F2 C H A P T E R 3 N U T R I T I O N .doc
- 1. CHAPTER 3 NUTRITION 营养学 Classes of Food 食物的种类/ 分类 Purpose 目的 and Classification 分类 of Food 1. Food is a basic need 基本需求 for all living organisms. 2. Food is needed for several 几个 purposes 目的 such as: (a) providing 提供 energy 能量 for the body cells 体细胞 (b) building 产生 new cells as well as repairing 修复 the damaged 毁坏 ones in the body (c) maintaining 维持 good health 健康 of the organism (d) maintaining 保持 the body temperature 体温 (e) enabling 确保 the organism to grow 成长 3. Food can be classified into seven classes.种类 Carbohydrates 1. Carbohydrate 碳水化合物 molecules contain the elements of carbon 碳, hydrogen 氢 and oxygen. 2. Carbohydrates are oxidized 被氧化 in the body cells to produce energy. 3. The following word equation 字等式 shows the process. 4. Carbohydrates can be found in four main forms: (a) Simple sugar 单糖 The examples are glucose 葡萄糖, galactose 半乳糖 and fructose 果糖. (b) Complex starch 淀粉 It is the carbohydrate stored 储存 in plants. (c) Glycogen 肝糖/糖原 It is the carbohydrate stored in animals. (d) Cellulose 纤维素 It is the main 主要 component 成分 of the plant's cell wall 细胞壁 that cannot be digested 消化 in the human digestive system 消化系统. 5. Functions of carbohydrates: (a) Provide energy 提供能量 (b) Enable living organisms to growth 成长 and reproduce 繁殖 (c) Form 形成 supporting 支撑 structure 结构 such as the cell walls 细胞壁 of plants QUICK FACTS 1
- 2. Rice has 78% of carbohydrates while bread, peanut and onion have 51%., 23%. and 9%. of carbohydrates respectively. Proteins 蛋白质 1. Protein molecules contain the elements of carbon, hydrogen, oxygen and nitrogen 氮. Some proteins also contain sulphur 硫磺 and phosphorus 磷. 2. Protein molecules are made up 由组成 of basic units called amino acids 氨基酸. 3. Proteins are grouped into two categories 分类: (a) Plant proteins are found mostly 大多数 in beans 豆类. (b) Animal proteins are present mostly in meat, fish and egg. 4. Functions of proteins: (a) The growth 成长 of new cells and repair 修复 the damaged body tissues (b) Form enzymes 酶, hormones 激素, haemoglobin 血红蛋白 and antibodies 抗体 in our body (c) Provide energy whenever 不论何时 necessary 需要 5. Protein deficiency 不足 can cause kwashiorkor 恶性营养不良. 6. Excess 超过的 proteins are excreted 排泄 from the body as urea 尿素 in the urine.尿液 Fats 脂肪 1. Fat molecules are made up of carbon, hydrogen and oxygen elements 元素. 2. There are two types of fats: (a) Plant fats such as margarine 人造奶油, coconut oil 椰油, olive oil 橄榄油 and peanut oil 花生油 (b) Animal fats such as butter, egg yolk,蛋黄 ghee 酥油 and milk 3. Animal fats have a higher cholesterol 胆固醇 level compared to plant fats. 4. Functions of fats: (a) Provide energy for the body (b) The layer of fat under the skin acts as 作为 a heat insulator 热绝缘体 to prevent 阻止 heat loss 消失 from the body (c) Help to transport 运输 fat-soluble 溶入脂肪的 vitamins (d) Protect 保护 some of the internal 内部 organs from injuries 受伤 (e) As a component 成分 of cells because cell membranes 细胞膜 are made from fats Vitamins 维生素/维他命 1. Vitamins are organic 有机 compounds 化合物 that are needed in small quantities.少许 2. They are needed to maintain good health and to protect 保护 the body from diseases 疾病. Lack of 缺少 vitamin will cause deficiency diseases.营养缺乏症 3. There are two types of vitamins (a) Fat-soluble 溶入脂肪 vitamins: vitamins A, D, E and K (b) Water-soluble 可溶水 vitamins : Vitamins B and C 2
