UGEB2380 The Chemistry of Life Lecture Notes - PDF

UGEB2380 The Chemistry of Life Academic Year 2024-25 Dr. Sam CK HAU (Department of Chemistry) Energy of Foods Foods and Drinks helps to define our culture and forms an integral part of our families and social lives Trans fats, saturated fats, omega-3s … … ? Food is the chemical fuel...

UGEB2380 The Chemistry of Life Academic Year 2024-25 Dr. Sam CK HAU (Department of Chemistry) Energy of Foods Foods and Drinks helps to define our culture and forms an integral part of our families and social lives Trans fats, saturated fats, omega-3s … … ? Food is the chemical fuel that provides the energy for the human engine; just like gasoline for the engines of motor vehicles 3 Energy, Heat and Work In our body, nutrients in food supply the chemical energy to do work and sustain life by a complex process “Metabolism” Energy - a capacity to do work and is typically measured in terms of its equivalent in heat A common unit used in quantifying heat or energy is the calorie (Latin: calor - heat) 1000 cal = 1 kcal = 1 Cal The technique of measuring the amount of heat is called calorie: the amount of heat (or energy) needed calorimetry to raise the temperature of 1 gram of H2O by 1℃ Calorie: 1000 calories, or 1 kilocalorie 4 Watts, Joules and Calories 1 watt = 1 Joule per 1 second 1 Joule = 0.24 calories 1 calorie = 4.2 joules Example: How many minutes could a 100-watt light bulb remain lit at full brightness if it consumed the energy content of a 275-Calorie candy bar? 5 The Human Body Metabolism - all the chemical reactions that take place in a living organism to support life Convert food into energy and physical substance of the body C12H22O11 + 12O2 sucrose 12CO2 + 11H2O + Energy Metabolism occurs more slowly than combustion and under much milder conditions 6 Nutrients Macronutrients The major components of our foods that provide us with energy and the materials that form our bodies Can be classified as three categories ✓ Fats & Oils ✓ Carbohydrates ✓ Proteins Micronutrients Nutrients we need in very small quantities, such as vitamins and minerals 7 Energy In and Out Energy In Fat and Oil - 9 Cal/g; Carbohydrates - 4 Cal/g; Proteins - 4 Cal/g Alcohol (ethanol) - 7 Cal/g Energy Out Uses energy through exercises and several forms of metabolism Two forms: 1) Thermic effect of food; 2) Basal metabolism Energy Stored Converting an excess of macronutrients to energy storage Long term storage: body fat; Short term storage: glycogen 8 Energy In and Out Thermic Effect of Food (TEF) Energy used in digesting and metabolising food Sense TEF - grow a bit warmer and find your heartbeat a bit faster after a meal Drowsiness after a large meal - diversion of blood from the brain to the digestive system; digestion takes momentary precedence over mental alertness 9 Energy In and Out Basal Metabolism Energy used to keep our hearts beating, our lungs expanding and contracting, and all other organs working to maintain life To measure, a person lies at rest after 12 hours fasting (i.e. Lying at rest - in the absence of exercises; 12-h fasting - precludes the energy used through TEF) A healthy adult - roughly 1 Calorie per hour per kg of body weight; increases with any body stress like illness or pregnancy 10 Fats and Oils Work both for and against life and good health Served as energy storage Helps insulate our bodies heat loss Form a protective cushioning around major organs Have no flavours but substances do add flavours are highly soluble in fat 11 Fatty acids Contains a long hydrocarbon (building up by Carbons and Hydrogens only) side chain and a carboxylic acid head Such chains can vary in: ✦ The number of carbons contained, therefore the length of the chain ✦ The number and nature of carbon-carbon double bonds in the chain Photo credit: https://www.savemyexams.co.uk/ 12 Saturated & Unsaturated Saturated fatty acid - a fatty acid with no carbon-carbon double bonds Chocolate’s rich, creamy, melt-on-tongue texture derives largely from the nature of its fat. Stearic acid (硬脂酸, H3C(CH2)16COOH) makes up about one-third of the fatty acid in chocolate Unsaturated fatty acid - a fatty acid with one or more carbon-carbon double bonds The fat/oil in avocados, almonds, and olive oil are rich in the mono- unsaturated fatty acid, oleic acid (油酸) O HO H 3C 13 Fats and Oils All fats and oils are triglycerides - molecules composed of a glycerol backbone connected to three fatty acids The fatty acid structures will affect the physical properties like the melting point 14 Photo credit: https://www.savemyexams.co.uk/ Fats A triglyceride with a high melting temperature and a tendency to exist as a fat (solid at room temperature) → long side chain and high degree of saturation Saturated side chains adopt a linear orientation Regularity allows for stronger associations between triglyceride molecules → more heat energy to disrupt these strong intermolecular forces → high melting point Triglycerides with saturated side-chains Strong temporary association between molecules Saturated side chains follow