, so the tooth resists wear. Notice that enamel covers only the crown, not the root.

4:08Beneath the enamel is dentine. Dentine is hard, like bone, but softer than enamel. It makes up most of the tooth, and it continues down into the root. In the centre of the tooth is the pulp cavity, filled with soft pulp. The pulp contains nerves and blood vessels. They enter the tooth through a small opening at the tip of the root. The blood vessels bring oxygen and nutrients to the living cells of the tooth.

4:31The nerves make the tooth sensitive, so you can feel pain, heat and cold. Finally, the root is covered by a thin layer called cement. Cement helps to attach the tooth to the jaw bone, holding it in its socket. So there are two outer layers to remember. Enamel covers the crown, and cement covers the root.

Stomach churning

4:48Now let us follow the food down to the stomach. The stomach has a thick wall made of muscle. The muscles in the wall contract and relax, again and again, which squeezes and churns the food. This churning breaks the food into even smaller pieces, and mixes it thoroughly with gastric juice, the digestive juice made by the stomach. So the function of the stomach in physical digestion is this.

5:09The muscular wall contracts to churn the food and mix it with gastric juice, breaking it into smaller pieces. Think of a washing machine, turning its load over and over in water. Be careful here. The stomach also contains hydrochloric acid and enzymes, and these are part of chemical digestion, which we cover in the next video. When a question asks about physical digestion in the stomach, the answer is the churning action of the muscular wall, not the acid.

Bile emulsification of fats

5:35For the extended syllabus, you also need to outline the role of bile. Bile is a green liquid made by the liver, stored in the gall bladder, and released into the small intestine. Fats and oils do not mix with water. In the watery contents of the small intestine, they tend to join together into large drops. A large drop has a small surface area compared to its volume, so lipase, the enzyme that digests fat, can only work on its outside.

5:59Bile emulsifies fats and oils. To emulsify means to break large drops of fat into many tiny droplets, which stay spread out through the watery liquid. Think of washing-up liquid on a greasy plate. The grease breaks up into tiny droplets that wash away in the water. Bile does something similar to fats. Why does this help? It is the same idea as chewing. Many small droplets have a much larger total surface area than one big drop.

6:24So emulsification increases the surface area of fats for the action of lipase, and chemical digestion of fats happens much faster. Notice that emulsification is physical digestion. The fat molecules in the tiny droplets are exactly the same as the fat molecules in the big drop. Nothing has been broken down chemically. Bile has a second job, to do with pH, but that belongs in the next video. This leads to one of the most common mistakes in this whole topic.

6:51Bile is not an enzyme. Bile does not digest fats, and it does not break fats down into fatty acids and glycerol. That is the job of lipase. Bile only breaks fats into smaller droplets, so that lipase has more surface to work on.

Exam answers and recap

7:06Let us try an exam-style question. Part A asks you to describe the function of the stomach in physical digestion, for two marks. Part B asks you to explain how bile helps the digestion of fats, for three marks. For part A, think about what the stomach wall is made of. A good answer is: the muscular wall of the stomach contracts to churn the food. This mixes the food with gastric juice and breaks it into smaller pieces, increasing the surface area for enzymes.

7:30One mark for the muscular wall churning, and one mark for mixing with gastric juice or breaking food into smaller pieces. Do not mention acid here. That is chemical digestion, and it would not earn the mark. For part B, the word explain means you need to give the reason, not just the fact. A good answer is: bile emulsifies fats. It breaks large drops of fat into small droplets.

7:53This increases the surface area of the fat for lipase to act on, so the fat is digested faster. Notice that the answer names the enzyme, lipase. Let us collect the common mistakes. The first is saying that physical digestion breaks food into smaller molecules. No, it does not. It breaks food into smaller pieces, and the molecules stay the same. Making smaller molecules is chemical digestion. The second is mixing up the tooth functions.

8:18Incisors bite and cut. Canines grip and tear. Premolars and molars grind and crush. The third is putting the nerves and blood vessels in the dentine or enamel. They are in the pulp, in the centre of the tooth. And the fourth is drawing enamel down the root. Enamel covers only the crown, and cement covers the root. The fifth is saying that bile is an enzyme, or that it digests fat.

8:39Bile emulsifies fat, which is physical. Lipase digests it, which is chemical. Let us recap. Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. It increases the surface area of food for the action of enzymes in chemical digestion. Humans have four types of teeth. Incisors bite and cut, canines grip and tear, and premolars and molars grind and crush. A tooth has enamel covering the crown, dentine beneath it, and a pulp cavity in the centre containing nerves and blood vessels.

9:12Cement covers the root, and the tooth is embedded in the jaw bone and the gums. The muscular wall of the stomach contracts to churn food, mixing it with gastric juice and breaking it into smaller pieces. And for the extended syllabus, bile emulsifies fats and oils into small droplets, increasing the surface area for lipase. In the next video, we will look at chemical digestion, and the enzymes that turn these small pieces into small, soluble molecules.