JWSS Biology Chapter 5A Nutriton in Humans

3E Science (Biology) Chapter 5 Notes 1 Chapter 5 : Nutrition in humans Learning Outcomes: Candidates should be able to: (a) describe the functions of the various parts of the digestive system: mouth, salivary glands, oesophagus, stomach, duodenum, pancreas, gall bladder, liver, ileum, colon, rectum, anus, in relation to ingestion, digestion, absorption, assimilation and egestion of food, as appropriate (b) describe the functions of enzymes (e.g. amylase, maltase, protease, lipase) in digestion, listing the substrate and end-products (c) state the function of the hepatic portal vein as the transport of blood rich in absorbed nutrients from the small intestine to the liver (d) state the role of the liver in: • conversion of glucose to glycogen and vice versa • fat digestion • metabolism of amino acids and formation of urea • breakdown of alcohol • breakdown of hormones (e) define a hormone as a chemical substance, produced by a gland, carried by the blood, which alters the activity of one or more specific target organs (f) outline how blood glucose concentration is regulated by insulin and glucagon (g) describe type 2 diabetes mellitus in terms of a persistently higher than normal blood glucose concentration due to the body's resistance to insulin or insufficient production of insulin (h) identify the risk factors of (e.g. unhealthy diet and sedentary lifestyle) and ways to manage type 2 diabetes mellitus (a) Describe the functions of main regions of the alimentary canal and the associated organs: mouth, salivary glands, oesophagus, stomach, duodenum, pancreas, gall bladder, liver, ileum, colon, rectum, anus, in relation to ingestion, digestion, absorption, assimilation and egestion of food, as appropriate Recap: i. What is nutrition? Nutrition is the process by which organisms obtain food and energy for growth, repair and maintenance of the body. In humans, nutrition consists of the following processes: • Ingestion (Food is taken into the body) • Digestion (Large food molecules are broken down into smaller, soluble molecules that can be absorbed into the body cells) • Absorption (Digested food substances are absorbed into the body cells) • Assimilation (Some of the absorbed food substances are converted into new protoplasm or used to provide energy)

3E Science (Biology) Chapter 5 Notes 2 ii. The human digestive system • It consists of the alimentary canal and the organs (liver, gall bladder and pancreas) associated with it. Organ Function Mouth Teeth-which break down large pieces of food into smaller pieces. This increases surface area of the food so enzymes can act on it more efficiently. Salivary glands- secrete saliva. Saliva contains enzymes amylase which helps in digestion of starch. Tongue- helps to mix food with saliva and roll them into bolus for easy swallowing. Oesophagus Liver Pancreas Stomach Ileum (Small intestine) Colon Duodenum Gall bladder

3E Science (Biology) Chapter 5 Notes 3 Oesophagus It is a narrow muscular tube which connects to the stomach. The wall of the oesophagus contains two layers of muscles. Longitudinal and circular muscles which work antagonistically to push food down to the stomach. Peristalsis, the rhythmic wave-like muscular contractions in the wall of the alimentary canal occurs here. Stomach It is a distensible muscular bag with numerous gastric glands that secretes gastric juice. Its main function is to store food temporarily as well as for digestion of proteins. Duodenum It is the first part of the small intestine. The linings contain glands which secretes digestive enzymes. Most of the digestion occurs in the duodenum. Ileum It is the last part of the small intestine. Most digested food is absorbed in the ileum. The walls of the small intestine are specially adapted to absorb digested food and water. Large intestine It is about 1.5 metres long and consists of the colon and rectum. The main function of colon is to absorb water and mineral salts from the undigested food. No digestion occurs in the large intestine. Faeces are stored temporarily in the rectum. Anus Faeces are expelled through the anus. Associated Organ Function Gall bladder Bile is stored temporarily in the gall bladder. When the gall bladder contracts, the bile flows into the duodenum via the bile duct. Liver The liver cells produce and secrete bile. Bile is an alkaline greenish-yellow liquid containing bile salts and bile pigments. Bile pigments give bile its color and are removed with the faeces or undigested matter. Pancreas The pancreas is connected to the small intestine via the pancreatic duct. The pancreas produces pancreatic juice which contains digestive enzymes. The pancreas also secretes hormones insulin and glucagon. These hormones play an important role in regulating blood glucose level in the body.

3E Science (Biology) Chapter 5 Notes 4 i. Digestion in humans • What is digestion? The process whereby large food molecules are broken down into smaller, soluble molecules that can be absorbed into the body cells. Digestion involves two processes: Physical digestion and chemical digestion Physical digestion involves mechanical break-up of food into small particles. This occurs: In the mouth, when you chew food by action of the teeth and tongue. In the stomach, where continual contractions and relaxations of the muscles in the stomach wall cause a churning action. This breaks up the food particles and mixes them with digestive enzymes. Physical digestion increases surface area to volume ratio of the ingested food so that digestive enzymes can act on the food more efficiently. Chemical digestion is the breaking down of large molecules in food such as proteins, starch and fats, into smaller soluble molecules which can be absorbed. This involves hydrolytic reactions catalysed by digestive enzymes. • Why does food need to be digested? i. To break down large complex food molecules into smaller, simpler soluble molecules that can pass through the partially permeable cell membranes. (Refer to diagram below)

3E Science (Biology) Chapter 5 Notes 5 Digestive processes along the alimentary canal Mouth • Salivary glands are stimulated to produce saliva • Saliva contains enzyme called salivary amylase which digests starch to maltose • pH of saliva is neutral • Chewing breaks down food into smaller pieces, this helps to increase surface area to volume ratio for faster digestion by amylase • The tongue rolls the food into small boli that are easy to swallow Stomach • Gastric glands are stimulated to produce gastric juice • Peristalsis in the stomach wall churns and breaks up food. Peristalsis also helps to mix the food well with gastric juice. • Gastric juice is a dilute solution of hydrochloric acid (pH 2) and enzyme pepsin. The hydrochloric acid helps: i. stop action of amylase by denaturing it ii. provides optimum pH for enzyme pepsin to work iii. kills harmful microorganisms in food • Food normally remains in the stomach for about three to four hours. The partly digested food becomes liquefied, forming chyme.

3E Science (Biology) Chapter 5 Notes 6 Small intestine 1. Chyme enters the small intestine. This stimulates accessory organs (pancreas and gall bladder) to do the following: a. pancreas produces pancreatic juice • Pancreatic juice contains enzymes pancreatic amylase, pancreatic lipase and trypsin (a peptidase). These are passed through pancreatic duct into the duodenum. b. The gall bladder releases bile. Bile is transported via the bile duct into the duodenum. • Bile helps to emulsify fats to tiny fat droplets. This increases surface area to volume ratio for faster digestion by enzyme lipase. 2. The epithelial cells of the small intestine produces intestinal juice containing enzymes m