Year 11 Human Nutrition (Digestive System) Notes PDF

HUMAN NUTRITION (DIGESTIVE SYSTEM) Definition of terms: 1. Digestion is the breaking down of large insoluble molecules into small soluble molecules that can be absorbed by the blood. It begins in the mouth with the mechanical and chemical breakdown of food and cont...

HUMAN NUTRITION (DIGESTIVE SYSTEM) Definition of terms: 1. Digestion is the breaking down of large insoluble molecules into small soluble molecules that can be absorbed by the blood. It begins in the mouth with the mechanical and chemical breakdown of food and continues through the digestive tract until nutrients are absorbed into the bloodstream and waste is eliminated. 2. Ingestion is the taking of substances, e.g. food and drink, into the body through the mouth. 3. Absorption is the movement of small food molecules and ions through the wall of the intestine into the blood and lymph. 4. Assimilation is the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells. 5. Egestion is the passing out of food that has not been digested or absorbed, as faeces, through the anus. Types of Digestion: a. Mechanical Digestion: It involves the physical breakdown of food into smaller pieces without chemical change to the food molecules (to increase their surface area). It begins in the mouth with chewing and continues in the stomach and small intestine through muscular contractions. b. Chemical Digestion: It involves the breakdown of large, insoluble molecules into small, soluble molecules by enzymes. It occurs mainly in the stomach and small intestine, where enzymes break down carbohydrates, proteins, and fats into simpler forms that can be absorbed. Parts of the digestive system and their functions: A. Mouth: Digestion begins in the mouth were we chew the food using our teeth and it is mixed with saliva, which contains the enzyme amylase. The salivary glands produce saliva which contains mucus. This makes the food slippery and easier to swallow. B. Oesophagus: A muscular tube that transports food from the mouth to the stomach through peristaltic contractions. Peristalsis involves a wave of muscular contractions that squeeze the food bolus carrying it down the oesophagus to the stomach or along the gut. C. Stomach: The stomach is a muscular organ which contracts in both directions, churning the food. Churning helps to physically break it down and also mixes it with gastric juices released by the stomach lining. Mechanical and chemical digestion continues, with the aid of gastric juice containing hydrochloric acid and enzymes. Hydrochloric acid carries out two important functions due to its low pH. i. Kills bacteria in food. The low pH denatures the enzymes in any harmful microbe in the food. ii. It provides the optimum/suitable pH for enzyme pepsin to act. D. Small Intestine: It consists of three parts: the duodenum, jejunum, and ileum. Pancreatic juice and intestinal enzymes are added to the food to break it smaller molecules. Absorption of nutrients takes place here. The small intestine is adapted for the efficient absorption of nutrients as it has: i. A very large surface area ii. A thin lining (one cell thick) so digested food can easily cross the wall into the blood hence, diffusion of nutrient is faster. iii. It has a folded inner lining with millions of tiny finger-like projections called villi (singular: villus) The villi is lined by epithelial cells which have microscopic projections called microvilli. This increases the surface area of the small intestine. E. Large Intestine (Colon): Absorbs water from undigested food, forming faeces. It is divided into the caecum, the appendix, the colon and the rectum. The caecum connects the small intestine to the colon. F. Rectum and Anus: Faeces are stored in the rectum and eliminated from the body through the anus. Other organs include: G. Liver: This produces bile which enters the duodenum along a tube called the bile duct. Bile carries out two important functions i. It breaks down large globules of fats into smaller globules thereby creating a larger surface area for the enzyme lipase to act on. This process is known as emulsification. ii. Bile is alkaline so it neutralises the acidic food from the stomach. H. Gall bladder: Stores bile before it is released into the small intestine. I. Pancreas: This is connected to the duodenum by the pancreatic duct. The pancreas produces a juice (pancreatic juice) which contains enzymes (amylase, trypsin and lipase) as well as an alkali (sodium bicarbonate) to neutralise the acidic food from the stomach. N.B: Amino acids and sugar molecules are absorbed through the villus wall into the blood capillaries inside the villus. These molecules are then transferred from the blood capillaries through the hepatic portal vein to the liver where deamination of amino acids takes place and excess glucose is converted to glycogen. Fatty acids and glycerol are absorbed into the lacteal inside the villus instead of the blood capillaries. Lymph fluid carries fatty acids and glycerol slowly through lymphatic vessels which eventually enter into the blood near the heart. As a result, fat doesn’t enter into the bloodstream too quickly. Intestinal Disease Cholera is a water-borne disease caused by the bacterium Vibrio cholerae. IGCSE FOCUS: Explain how the cholera bacterium brings about dehydration, diarrhoea and loss of salts from the blood in the intestine. ANSWER: The cholera bacterium produces a toxin that causes chloride ions to pass out of the cells into the intestine (lumen). The accumulation of ions in the lumen reduces its water potential. This causes water to move out of the cells and from the blood into the lumen by osmosis. The movement of ions and water from the blood into the lumen of the small intestine causes diarrhoea (loss of water in faeces) and dehydration Diarrhoea caused by cholera can be treated by using ORT (oral rehydration therapy) ORT solution contains: i. Water to rehydrate the blood and tissues. ii. Sodium ions to replace the ions lost from the blood and tissue fluid iii. Glucose to provide energy for the active uptake of sodium ions from the small intestine. iv. Other ions like potassium and chloride to replace the ions lost in diarrhoea. Digestive Enzymes Enzymes Functions Site of secretion Amylase Breaks down starch to maltose i. Mouth or salivary gland ii. Pancreas  Pepsin A protease that breaks down protein into Stomach polypeptides Trypsin A protease that breaks down protein and Pancreas polypeptides into peptides/amino acids Lipase It breaks down fats into fatty acids and glycerol Pancreas Maltase It breaks down maltose into glucose Small intestine Sucrase It breaks down sucrose into glucose and fructose Small intestine Peptidase A protease that breaks down peptides into amino Small intestine acids THE TEETH The human mouth contains four types of teeth: 1. Incisors: These are the front teeth used for cutting and biting into food. 2. Canines: Also known as cuspids, canines are sharp, pointed teeth used for tearing food. 3. Premolars: Premolars have flat surfaces and are used for crushing and grinding food. 4. Molars: Molars are the large, flat teeth at the back of the mouth responsible for further grinding and chewing food. Study chapter 7 of your IGCSE Biology textbook for a better understanding of this concept.