- 3. 4. The information about vitamins are given in the table below. Vitamin Source 来源 Function 功能 Effect 影响 of deficiency 缺乏 A Milk, fish liver oil 鱼肝油, eggs, carrots, tomatoes • Promotes good vision 视力 at night or in dim 暗淡的 night • Keeps the skin healthy • Night blindness 夜盲 症 • Skin becomes dry and scaly 多鳞 B Nuts 坚果, yeast, eggs, cereals 谷物, milk • Maintains healthy nervous system • Important for the release of energy from carbohydrates • Beriberi 脚气病, • Pellagra 糙皮病 • Dermatitis 皮炎 C Fresh fruits, green vegetables 青 菜 • Maintains healthy gums 牙龈 • Helps in healing 恢复 wounds 伤口 •improves the body’s resistance to diseases. • Scurvy 坏血病 (bleeding 出血 gum) • Get infections 感染 easily D Milk, eggs, fish liver oil, dairy products, formed in the skin under the sunlight. • Assists in 帮助 the absorption 吸收 of calcium and phosphorus • Supports growth of strong teeth and bones 骨头 • Rickets 软骨病 (bones become weak) • Dental decay 牙蛀 E Vegetable oil, whole grains, nuts • Maintains a healthy reproductive system • Sterility 不孕 •poor nerve and muscles function K Green vegetables, tomatoes, egg yolk 蛋黄, liver • Helps in blood clotting 血凝固 • Prolonged 长时间 bleeding 出血 Minerals 矿物质 1. Minerals are inorganic 无机 substances that are needed in small quantities to carry out 进行 body processes and to maintain 维持 the health 健康 of the body. 2. Our body needs more calcium and phosphorus than iron, iodine 碘, sodium and fluorine 氟. 3. The information about minerals are given in the table below. Mineral Source Function Effect of deficiency Calcium 钙质 Milk, eggs, cheese 乳酪, green vegetables, anchovies 凤尾鱼 • Builds healthy bones and teeth • Assists 帮助 blood to clot 凝固 • Normal 正常 functioning of muscles and nerves • Rickets 软骨病 in children, bones and teeth become brittle 脆, blood difficult to clot • Osteoporosis 骨骼疏松症 in aldults • Prolonged 长久 bleeding 流血 Phosphorus Eggs, cheese, meat, milk • Forms healthy bones and teeth • Helps in muscle 肌肉 •Rickets • dental decay (tooth decay)龋齿 • Weak muscle 3
- 4. contraction 收缩 Iron 铁 Meat, liver, eggs, green vegetables • Formation of haemoglobin 血红素 in the blood • Anaemia 贫血症 Iodine Seafood 海产, seaweed 海草, iodised salt • Produce thyroxine 甲状 腺素 hormones • Goitre 甲状腺肿 (swelling 肿 of the thyroid gland) Sodium 钠 Salt, meat, cheese 乳酪 • Normal functioning of nerves 神经细胞 • Balances body fluid • Muscular cramps 抽筋 Fluorine Drinking water, fish, vegetables • Strengthens 加强 the tooth enamel 珐琅 • Dental decay 牙蛀 Potassium 钾 Cereals 谷类, fish, meat, nuts, banana • Normal functioning of nerves • Maintain the balance of fluid • Weak muscles, paralysis 瘫痪 Water 1. Water constitutes 构成 about 70% of our body weight, which is why water is needed the most compared to other food classes. 2. Water is important to the body for several reasons. (a) As a medium 媒介 for various chemical reactions in the body. (b) As a solvent 溶剂 for transporting digested food, oxygen and hormones as well as the excretory products 排泄物 such as urea and carbon dioxide. (c) Maintaining 维持 body temperature. (d) Maintaining the concentration 浓度 of blood. Fibre 纤维 ( Roughage 粗粮) 1. Fibre consists of fibrous material from plant cell walls (cellulose) that cannot be digested 不能被消化 in the human digestive system. 2. Fibre helps in the movement of food through the small and large intestines 大小肠, that is the peristalsis 蠕 动 process in the intestines 肠. HOT TIPS Fibre can prevent constipation.便秘 Analyzing 分析 Nutrient Content in Food (Food Test) Nutrient content can be determined by carrying out food tests on various food samples. Nutrient Reagent Changes of colour Beginning End Starch 淀粉 Iodine solution 碘液 Brown 褐色 Blue-black 黑蓝 Glucosep Benedict's solution Blue Brick-red 砖块红 precipitate 凝结 Proteins Millon's reagent White precipitate Brick-red precipitate 4
- 5. Fats Ethanol 酒精 and distilled water 蒸馏水 Cloudy 朦胧的 solution Milky 浑浊的 emulsion 乳 状液 Activity 1 Iodine test for starch Procedure 1. Add 3 cm3 of bread 面包 solution into a test tube. 2. Add three drops of iodine solution into the bread solution. Observe the colour change. 3. Repeat all the steps using a sample of distilled water 蒸馏水 as a control test 对照实验. Observation The colour of the iodine solution changes from brown 褐色 to blue-black 深蓝色的 in the bread solution but does not change in the distilled water. Inference Starch turns 变 the colour of iodine solution to blue-black. Benedict’s test for glucose: (a) Add Benedicts solution into glucose solution and heat in a boiling bath. (b) A brick-red precipitate is formed 形成 if glucose is present.出现 5
