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Transcript
1 PROTEIN What are proteins? Proteins are complex substances that are essential for the structure and function of all living cells. Our muscles, cartilage, ligaments, skin and hair are made up of protein. Proteins are very important for body building. Functions of proteins 1. For ______________________ of body cells to form ______________________. E.g. muscles, bones, teeth, skin, lungs, blood. 2. For ___________________ and _________________ of worn-out or damaged cells E.g. to heal a cut, bruise or broken bones, to replace worn-out skin 3. To form _______________________________in the body. E.g. Antibodies – to protect against diseases. Enzymes – to digest food Hormones – to regulate growth and other body functions Haemoglobin – to carry oxygen in the blood Myoglobin – in the muscles for storing oxygen. Fibrinogen – to help blood clot 4. To regulate the amount of body fluid in and around the tissues. 5. To supply heat and energy if there are not enough of other energy food in the diet (carbohydrate and fat). What is the elemental composition of protein? Protein is made up of: _____________________________ _____________________________ _____________________________ _____________________________ It also contains small amounts of: _____________________________ _____________________________ What is the chemical composition of protein? F&N-Protein-Year 9 2 Proteins are made up of basic units called __________________ ________________. There are at least 22 naturally occurring amino acids. Different proteins consist of a varying number and combination of these amino acids. When two amino acids combine, they form a structure known as a ______________. A strong bond known as a peptide bond forms between the amino acids. If three or more amino acids are linked by peptide bonds, a _________________ is formed. The amino acids link with one another to form long coiled chains. A protein molecule may be made up of one or more of such chains. Types of amino acids There are two types of amino acids - ___________________ _______________ ________________ and __________________________ _________________ __________________. Essential amino acids Certain amino acids cannot be manufactured by the body and must be obtained from food. They are known as ____________________ __________________ _____________. In children, ________________ essential amino acids are required for growth and repair. In adults, ___________________essential amino acids are required for repair and maintenance. Non-essential amino acids Non-essential amino acids can be made by the body. Hence, it is not necessary to obtain these amino acids from food. Biological Value of Protein Biological value of protein is a measure of how nourishing a protein food is. The biological value of a protein food is measured by the number of essential amino acids it contains in proportion to the body requirements. Types of protein F&N-Protein-Year 9 3 High Biological Value (HBV) Protein High biological value (HBV) protein contains _______ the ________________ ___________ _______________ in the ____________ proportion needed by the body. They are known as ______________ protein and most of them are from ___________ sources. Examples eggs, human milk have the highest biological value followed by meat, fish, cheese and cow’s milk. Low Biological Value (LBV) Protein Low biological value (LBV) protein _______ _______ contain ______ the ________________ ________________. They are lacking in one or more of the essential amino acids. They are called ______________________protein. Most of them are from _________________________sources. Examples: cereals, pulses (peas, beans, lentils), nuts and vegetables. Exception Soya beans from plants contain proteins of high biological value. Gelatine from animal source contains no essential amino acids. What are complementary proteins? LBV proteins are also important. They can be combined in such a way that the essential amino acids lacking in one type of food present in other. In other words, the protein content of one food compensates for the other’s deficiencies. This is called complementary protein. Examples: eating bread with cheese, spaghetti and meatballs, rice and fish curry. Sources of Protein Meat Beef Mutton Veal Venison Pork Poultry Offal Animal Proteins Seafood Fish Prawns Crabs Lobster Crayfish Mussels Oysters Plant Protein F&N-Protein-Year 9 Farm Products Milk Dairy products such as cheese and yoghurt Eggs 4 Pulses Soya beans Lentils Peas Legumes Long beans Broad beans French beans Nuts Peanuts Almond nuts Cashew nuts Requirements of Protein The recommended intake of protein each day is 1gram for each kilogram of body weight. For example, if a person weighs 70kg, he will require 70g protein each day. More is required for children, teenagers, pregnant and nursing mothers. Babies and children require a lot of protein as they are growing rapidly. An average child requires 30-50g per day. Adolescents require protein for their rapid spurt of growth, about 60-75g per day. Pregnant women require more than normal to cater for the growing baby (90g per day). Nursing mothers require more than normal for milk production during breast-feeding (lactation). Extra protein is also required during illness, convalescence and after surgery. Effects of Deficiency 1. Degeneration (wasting) of the body as worn-out cells are not replaced. 2. Retardation of growth in children. 3. Malfunctions of various organs due to hormone and enzyme deficiency. 4. Lack of anti-bodies, which makes the body more susceptible to disease. 5. Possible obesity, due to overindulgence in high-calorie foods. 6. Severe deficiency (e.g. in underdeveloped countries) results in weight loss and disease. Two protein deficiency diseases are Marasmus, caused by a lack of protein and calories and Kwashiorkor, a protein deficiency which affects children. Proteins in Food Preparation Denaturation When proteins undergo a change in their structure, the process is known as denaturation. When denaturation occurs, the bonds that hold the shape of the coiled polypeptide chains break. The original structure of the protein is replaced by a looser, less compact structure. Denaturation often affects the original properties of the protein. Heat is the most common means of denaturing proteins. The extent and speed of protein denaturation depends on the structure of the protein and the temperature applied. Proteins F&N-Protein-Year 9 5 can also be denatured by pressure, presence of chemicals or by mechanical means such as beating, whipping and kneading. Coagulation Coagulation is the reaction in which a protein changes from a liquid state to a semisolid or solid state in the presence of heat, pressure or chemicals. In coagulation, protein molecules unfold, collide with each other and clump together to form a solid. Coagulation is an irreversible reaction. Examples of coagulation in food are: Cheese, when heated to about 70°C becomes rubbery and tough. Milk proteins coagulate as milk is heated and form a ‘skin; on its surface. The protein albumin in egg white denatures and coagulates at about 60°C and the egg white turns solid and opaque. The egg yolk coagulates at 70°C and on further heating becomes hard and crumbly. Maillard Browning Maillard browning is a complex chemical reaction that occurs when foods containing sugars and proteins are heated. Upon heating, the proteins in the food denature and react with the sugars to form several compounds, giving rise to a brown colouration, usually seen on the surface of the food. As a result, the flavour and aroma of the food are greatly enhanced. This reaction generally occurs at temperatures of 150°C and above. This is why productions of Maillard browning are typically found on the surface of the food being cooked, where the temperature is high. Effects of Maillard browning can be seen on bread crust and the surface of roasted meat. Foaming Foam is formed when air is incorporated into a protein such as egg white. When egg white is whisked, the agitation causes the protein albumin to denature. The molecules of the protein stretch and join together loosely. Air bubbles beaten into the egg white are trapped between these protein molecules. This makes the foam stable and elastic. The trapped air bubbles also increase the volume of the mixture. Examples of foods that incorporate foam are whipped cream and meringues. Protein Written Exercises Rewrite the questions on lined paper. Write the answer after each question. 1. Name the elemental composition of protein. 2. What is meant by high biological value protein? 3. What is meant by low biological value protein? 4. What are complementary protein? Give examples. 5. Give three functions of protein. Give examples. 6. What is denaturation? F&N-Protein-Year 9