IP2 Topic 7 Nutrition in Humans Notes PDF

Topic 7 NUTRITION IN HUMANS (ENZYMES AND DIGESTIVE SYSTEM) Learning Outcomes By the end of this topic, students should be able to: (a) Describe and carry out tests for [O levels] starch (iodine solution)...

Topic 7 NUTRITION IN HUMANS (ENZYMES AND DIGESTIVE SYSTEM) Learning Outcomes By the end of this topic, students should be able to: (a) Describe and carry out tests for [O levels] starch (iodine solution) reducing sugars (Benedict’s solution) protein (biuret test) fats (ethanol emulsion) (b) Explain what is meant by digestion and its importance [IP] (c) State that large molecules are synthesised from smaller basic units [O levels] starch from glucose polypeptides and proteins from amino acids fats from glycerol and fatty acids (d) Explain enzyme action in terms of the ‘lock and key’ hypothesis [O levels] (e) Explain the mode of action of enzymes in terms of an active site, enzyme-substrate complex and enzyme specificity [reduced O levels] (f) Describe the functions of main regions of the alimentary canal and the associated organs: mouth, salivary glands, oesophagus, stomach, small intestine, pancreas, gall bladder, liver, large intestine, rectum, anus, in relation to ingestion, digestion, absorption, assimilation and egestion of food, as appropriate [O levels] (g) Describe peristalsis in terms of rhythmic wave-like contractions of the muscles to mix and propel the contents of the alimentary canal [O levels] (h) Describe the functions of enzymes (e.g. amylase, maltase, protease, lipase) in digestion, listing the substrates and end-products [O levels] (i) Describe the structure of a villus and its role, including the role of capillaries and lacteals in absorption [O levels] Use the knowledge gained in this section in new situations or to solve related problems. Page 1 of 13 1. The importance of food Food serves as our source of energy for daily activities and cellular processes such as respiration and muscular contraction. In addition, it also provides us with the essential nutrients for growth of the body and to repair worn-out body parts. ❖ Functions of nutrients in the foods The table below shows the importance of the various nutrients found in food. Types of nutrient Uses Sources Carbohydrate Primary source of energy Rice, noodles and potato 1. Growth and repair 2. Needed for cellular reactions Fish, meat, egg, beans and Protein dairy products 3. Chemical messengers in body 1. Source of energy Fat Butter, oil and nuts 2. Insulation against the cold Needed in small amounts to maintain Fruits, vegetables, dairy Vitamins and minerals health products I. Do you know? Fats, proteins and carbohydrates are important biological molecules with characteristic structures and bonds. They are huge structures made up of specific monomers. Recall learning about atomic structure and chemical bonding? You will learn more about these structures in IP3 Biology. Page 2 of 13 ❖ Tests for various nutrients (Food Tests) LO (a) Test for presence of starch – Iodine test i. To the solid sample / 2.0 cm3 of liquid sample, add a few (2 – 3) drops of iodine solution. ii. Observe any colour change. (Presence) The mixture turned from yellow to dark blue. (Absence) The mixture remained yellow. Test for presence of reducing sugar – Benedict’s test i. To 2.0 cm3 of the sample, add an equal volume (i.e. 2.0 cm3 ) of Benedict's solution. ii. Mix well with a glass rod and place in a boiling water bath for 3 minutes. iii. Observe the colour change. Blue Green Yellow Orange Brick-red (Presence) A green / yellow / brown / orange / brick-red precipitate is produced. Note: always record initial and final colour only. Final colour of precipitate could be any colour in the above mentioned range, depending on amount of reducing sugar present. none trace low moderate high (Absence) The mixture remained blue. increasing amounts of reducing sugar Test for proteins – Biuret test i. To 2.0 cm3 of the sample, add an equal volume of 5% sodium hydroxide solution. ii. Add 1% copper(II) sulfate solution drop by drop, shaking after every drop. iii. Observe any colour changes. (Presence) The mixture turned from light blue to violet. (Absence) The mixture remained light blue. Test for lipids – Ethanol-emulsion test i. [First observation] To 2 drops of the sample, add 2.0 cm3 of ethanol and mix well with a glass rod. (If the substance is a solid, grind it first before adding ethanol. Then allow any solid particles to settle before proceeding to the next step.) ii. Decant the ethanol into another test-tube containing 2.0 cm3 of water. iii. [Second observation] Shake the test-tube thoroughly and check for cloudiness. (Presence) First observation: Homogeneous (or clear) mixture was formed with ethanol. Second observation: White emulsion was formed when water was added. (Absence) First observation: Homogeneous (or clear) mixture was formed with ethanol. Second observation: Mixture remained homogeneous (or clear)/ no white emulsion formed when water was added. Page 3 of 13 2. The importance of digestion LO (b), (c) Most food (such as starch, proteins and fats) that we eat consist of large, complex and insoluble molecules which cannot diffuse through our intestinal walls and into the bloodstream ❖ What is digestion? It is a process of breaking down complex and insoluble food molecules into small soluble molecules, which can be effectively absorbed into the body. 1. Starch will be broken down into glucose (a simple sugar) 2. Proteins and polypeptides will be broken down into amino acids 3. Fats will be broken down into glycerol and fatty acids Digestion can be classified into two groups: physical and chemical digestion. The table below shows the difference between these two types of digestion. Physical digestion Chemical digestion Involves breaking up the foods into smaller pieces Involves breaking down of large, complex food to increase its surface area to volume ratio, molecules into smaller and simpler molecules producing a larger exposed surface area for with the help of an enzyme enzymes to act on subsequently. No change in the chemical structure of the food. Change in the chemical structure of the food Caused by: Caused by enzymes 1. chewing action in the mouth 2. churning and squeezing action by the stomach 3. action of bile on fats in the small intestine (Fig. 1) Fig. 1: Action of bile on fats Page 4 of 13  3. Enzymes and its properties LO (d), (e) Definition Enzymes are biological catalysts made up of special proteins. They speed up specific chemical reactions and remain chemically unchanged at the end of the chemical reaction. Catalysts speed up the rate of chemical reactions. These reactions may be anabolic (substances synthesized) or catabolic (breakdown of substances) in nature. Enzymes are specific in their action. In other words, each enzymes works on one type of molecules or a group of similar molecules in a chemical reaction. Only a small portion of the enzyme molecule comes into direct contact with the substrate. This region is called the active site of the enzyme. Binding with the substrate occurs here. The active site has a specific “shape” which is complementary to the shape of the substrate. (Fig. 2) Enzymes that help in digestion (a catabolic reaction) are called digestive enzymes. The table below shows some examples of digestive enzymes and their respective functions. Digestive enzyme Function Amylase Digest starch (amylose) into maltose. e.g. salivary amylase, pancreatic amylase Maltase Digest maltose into glucose. Lipase Digest fats into fatty acids and glycerol Protease Digested proteins into polypeptides (e.g. pepsin and trypsin) Digest polypeptides into amino acids (e.g. erepsin) Fig. 2: Structure of an enzyme II. Do you know? Enzymes, being largely organic in nature, are known as biological catalysts. Do you know that non-biological catalysts exist too? Non-biological catalysts are made up of transition metals or compounds of transition metals and they play an important role in a wide range of applications. For example, the conversion of harmful gases into less hazardous ones using the catalytic converter makes use of catalysts such as platinum and rhodium. You will learn more about the action of catalysts in IP4 Chemistry. III. Do you know? In Biology, enzymes alter the rate of reaction by lowering the activation energy. Activation energy is the energy that a reaction needs to overcome for it to proceed on. You will learn more about enzymes and activation energy in IP3 Biology. Page 5 of 13 ❖ Mode of Action of Enzymes What is the lock-and-key hypothesis? 1. According to the lock and key hypothesis, an enzyme reaction depends on the active site. The substrate is imagined to be like a ‘key’, whose shape is complementary to the shape of the active site (the keyhole) of the enzyme (the ‘lock’). The shape of substrate or substrates fit exactly into the shape of the active site. 2. When the substrate binds to the enzyme’s active site, an intermediate enzyme-substrate complex is formed. 3. Once the reaction is completed, the products formed no longer fit into the active site. They leave the active site and now the active site is free to receive further substrate molecules. IV. Do you know? In IP3 Biology, we will be exploring another hypothesis known as the induced-fit hypothesis in which the substrate may not be entirely complementary to the active site. The active site then alters its shape to allow the substrate to fit snugly for the enzymatic reaction to take place. c Page 6 of 13 4. Nutrition Nutrition is the process of taking in food, absorbing the nutrients and using them for growth and development. In animals, nutrition consists of the following processes: Ingestion − food is taken into the body Digestion − breaking down complex insoluble food molecules into small soluble molecules Absorption − digested food substances are absorbed (diffused) into the body cells Assimilation − some of the absorbed food substances are used to provide energy or converted into new protoplasm Egestion – removal of undigested food in the form of faeces 4.1 The human digestive system LO (f) The