Key Stage 4

1. Key Stage 4 Digestion

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3. The 7 food groups represent large chemicals. These chemicals are often chains of smaller, more useful chemicals, joined together. One example is Carbohydrates. Carbohydrates are made of long chains of identical small sugar molecules. Carbohydrate Sugar molecule

4. The small sugar molecules are very useful. The body can use them to make… • So the body has to break these large food molecules up into smaller pieces. ENERGY However!

5. Problem One • It is not enough to cut these foods up into pieces using a knife. This would not release the sugars. • i.e. cutting a loaf of bread (carbohydrate) into slices does not produce sugar. This is because we can’t release sugars from carbohydrates by physically breaking them up.

6. The chain of sugars is held together by… • Chemical bonds. Chemical bonds require a chemical technique if they are to be broken.

7. Problem 2 The food we start with is often large in size. Being large, the food tends to be unable to dissolve. We say it is large and insoluble. This is a real problem. The food needs to be soluble so that it can dissolve in the blood and thus, be transported around the body. The smaller the food, the more likely they will dissolve. So the digestive system has to cope with both these problems.

8. Remember…. • The digestive system, being an organ system, is made of a group of organs all working together. • i.e. Each organ has a particular function but together they get the job done. The only visible parts of the digestive system are the entry and exit points Mouth Anus

9. Food looks very different when it leaves, compared to when it enters! Therefore the body must be doing something to the food during its journey. It is digested. This means it is broken down. However, breaking something up happens in 2 ways. As we know all food has a physical shape and is made of chemicals. These chemicals are held together by chemical bonds.

10. It must be broken down using physical and chemical methods. physical chemical This allows it to pass through the small diameter of the digestive system. This allows useful chemicals to be released and dissolve in the blood. To be broken down physically, the shape of the food must be changed. To be broken down chemically, the bonds must be broken.

11. Our digestive system uses both these methods of digestion. • Chemical Digestion • Physical Digestion • As we move through the digestive system, we will see one or both of these methods in action at any one time. • So, in summary so far…

12. Digestion is the chemical and physical breakdown of largeinsoluble molecules into smallsoluble molecules. Let’s take a close look at how this happens… Open wide!

13. All food enters our digestive system through the mouth and waste material leaves through the anus. Mouth If we stretch the digestive system out, we can see that it is really one long tube with an opening at each end. However, it is a 9m tube! Anus

14. But how does a 9m tube fit into a space, which is less than a metre long? It is extremely folded ! • In addition, the tube passes through organs on its route from the mouth to the anus.

15. The Digestive system

16. Let’s start in the mouth. • As food moves through the system, it passes through these organs. • Each organ has a slightly different effect on the food.

17. The mouth is where digestion begins. It is therefore where we first meet chemical and physical methods of digestion. We will consider physical digestion first. If you look in the mirror and smile, you immediately notice your teeth. You will also realise that your teeth are different shapes. You have 4 basic types of teeth.

18. Each is designed to do a different job. Premolar Molar Canine Incisor Central Incisor Latent Incisor Canine 1st Premolar 2nd Premolar 1st Molar 2nd Molar 3rd Molar

20. The shape and size of each tooth is related to the function they have in digesting (breaking down) food. If we look at the teeth of other living organisms they too have these 4 types of teeth. However, the size and shape maybe different. This is because other organisms may have a different diet.

21. Canine • Sharp pointed teeth, which are used to bite and tear food. • Incisors • Small rectangular shaped teeth, which are found between the canines. They are used for cutting food. • Premolars • Found behind the canines and are used to grind soft food. • Molars • Found behind the premolars and are used to grind hard food.

22. Together, these teeth deal with most foods that we put into our mouths. Although they break the food up, they do not effect the chemical bonds. Therefore, they are physically digesting the food. Once the teeth have digested the food, it may be small enough to be swallowed. However, some food can be sharp and it would be uncomfortable to swallow. The food also needs chemically breaking down. Therefore, the mouth produces a substance that solves both of these problems at the same time.

23. These glands (a special type of tissue) produce saliva, a sticky liquid. • As mentioned, the saliva has two jobs. • Being a liquid, it softens the food and allows the digested food to be rolled into a ball just before it is swallowed. • It also contains a chemical known as an enzyme.

24. What is an enzyme? Enzymes are chemicals, which act to speedup chemical reactions. They are produced from glandular tissue, which is found all over the body. In order to understand how an enzyme works, you have to think of it as having a particular shape. We will use the shape below to represent on particular enzyme.

25. Somewhere on the surface of the enzyme is an important region known as the active site. Enzyme In order for an enzyme to be able to speed up a reaction, it must attach to the chemicals that are reacting. It does so using its active site.

26. The red area on these two reacting chemicals represents the area where the active site of the enzyme will attach. The enzyme will attach to both at the same time. + Enzymes are very specific • Enzymes can only speed up certain reactions. If the shape of the reacting chemicals does not match the shape of the active site, the enzyme will not be able to work.

27. Therefore, enzymes are specific to certain reactions. As well as being specific to certain reactions, enzymes are very particular about the environment that they work in. To understand this, think of how you do homework. You probably have a certain place to work, or you work at a certain time, you may like listening to music whilst you work or else you can only work if it is completely silent. Well, different enzymes work best in different conditions. If the condition is wrong, their active site can change shape. One particular enzyme works best in acidic conditions (pH less than 7). If the pH rises and the conditions become alkaline, the enzyme changes shape and stops working. It can no longer fit with the reacting particles of the chemical reaction.