- 6. Millon’s test for proteins: (a) Add Millon’s reagent into the albumen 蛋清 solution and heat in a water bath. (b) A red precipitate is formed if protein is present. Alcohol-emulsion test for fats: (a) Add cooking oil into ethanol, then shake 摇晃 the mixture 混合物 vigorously 激烈. Add an equal 相 同 amount of distilled water into mixture. (b) A cloudy 混浊的 white solution or an emulsion 乳浊液 is formed. The emulsion contains tiny 细小 fat globules 滴 in the water. Fats dissolve 溶解 in ethanol but not in water. 3.2 The Importance of Balanced Diet 均衡的饮食 1. Diet 食物 means food that is consumed 吃 daily. 2.A balanced diet is food that contains all food classes in the right 对的 quantities and proportions 比率. 3.A balanced diet comprises 包含 of about 70% of carbohydrates, 15% of proteins, 15% of fats, vitamins, water, minerals and fibre. 4. A balanced diet (a) provides energy for daily activities 日常活动 (b) prevents infectious 传染的 diseases (c) provides all nutrients needed for growth 5. An unbalanced diet can lead to 导致 several types of diseases such as kwashiorkor, anaemia, beriberi, scurvy, rickets and many others. Food Pyramid 食物金字塔 and Plate Portion 1 The food pyramid is a guide 指 导 which shows the relative amounts different food intake 摄 入 according to 根据 classes of food. 2 The plate portion is a guide about the intake of a certain 某种 quantity classes of food according to its portion size on the plate.盘 6
- 7. Use healthy oil (such as olive oil 橄榄油 and palm oil 棕油) to cook and on salads; limit 限制 the intake 摄取 of butter 牛油, avoid 避免 trans-fat (processed 提炼的 vegetable oil). Drink water, tea or coffee (less or without sugar); limit the intake of milk (1 to 2 servings a day) and juice (1 small glass a day); avoid sugary 有糖的 drinks Eat many and various 不 同 types of vegetable; reduce 减少 the intake of potatoes Eat a variety of whole grains 全谷 (such as bread and whole wheat cereals and brown rice); reduce refined 被提炼的 grains (white bread and white rice) Eat plenty of 许多 fruits Choose fish, poultry 家禽, beans, grains; reduce red meat 红肉 and butter. Avoid processed meat. Figure 3.6 Plate portion (Source: Healthy Eating Plate & Healthy Eating Pyramid, Harvard School of Public Health) Factors that Determine a Balanced Diet 7
- 8. Age 年龄 • A child 小孩 needs more carbohydrates and proteins than an adult 成人 for continuous 持 续 growth. • A teenager 少年人 needs more carbohydrates to maintain an active lifestyle 生活方式. They also need more proteins to build new cells and calcium to build strong bones and teeth. Gender 性别 • Men 男人 need more energy than women from the same age group. • Heat loss is faster in men than in women because men have a lower body fat content.容 量 Physical activity 身体活动 and type of occupation 职业 • An active 活跃 person needs more energy. • A person who does heavy 重的 work such as farmer 农夫 needs more energy than a person who does lighter 比较轻的 work like a secretary.秘书 Factors that Determine a Person’s Balanced Diet Body size 体型 • A person who has a larger body size needs more energy than a person who has a smaller body size. State of health 健康情况 • A person who is recovering 复 原 from illness 疾病 needs more of certain nutrients. • A pregnant 怀孕 woman needs more energy than a woman who is not pregnant. Climate 气候 • People living in cold countries need more energy to keep their body warm.温暖 • People that live in warmer climates do not need as much The Calorific Value 热值 of Food 1. The calorific value or energy value of food is the quantity of energy released 释放 when one gram of food is oxidized 氧化 completely 完全地. 2. The calorific value of food is measured in calorie (cal) or joule (J) unit. 1 calorie 卡路里 (cal) = 4.2 joules (J) 1 kilojoule 千焦耳 (kJ) - 1 000 joules 焦耳 Table 3.4 Calorific value of main food classes Food class Energy value (kj g– 1) Energy value (kcal g – 1 ) Carbohydrates 7 4 Proteins 17 4 Fats 37 9 3 Food labels provide information about the nutritional facts and total calories for each serving. 4 The following example shows the energy value of a meal consumed by a person. Food Energy value (kj/100 g) Chicken rice Fried egg Mustard 150 g 100 g 50 g Chicken rice 1000 Mustard 100 Fried egg 315 8