diagram shows the human digestive system which is made up of the alimentary canal and the various accessory organs, such as the gall bladder, liver and pancreas. Mouth Salivary glands Tongue Oesophagus Stomach Gall bladder Pancreas Liver Large intestine Small intestine Anus Fig. 5: The human digestive system The alimentary canal (or also termed as ‘digestive canal’ or ‘gut’) is the whole passage which food passes through the body consists of mouth, oesophagus (also known as gullet in primary school, which is no longer an acceptable term in IP Science), stomach, small intestine, large intestine (also known as colon), rectum, and anus. II. Do you remember? In IP1 under the topic of Cells, you learnt that an organ system is a group of organs that work together to perform one or more functions. The human digestive system consists of the alimentary canal (the mouth, oesophagus, stomach, small intestine, large intestine, rectum and anus) plus the accessory organs of digestion (salivary glands, pancreas, liver, and gall bladder). They play a role by adding secretions and enzymes that aid in the digestion process. Page 7 of 13 4.2 The Alimentary Canal LO (f), (g), (h) (a) Mouth In the mouth, chewing first takes place to break the food up into smaller pieces. This physical digestion (also termed as ‘mechanical digestion’) increases surface area to volume ratio of the food so that the digestive enzymes can act on it more efficiently. The saliva in the mouth: (i) contains salivary amylase which breaks down the starch into simple sugars called maltose (chemical digestion) (ii) moistens and lubricates the food for easy swallowing The tongue in the mouth: o rolls the food into a small ball (bolus) and pushes it to the back of the mouth before it enters the oesophagus. (b) Oesophagus LO (g) Rhythmic contraction and relaxation of the muscles in the wall of the oesophagus produces a wave-like motion (also termed as peristalsis), which pushes the food along and into the stomach. No (new) chemical digestion takes place here except for the action of salivary amylase from the mouth. V. Do you know? Peristalsis also occurs in the small and large intestines. It ensures the smooth and efficient removal of undigested matter in the large intestine (egestion). This is necessary to avoid excessive dehydration of the Fig. 6: Action of peristalsis in the oesophagus undigested matter, which could cause constipation. Page 8 of 13 (c) Stomach Further physical digestion of foods occurs by the churning and squeezing action of the stomach. It contains a gastric gland which produces gastric juice – a mixture of proteases and hydrochloric acid (pH 2). Proteins in the food are broken down into smaller chain protein by proteases (chemical digestion): activated by hydrochloric acid Inactivated protease Active protease protease Proteins Polypeptides The hydrochloric acid (i) provides an acidic medium optimal for enzyme (protease) activity (ii) kills bacteria (harmful microorganisms) in the food When the food leaves the stomach, it has become a thick liquid which is released in small amounts at different intervals of time into the small intestine. VI. Do you know? The stomach releases and contains hydrochloric acid. Sometimes, excess acid in the stomach causes a condition known as heartburn. One of the remedies to relief heartburn is through the consumption of antacid tablets, which contain the compound sodium hydrogencarbonate (NaHCO3). Hydrogencarbonates react similarly as carbonates. Using what you have learnt in the topic Acids and Bases, how do you think antacid tablets help to relief heartburn? Construct an equation to represent the reaction that happens. What other role(s) do/ does hydrochloric acid play in the stomach? The acidic environment in the stomach denatures salivary amylase in the bolus of food. (d) Small intestine It is connected to the stomach, liver (through the gall bladder) and pancreas. Complete digestion of the proteins, fats and carbohydrates (Table 1) occurs here due to the action of: o digestive juices by the intestinal glands and the pancreas (chemical digestion) o bile from the gall bladder (produced by the liver) (physical digestion) Towards the lower end of the small intestine, the digested food that passes will be absorbed into the bloodstream while the undigested food (such as fibre, Fig. 7: Small intestine and water and mineral salts) will enter the large intestine. neighbouring accessory organs Page 9 of 13  How are the digested food substances absorbed into the bloodstream? Diffusion or active transport III. Do you remember? Recall what students have learnt in IP1: Diffusion and active transport are ways in which substances that move in and out of the cell. Active transport require energy for substances to move into the cells. Since active transport require energy, what organelles do you expect to be present in the epithelial cells of the small intestines for this to occur? ❖ Roles of accessory organs in digestion – liver, pancreas and gall bladder The liver produces bile (an alkaline greenish-yellow