Digestion The Processes Involved and Key Terminology for AS Level © British Nutrition Foundation 2002

What is Digestion? Digestion is both the physical and chemical breakdown of large food particles into smaller ones in order that they become more soluble and easier to absorb by the body. Not all of the foods that we eat need to be broken down e.g. water and simple sugars such as glucose and many vitamin and mineral salts. All foods that we eat cannot be used by the body until they have passed into the bloodstream a process called absorption. Here they are distributed to all the cells in the body where they sustain the complex process of metabolism.

Digestion © British Nutrition Foundation 2002 The process of digestion is a complex one. It starts in the mouth and continues throughout the intestinal tract until the actual absorption of food consumed takes place or until any waste or by products are passed from the body in faeces or urine. The chemical processes involved in digestion are brought about by enzymes. These enzymes speed up digestion meaning that the whole process takes just a few hours. Each stage of digestion involves hydrolysis and is carried out by hydrolysing enzymes or hydrolase's. Some less dense food molecules only need one or two stages of hydrolyses to make them ready for absorption; where as Protein requires a large amount of hydrolytic steps before breakdown is complete.

The Digestive Process The digestive Process involves a very small number of different enzymes or hydrolase's which breakdown proteins, carbohydrates and fats – their names can be seen below. NameFood Group (Substrate) End Product Ready for Absorption. AmylaseStarchMaltose MaltaseMaltoseGlucose LipaseFatsFatty Acids and Glycerol PeptidaseProteinsAmino Acids

Hydrolase's Peptides – can be conveniently sub divided into Exopeptidases which split off amino acids from the ends of protein molecules and Endopeptidases which breakdown the inside of the protein molecules. Lipases – only a few lipases are required for the breakdown of fats. Amylases – that are required for the breakdown of sugars also require quite a few steps before absorption can take place. A whole series of hydrolase's are required for the breakdown of Proteins and carbohydrates as they are so large and complex.

The Stages of Digestion Mouth ╬ Oesophagus ╬ Stomach ╬ Small Intestine ╬ Large Intestine

The Mouth © British Nutrition Foundation 2002 The teeth and jaws crush and grind the food into small particles (physical digestion) and mix them with salvia. The saliva contains an enzyme called Salivary Amylase and sodium chloride which activates the enzyme. The initial hydrolysis of cooked starchy food begins in the mouth and continues as the food travels down the oesophagus and into the stomach. The Salivary Amylase is deactivated in the stomach as it cannot tolerate acid environments. Enzyme Action: Salivary Amylase breaks down starch to sugars

The Oesophagus © British Nutrition Foundation 2002 The food passes down the tube which connects the mouth to the stomach. This tube is called the oesophagus. The food is squeezed along the oesophagus by a process called peristalsis where the muscles contract to push the food along. It takes about 3-6 seconds for food to go from the mouth to the stomach.

The Stomach © British Nutrition Foundation 2002 The stomach is a kind of mixing tank with thick, muscular walls which churn the food into a liquid called chyme. The stomach lining contains cells which produce a liquid called gastric juice. The gastric juice contains the enzyme pepsin and an acid called hydrochloric acid which pepsin needs to work effectively. This acid also kills most of the bacteria which are present in food. Enzyme Action: Pepsin breaks down food - Protein to Peptides and Amino Acids The rhythmic churning of the stomach also called peristalsis mixes the food with the gastric juice.

Digestion in The Stomach cont… The acidity of the chyme increases. The endopeptidase called pepsin converts the inner part of protein into simpler molecules called peptones. The other enzyme in the gastric juice is rennin which causes milk to clot. The production of Gastric Juice is stimulated by the appearance, taste and smell of food. However it too may be inhibited by excitement, depression, anxiety and fear. Food usually stays in the stomach for 1-4 hours, but liquids may pass through in a few minutes. The peristaltic action moves the chyme into the lower region of the stomach where the pyloric valve opens at intervals allowing small portions of Chyme to leave the stomach and enter the duodenum (small intestine).

The Small Intestine – The main stage of digestion!! © British Nutrition Foundation 2002 The small intestine is a tube about 6 meters long. Chyme is squirted into the duodenum from the stomach. As it enters the small intestine the chyme is mixed with a digestive juice from the pancreas called pancreatic juice. Another substance called bile, which is made in the liver, is also mixed with the chyme; and finally intestinal juice which produced in the lining of the duodenum. Pancreatic juice is alkaline which helps to neutralise the acid from the stomach. It also contains several enzymes which help to break starch, fat and protein. Pancreatic Amylase found in the pancreatic juice breaks down starch converting it to maltose.

The Small Intestine cont.. Pancreatic Lipase partly hydrolyses or breaks down some fat molecules converting them to simpler forms for absorption. The endopeptidases trypsin and chymotrypsin among others carry on the breakdown of proteins into peptones. Bile contains bile salts which break down fat into tiny droplets. This process helps lipase split the fat droplets into fatty acids and glycerol. Maltase, Lactase and Sucrase break down the double sugars maltose, lactose and sucrose into absorbable forms. Exopepetidases break down the ends of the protein molecules into dipeptides which contain only two amino acids. These are then broken down into single absorbable amino acids by the enzyme dipeptidases.

Absorption in The Small Intestine © British Nutrition Foundation 2002 The chyme is pushed along the small intestine by the action of peristalsis. As the chyme is digested, vitamins, minerals, amino acids, fatty acids, glycerol, peptides and sugars are released. These are able to enter the bloodstream through the intestinal wall. This is called absorption. The lining of the small intestine is folded and has little finger-like projections called villi which increase the area for absorption. The surface is only one cell thick which speeds up the absorption of nutrients. The amino acids and sugars then pass from the villi wall into the bloodstream and go to the liver. Most of the fatty acids and glycerol go to the lymphatic system (lymph glands) from where they are resynthesized and reformed into fat molecules that are more suitable for use in the body. The nutrients are then transported in the blood to all the cells of the body. In this way the cells get the nutrients they need. Some nutrients provide energy and others are used to repair cells or build new cells. The chyme, which is now less fluid because some water has been removed, passes into the large intestine.

The Large Intestine © British Nutrition Foundation 2002 About 7-9hrs after a meal food that has not been digested in the duodenum passes through the ileocaecal valve and into the large intestine. The large intestine is a long tube, inhabited by bacteria. The remains of chyme contain a mixture of substances which have not been digested. These include fibre, some vitamins and minerals and water. Some of the bacteria in the large intestine break down the fibre to fatty acids and gas. Other bacteria produce vitamin K and some B Vitamins. This bacterial action is not on a large scale but small molecules formed can be absorbed through the walls of the large intestine and into the blood.

The Main Function of The Large Intestine The removal of water from the fluid mass as it passes through the large intestine. By the time it reaches the end of the tube known as the anus it is a semi solid form known as faeces. The faeces contain – undigested food material, residues from digestive juices, large numbers of both living and dead bacteria and water. After having been in the large intestine for about 20hrs these material are passed out of the body. This time can be reduced if the diet is high in fibre because of the increased bulk.

Student Task 1.Using Food Science Nutrition and Health p.23 complete the diagram summary of the digestive process. 2.Use Food Science Nutrition and Health p.22 to complete the handout / diagram of the digestive system. 3.Past Paper Questions on digestion.

Mrs. C Simpson Food Science Nutrition and Health. British Nutrition Foundation. For further information, access: © British Nutrition Foundation 2002