- 9. Curry puff Coffee 2 pieces 1 cup Curry puff (1 piece) 150 Coffee (1 cup) 294 Calculation Energy value = (1 000 + 500) + (315) + 50 + (2 X 150) + 294 =1 500 + 315 + 50 + 300 + 294 = 2 459 kJ/g QUICK FACTS The calories in a bowl of cooked rice, equal to the calories in 10 teaspoons of sugar. As too much rice interferes 干预 with the absorption of zinc 锌 and iron, we should eat less rice or we will feel lethargic 昏睡 instead of energetic 精力充沛. A Balanced 平衡的 Diet 饮食, Exercise and a Healthy Lifestyle 生活习惯 1 A balanced diet and regular 有规律的 exercises can maintain body health. 2 Practice healthy eating habit 饮食习惯 by (a) having a balanced diet (b) drinking at least 至少 8 glasses of water every day (c) reducing fats, sugar and salt intake 摄取 (d) eating plenty of 很多 vegetables and fruits (e) eating moderately 适度地 3 An intake 摄取 of unbalanced diet leads to many health problems. a). Obesity 肥胖 and diabetes 糖尿病 (excess 多余的 of fats and carbohydrates) b). Blocked 阻塞 blood vessels, high blood pressure 高血压 and heart diseases (excessive fats) c). Kidney 肾 and liver 肝 problems (too much proteins) d). Anorexia 厌食症 （low calorie diet) e). Kwashiorkor (deficiency 缺乏 of proteins) f). Malnutrition 营养不良 (lack 缺乏 of certain 某种 nutrients in the diet) g) high blood pressure 高血压( too much salt) 4. Obesity is related to many health problems such as heart disease, high blood pressure and diabetes. 5. One way to determine 决定 whether the weight of an individual is in normal or healthy category is by calculating 计算 the Body Mass Index 体重指数 (BMI) as follows: (a) Measure body weight in kilogram (kg) and height in meter (m). (b) Formula for BMI: (c) Refer to the Body Mass Index (Table 3.5), to determine the category of body weight based on the calculation in 5(b). Table 3.5 Body Mass Index BMI Category 分类 Less than 20.6 Underweight 重量不足 20.7 - 26.4 Normal 26.5 - 30.9 Overweight 过重 31.0 - 45.2 Obesity 肥胖 9
- 10. More than 45.3 Dangerous 危险 Human Digestive System 消化系统 1 Digestion is the process of breaking down 分解 complex food molecules into smaller and simpler molecules. 2 Digestion occurs in the digestive tracts.消化道 3 There are two types of digestion. (a) Physical digestion is the breaking down of large pieces 大片 of into smaller pieces using teeth and the peristalsis 肠蠕动 actions along the digestive tract. (b) Chemical digestion is the breaking down of complex food molecules into small soluble molecules by digestive enzymes 酶. 4. Chemical digestion takes place 发生 in various parts: (a) The digestion of starch by salivary amylase 唾液淀粉酶 in the mouth. (b) The digestion of proteins by protease 蛋白酶 in the stomach. (c) The digestion of fats by lipase 脂肪酶 in the small intestine.小肠 (d) The digestion of carbohydrates, proteins and fats in the small intestine. Figure 3.7 The chemical digestion by digestive enzymes in the digestive tract 5 Complex food molecules are broken down into small molecules so that these molecules can dissolve and diffuse 渗透 through the wall of small intestine. Table 3.6 End products 最后产物 of carbohydrates, proteins and fats Complex molecules Simple molecules (end products of digestion) Enzymes Carbohydrates Glucose Amylase Proteins Amino acids 氨基酸 Protease Fats Fatty acids 脂肪酸 and glycerol 甘 油 Lipase 6. The digestion process takes place 发生 in the digestive system: 10