liquid), which is stored temporarily in the gall bladder and flows into the small intestine via the bile duct. The bile helps to: (i) neutralize the hydrochloric acid from the stomach; (ii) break up the fat into smaller fat droplets by emulsification. Emulsification increases the surface area to volume ratio of the fats for efficient digestion by lipase. (physical digestion) The pancreas produces pancreatic juice, which contains pancreatic amylase, protease (trypsin) and lipase. It flows into the small intestine by the pancreatic duct. (chemical digestion) In the small intestine, the intestinal glands secrete intestinal juice. Intestinal juice contains maltase, protease (erepsin) and lipase. (chemical digestion) LO (h) Table 1: Summary of Chemical Digestion in the Mouth, Stomach & Small intestine Digestive Organ Secretion Source Enzyme Functions of enzyme Mouth Saliva Salivary Salivary Digest starch into maltose glands amylase Stomach Gastric Gastric *Protease Digest proteins into polypeptides juice glands (pepsin) Small intestine Pancreatic Pancreas *Protease Digest proteins into polypeptides juice (trypsin) Pancreatic Digest starch into maltose amylase Lipase Digest fats into fatty acids and glycerol Intestinal Intestinal Maltase Digest maltose to glucose juice glands *Protease Digest polypeptides into amino acids (erepsin) Lipase Digest fats into fatty acids and glycerol *specific names of proteases are not required at lower IP level Page 10 of 13  VII. Do you know? Glucose, maltose and starch are examples of monosaccharides, disaccharides and polysaccharides respectively. As the term "polysaccharide" suggests, starch consists of hundreds to thousands of individual glucose molecules bonded together. On the other hand, maltose, being a "disaccharide", is made up of two individual glucose molecules bonded together. You will learn more about this in IP3 Biology. (e) Large intestine (also termed as the ‘colon’) In the large intestine, water and mineral salts are absorbed from the undigested food material. No digestive enzymes are secreted here. Faeces (solid waste) is stored temporarily in the rectum. o Faeces will be forced out through the anus during the process of egestion (or defecation). Fig. 8: Human Digestive System Why is it important for us to eat foods that are rich in fibre when they cannot be digested in the human digestive system? It helps in adding bulk to food to facilitate its movement through the alimentary canal. Page 11 of 13 4.3 Absorption LO (i) Digested products are absorbed into the blood (glucose, amino acids) and lymph (fatty acids and glycerol) at the small intestine. o Glucose and amino acids diffuse into the blood capillaries of the intestinal villi while fatty acids and glycerol diffuse into the lacteal (fine lymphatic vessels). (Fig. 9) [Note: lymphatic vessels are part of the lymphatic system – which plays a role in human’s immunity] Water is absorbed in both the small intestine and large intestine; but most absorption of water takes place at the small intestine. Fig. 9: Structure of inner walls of small intestine villi a fold within the the small intestine Fig. 10: Photomicrograph of intestinal villi Page 12 of 13  LO (i) ❖ How is the small intestine adapted for absorption? Structure Function 1 Small intestine is long Provides sufficient time for absorption 2 Walls of the small intestine have numerous finger-like Increases surface area to volume ratio projections called villi (singular: villus) for efficient absorption of digested food Epithelial cells of the villi have numerous microvilli substances (Fig. 9) 3 Wall of villi (epithelium) is one-cell thick Short diffusion distance for efficient absorption of digested food substances to happen by diffusion and active transport 4 Each villus is richly supplied by blood capillaries Glucose and amino acids are constantly transported away from the small intestine in the blood, hence maintaining a steep concentration gradient for absorption (by diffusion) of digested food substances 4.4 Assimilation Assimilation involves processes in which absorbed food substances (e.g. glucose, amino acids) are used by the living cells or converted to complex substances by the body. For example: o glucose is used in respiration to release energy o amino acids are used to build new proteins like antibodies, hormones and enzymes The liver is important in assimilation. For example, it converts excess glucose into glycogen (a complex carbohydrate used for storage) and amino acids into proteins. 4.5 Egestion The passing out of food that has not been digested and/ or absorbed, through the anus, in the form of faeces. o Undigested food is mostly cellulose in humans since the body lacks the enzymes to digest it. VIII. Do you know? The liver also plays an important role in a metabolic process called deamination, which helps remove excess amino acids from the blood. In this process, the nitrogen-containing part of amino acids (NH2) is converted to form urea that is removed from the body in the form of urine. (KIV: IP3 Excretion) Page 13 of 13