28. pH 10 • Which enzymes are made by the digestive system? • The bulk of the food that enters the digestive system is from the three main food groups: Proteins Carbohydrates Fats

29. Therefore, it is not surprising that the digestive system has glands that produce enzymes that digest these three types of food. Remember that the shape of the chemicals within the different food groups will be different. Therefore the shape of the enzymes that digest these chemicals will also be different. This means that an enzyme that digests fats will not be able to digest proteins. Let us look at the chemical structure of Fats, proteins and carbohydrates to see why they require different enzymes for their digestion.

30. Carbohydrates As we mentioned previously, carbohydrates are chains of identical sugar molecules. The enzyme that digests carbohydrates must be able to break the chemical bonds between the individual sugar molecules. Sites of enzyme attack Sugar The product of the chemical breakdown of carbohydrates is sugar. The sugar is known as Glucose. An enzyme that digests carbohydrates is known as a Carbohydrase.

31. The digestion of carbohydrates can be represented by the following equation. Carbohydrase Carbohydrates Sugars

32. Proteins As with carbohydrates, proteins are made of chains of chemicals. However, instead of the chain containing identical molecules, in protein these molecules are different. Protein is made up of chains of aminoacids. There are over 20 different kinds of amino acid.

33. Imagine a bead necklace made up of over 20 different kinds of bead. Site of enzyme attack Amino Acids The enzymes that digest proteins must be able to break the chemical bonds between the different amino acids. An enzyme that digests protein is known as a Protease.

34. The digestion of proteins can be represented by the following equation. Protease Protein Amino Acids

35. Fats Fats are made up of a molecule of glycerol phosphate attached to three fatty acid molecules. The enzymes that digest fats must be able to break the chemical bonds between the glycerol phosphates and the fatty acids. Fats are also known as Lipids. Its structure can be represented using the following diagram. Fatty Acids Glycerol Phosphate Site of enzyme attack

36. The enzymes that digest fats must be able to break the chemical bonds between the glycerol phosphates and the fatty acids. Fats are also known as Lipids. An Enzyme that digests fat (lipid) is known as a Lipase The digestion of fats can be represented by the following equation. Lipase Fat Fatty Acids +Glycerol Phosphate

37. Of these three enzymes, the only one that is released within the mouth is Carbohydrase. This is partly because the conditions within the mouth are suitable for Carbohydrase action. It works best within an alkaline (pH > 7) environment. Let’s represent the three food groups using colours. Carbohydrates Fats Proteins

38. The digestive action of the mouth can be summarised in the following diagram. Physical Digestion Chemical Digestion The food could now pass down either the Trachea (windpipe) or the Gullet/Oesophagus. Physically digested Sugars Chemically and physically digested

39. Swallowing and Peristalsis

40. Swallowing Notes The semi-digested food has been formed into a ball by the tongue and is now swallowed. The ball firstly moves to the pharynx, the region at the back of the mouth where there is a junction between two pipes. It needs to pass down through the Gullet. To ensure that it does not fall into the trachea and thus block our breathing, a small flap moves to cover the tube.

41. Swallowing Notes • The flap is known as the Epiglottis. If ever food accidentally got into the trachea, we would choke and try to cough it back out. • How do we swallow? • Once the ball of food has passed through the top of the gullet, it is forced downwards by muscle contraction.

42. The swallowing process finishes with a muscle known as a sphincter relaxing and the food passes into the stomach. The sphincter muscle found at the entry and exits points of the stomach acts rather like a drawstring on a bag. It allows the stomach to shut off both entry and exit points and hold food inside it.

43. The Stomach The stomach is basically a muscular bag, filled with Hydrochloric Acid (HCl). cross section of stomach food enters from the gullet muscle tissue glandular tissue makes: Hydrochloric Acid, Mucus and Protease enzyme liquids mix with the food digested food leaves

44. When the food enters the stomach. The sphincter contracts behind it. The food is then subjected to a coordinated attack. Firstly the the hydrochloric acid …attacks any microbes (bacteria) that may have been swallowed accidentally when the food was eaten.

45. Secondly, the hydrochloric acid provides the perfect conditions for Protease Enzyme …to work in. These enzymes begin digesting the proteins in the swallowed food. These proteins are broken down to release the amino acids. Protein Protease

46. Protease enzymes work best under acidic conditions (pH < 7) Amino Acids Thirdly, the muscular tissue of the stomach has the ability to contract and relax and in doing so, physically grinds the food inside it. The mucus is produced to protect the lining of the stomach from the acid. If the mucus were not present, the hydrochloric acid could actually digest the tissue that had made it.

47. Presence of HCl Muscle and Glandular tissue Wall of stomach is protected Layer of Mucus The hydrochloric acid, mucus, food and enzyme solution is given the name, Gastric Juice.

48. A summary of the digestive action of the stomach can be represented in the diagram below. Protein Sugar Fat Carbohydrate Physical Digestion Chemical Digestion Muscular tissue of stomach Protease enzyme and Hydrochloric Acid Sugars Fats Carbohydrates Amino Acids Proteins

49. It is clear that the large insoluble foods are slowly being digested to produce the small soluble foods. Carbohydrates Amino Acids Proteins It is also clear that the Fat has yet to be chemically digested. This happens in the next section of the digestive system.

50. The Small Intestine The name of this organ is deceiving. Although it is called small, it can stretch up to 6 metres in length. In order to fit into the body, it is heavily folded. When the gastric juices leave the stomach, they pass into the small intestine. small intestine