- 11. (a) A long tract 长道 (8 m) called the alimentary canal 消化道, which starts from the mouth, followed by the oesophagus 食道, stomach 胃, duodenum 十二指肠, small intestine 小肠, large intestine 大肠, rectum 直肠 and anus 肛门. (b) Other organs that are involved in the digestion process, namely 也就是 the liver 肝 and pancreas 胰. Sequences of the parts of the digestive tract involved in food digestion, absorption and defecation. 5. The information about parts of the digestive system and their functions are given in the following table. Part Function Mouth 口 • Breaks down food particles into smaller pieces 小片. • Salivary glands 唾腺 secrete 分泌 amylase enzyme 淀粉酶 to digest starch 淀粉. • Incomplete 不完整 digestion of cooked starch produces maltose 麦芽糖. Oesophagus 食道 • Peristalsis 蠕动 helps to move the bolus 圆块 (ball-shaped) food from the mouth to the stomach along the oesophagus. Stomach • Retains 留住 the food. • Gastric juices 胃液 are secreted to begin the digestion of proteins. • Secretes acid 酸 that neutralizes 中和 the alkali 碱 in saliva 口水, as well as to kill bacteria in food. Duodenum 十二指肠 • Liver produces bile 胆汁 and pancreas produces pancreatic juice 胰汁 which is released into the duodenum. Small intestine 小肠 • Intestinal juice 肠液 is secreted to complete the digestion of proteins, carbohydrates and fats. • End products 终产物 of digestion are absorbed 吸收. Large intestine 大肠 • Water and minerals are absorbed. • Faeces 粪便 is formed 形成. 11
- 12. Rectum 直肠 • Faeces is stored 储藏. Anus 肛门 • Defecation 排便 to remove 去除 faeces from the body. Flow of Food Particles in the Alimentary Canal The digestion process starts when food is taken into the mouth and ends when the digested food is absorbed into the blood. The process takes about 18 hours to complete. Digestion in the Mouth 1. Food is chewed 咀嚼 and grinded 磨 into small particles by the action of the teeth. 2. The smaller particles of food have a bigger surface area 表面面积 for the enzymes to react upon. 3. The salivary glands secrete saliva that contains the enzyme amylase 淀粉酶. Amylase needs an alkaline 碱性 的 medium to function. 4. Starch in food is digested and converted 转换 into maltose 麦芽糖 (sugar). 5. The saliva also makes the food particles softer 柔软 and easier to be swallowed 吞 through the throat 咽喉. The food particles are shaped into a lump 团块 called bolus 圆块. 6. The bolus is pushed down to the stomach by the contraction 收缩 and relaxation 放松 of the walls of the | oesophagus 食道 through peristalsis 蠕动. 7. Peristalsis occurs along the oesophagus and no digestive enzymes are secreted in this part of the alimentary canal. Digestion in the Stomach 1. The gastric glands 胃腺 in the stomach walls secrete gastric juice.胃液 2. The food bolus is mixed with the gastric juice. 3. The gastric juice contains hydrochloric acid 盐酸 and digestive enzymes.消化酶类 4. The functions of hydrochloric acid are to: (a) stop the reaction of amylase in the saliva (b) create 创造 an acidic 酸性的 medium for the reaction of enzyme in the gastric juice (c) kill bacteria 杀菌 in the food 5. Digestion of proteins starts in the stomach. Enzymes in the gastric juice coagulate 凝固 liquid milk protein into a solid called casein 酪蛋白. Enzyme protease 蛋白酶 digests proteins into polypeptides 多肽. 12
- 13. 6. The partially 部分地 digested food known as chyme 半流体的消化物 is then pushed 推 into the duodenum. Digestion in the Duodenum 1. The first part of the small intestine is called the duodenum. The liver and pancreas are connected to it by ducts 管. 2. The chyme is mixed with the bile and pancreatic juice. 3. Bile 胆汁 is a greenish 微绿的 fluid that is secreted by the liver and stored 储存 in the gall bladder 胆囊. 4. The functions of bile are to: (a) neutralize the acidity of chyme from the stomach (b) prepare 准备 an alkaline medium for the action of the pancreatic juice (c) break fat molecules into small droplets 小滴, so that they are easily digested. This process is called the emulsification 乳化作用 of fat. 5. Pancreatic juice, which is alkaline, is produced by the pancreas. 6. There are three types of enzymes in the pancreatic juice: (a) The lipase enzyme 脂肪酶 digests fats into fatty acids 脂肪酸 and glycerol 甘油. (b) The amylase enzyme digests starch into maltose. (c) The protease enzyme digests proteins into polypeptides. 7. Apart from secreted bile, the liver also: (a) controlling 控制 the sugar level in the blood (excess 多余的 glucose is converted into glycogen 肝糖 and is stored in the liver). (b) converted excess amino acids 氨基酸 into urea 尿素. Urea is expelled from 排出 the body in the form of urine 尿液. (c) store vitamins, such as vitamins A, D and minerals such as iron. Digestion in the Lower Part 下面部分 of the Small Intestine 1. From the duodenum, food enters the lower part of the small intestine. 2. The intestinal glands that are located in the intestinal wall secrete intestinal juice 肠液. 3. The intestinal juice completes the digestion of carbohydrates, fats and proteins. 4. Food digestion ends in the small intestine. 5. The end products of digestion for: (a) carbohydrates are glucose (b) proteins are amino acids (c) fats are fatty acids and glycerol Summary of Digestive Process Organ Glands Secretion Enzyme Chemical process Notes Mouth Salivary glands Saliva Salivary amylase Starch + water → maltose Digestion of carbohydrate begins Stomach Gastric glands Gastric juice Pepsin Protein + water → peptone + polypeptide Digestion of proteins begins rennin Casinogen → casein 13
- 14. Duodenum Pancreas Pancreatic juice Amylase Starch + water → maltose Digestion of lipids begins. Digestion of protein and carbohydrates Trypsin peptone + polypeptide + water → peptides Lipase Fats + water → fatty acids + glycerol Ileum Intestinal glands Intestinal juice Erepsin (peptidase) Peptide + water → amino acid Digestion of carbohydrates, proteins and lipids is complete. Maltase Maltose + water → glucose + glucose Lactase Lactose + water → glucose + galactose Sucrase Sucrose + water → glucose + fructose lipase Fats + water → fatty acids + glycerol Large Intestine 大肠 1. The undigested 未消化的 food enters the large intestine. 2. Water from the undigested food is absorbed by the large intestine. 3. Rectum 直肠 stores the remaining 剩余的 solid (faeces 粪便). Faeces are expelled from the body through the anus 肛门. To investigate the action of salivary amylase on starch Variables Manipulated : Presence of saliva Responding : Presence of simple sugar Constant : Volume of saliva, percentage of cooked starch suspension Hypothesis: Salivary amylase breaks down starch into simple sugars. Procedure 1 The contents in test tube A and B are prepared as in Figure 3.9. 2. The contents from each test tube are tested with iodine solution at 1 minute interval 间断 over 5 minutes. 3. After 5 minutes, the contents in each test rube are tested with Benedict's solution. 14
- 15. Discussion 1 Test tube A serves as 作为 a control 核对实验 for this experiment. 2 All test tubes are heated in water bath at 370 C which is the optimum 最佳的 temperature for enzymatic action. 3 Test cube B contains saliva that has salivary amylase to break down starch into simple sugars. Conclusion Salivary amylase breaks down starch into simple sugars. The hypothesis accepted. 3.4 Process of Absorption and Transportation of Digested Food Products and Defecation 1 Absorption 吸收 is the diffusion of end products 最后产物 of digestion through walls of small intestine into blood capillaries.微血管 2 The characteristics 特点 of the walls of the small intestine adapted 适应 for absorption: • The small intestine is long (about 6 metres) to ensure the digested food is absorbed before it reaches 到 达 large intestine. • The inner surface of the ileum is covered with millions 数百万 of finger-like 像手指 projections 突出物 called villi 绒毛(singular, villus). The villus increases the surface area 表面面积 for the rapid 迅速 absorption of digested end products. • The walls of the small intestine have many blood capillaries to transport 运输 digested food. • The intestinal wail is very thin, only one-cell thick so char digested food can easily and rapidly diffused across blood capillaries. 3 Absorption of end products via villus occurs as follows: • Amino acids and glucose diffuse across epithelial cells into blood capillaries. • Fatty acids and glycerol diffuse across epithelial cells into lacteal.乳糜管 15
- 16. EXPERIMENT 3.2 To investigate the absorption of digested food using a Visking tube 肠衣装管 Problem statement： What substance can diffuse through the Visking cube? Variables： Manipulated: Content of the Visking tube Responding : Change in colour of iodine and Benedict’s solution Constant : Temperature of water bath (37 °C) Hypothesis：Only small molecules (such as simple sugars) can diffuse through the tube Visking. 1 The apparatus are prepared as in Figure 3.11. 2 Iodine and Benedicts tests are conducted on the sample of distilled water to determine the presence of starch and glucose. 3 After 30 minutes, samples of distilled water are taken again from the boiling tube and step 2 is repeated. 1 16
- 17. Discussion 1 The results show that glucose can diffuse through Visking tube, but starch cannot, because glucose molecules are small while starch molecules are too large. 2 In the human digestive system, (a) the Visking tube represents 代表 the small intestine. (b) water in the boiling tube represents blood. 3 In the small intestine, glucose molecules diffuse through the villi into the bloodstream. Conclusion Only small molecules (such as simple sugars) diffuse through the Visking tube. The hypothesis is accepted. Process of Transporting the Products of Digestion 1 All systems work together to ensure the digested food molecules reach the body cells for assimilation.吸收 (a) The digestive system: Absorbs 吸收 simple molecules of digested products through the villus (b) The respiratory system: Supplies 提供 oxygen (c) The circulatory 循环 system: Transports oxygen and simple molecules of digested products to body cells: (i) Amino acids and glucose are transported to the liver 肝 and then to the body cells. (ii) Fatty acids and glycerol are transported to the heart and then to the blood circulatory system to be transported to body cells. 2. The following diagram summarizes the transport of digestive products. Assimilation process 同化，吸收 1 Assimilation is a process of converting 转换 simple molecules of digested products into complex 复杂 molecules that can be used or stored 储存 in body cells. Assimilation process 17
- 18. Amino acids In liver Amino acids are used to build 产生 new cell proteins. In body cells Amino acids are used to build and repair damaged 已毁坏的 tissues.组织 Glucose In liver and body cells Glucose is oxidized 被氧化 through cellular respiration 细胞的呼吸作用 to produce energy, water and carbon dioxide. Fatty acids and glycerol Excessive 多余的 fats are stored in adipose 脂肪的 tissues under the skin which act as 作为 stored energy.储存能量 Defecation 排便 1. Reabsorption of water and the formation of faeces 粪便 occur in the large intestine. 2. Undigested 没有消化的 food and water will be passed down to the first part of the large intestine, the colon 结肠. 3. The size of the undigested product is reduced to one-tenth of its original volume when water is reabsorbed. 4. When faeces are formed, they are kept In the rectum 直肠, that is the last part of the large intestine. 5. Defecation is the process of removing 去除 faeces from the body through the anus 肛门. 6. Fibre 纤维 helps to avoid 避免 constipation 便秘 by absorbing water as it passes along the alimentary canal. 1. A mass of undigested food that are not absorbed in the small intestine move along the large intestine. 2. In the large intestine, water is reabsorbed 4. The process of removing faeces from the body is known as defecation. 3. The undigested food becomes faeces. Faeces are stored in the rectum before being pushed out through the anus. 7. If there is lack of fibre in our diet, the undigested food moves very slowly through the large intestine and too much water is reabsorbed into the blood. 8. Therefore, faeces become hard and dry. This condition is called constipation 便秘 18