more linear patterns, allowing for stronger intermolecular interactions, which result a higher melting temperature 15 Oils A triglyceride with a low melting temperature and a tendency to exist as a oil (liquid at room temperature) → short side chain and low degree of saturation Unsaturated side chains adopt a bent orientation Irregularity allows for weaker associations between triglyceride molecules → less heat energy to disrupt these weak intermolecular forces → low melting point Triglycerides with unsaturated side-chains Unsaturated side chains have kinks, which prevent close, orderly association between molecules, and gives Weaker temporary association rise to a lower melting between molecules temperature 16 Fats and Oils Nature is not that neat and simple Fats and oils of our diets are complex mixtures of triglycerides containing a variety of side chains: long, short, fully saturated, monounsaturated, polyunsaturated Photo Credit: European Food Information Council 17 Fats and Oils Extent of unsaturation can be determined by a measure called “iodine number” - the number of grams of iodine C＝C bond(s) on the side chain that can add to 100 g of of triglycerides (colorless) triglycerides H H I I + C C Coloured Mixture Higher iodine number - more unsaturated; lower Iodine (I2) Dark Purple iodine number - less unsaturated & more fully Addition Reaction saturated H H I C C I Colourless Mixture 18 Fat, Cholesterol and Heart Disease The body requires cholesterol like estrogen (雌激素) and testosterone (睪 丸激素) to aid in digestion, produce Vitamin D and hormones. High levels of cholesterol are closely associated with atherosclerosis (動 脈硬化) - build-up of fat deposits on the inner lining of arteries → reduce blood flow and leads to high blood pressure and heart disease H 3C H 3C CH3 H 3C H 3C HO Cholesterol is a hydrophobic molecule (very little tendency to dissolve in water), and is classified as a steroid because of its core molecule structure of four rings 19 Fat, Cholesterol and Heart Disease Transport of cholesterol through the blood Wrap packets of cholesterol in a sheath proteins and triglycerides (more hydrophilic in the surface) - lipoprotein HDL: high density lipoprotein; LDL: low density lipoprotein HDLs are beneficial by removing cholesterol deposits from the wall of arteries and transport to the liver for disposal!; LDLs deposit cholesterols on arterial walls" 20 Photo credit: Healthline  Fats and Oils in Our Diet Triglycerides rich in mono-unsaturated and polyunsaturated fatty acids - more healthier, lowering the risk of heart disease Those rich in saturated fatty acids or trans-fatty acids - potentially harmful Trans-fatty acids - unintended byproduct of the catalytic hydrogenation of vegetable oil or high temperature cooking Catalytic Hydrogenation H H H H Cis C＝C bond + Hydrogen atoms on H H C C same side of the C C Vegetable Oil H 3C CH3 double bond H CH3 Trans C＝C bond Hydrogen atoms on Catalyst C C opposite side of the H 3C H double bond H H Process used in food industry to harden The C＝C bonds found in vegetable oils H C C H vegetable oils in order to are typically in the cis configuration convert them into semi- solids for margarines 21 Fats and Oils in Our Diet Essential fatty acids (EFAs) Necessary for good health Must come from the diet and cannot produce by our bodies e.g Omega-3 fatty acid - an unsaturated fatty acid with a C＝C occurring between the third and fourth carbons from the end of the chain O C HO H H H H H C C C H C C C C H H C H C H C 2 C C CH2 3 H C C H 1 CH3 H H Lower the risk of heart disease, and rich in salmons, other fatty fishes, walnuts and flaxseed (亞麻籽) 22 Carbohydrates A class of macronutrients including sugars and starches that is an important energy source for organisms Composed exclusively of three elements: carbon, hydrogen and oxygen Ratio of hydrogen to oxygen atoms in all carbohydrates is almost exactly 2:1, as it is in water (H2O) In early days, such chemicals were suggested to be the hydrates of carbon 23 Types of Carbohydrates Classified according to the number of connected sugar molecules, called saccharides The smallest, simplest carbohydrates are the monosaccharides CH2OH C O H HC OH CH Gluocose H OH C C OH H OH CH2OH OH O C CH Fructose C OH H CH2OH OH CH2OH OH C O H HC OH CH Galactose H C C OH H OH 24  Types of Carbohydrates Remove a –OH group from one of the monosaccharides and a –H group from the other, two monosaccharides will join into one by a covalent bond (glycosidic linkage) and become a disaccharide This process is named as dehydration; on the other hand, when one disaccharide split into two monosaccharides is hydrolysis Gluocose Fructose CH2OH CH2OH C O OH H O HC OH CH C H CH H OH C C OH H O C C H H CH2OH OH OH (+H2O) Hydrolysis Dehydration (-H2O) CH2OH CH2OH C O OH H O HC OH CH C H CH H OH C C O C C H H CH2OH OH OH Glycosidic Linkage 25 Carbohydrates and Diet High glycemic-index food (rich in carbohydrates) - associated with high risk of diabetes and obesity (i.e. refined sugars in soft drinks and the flours in white bread) Low glycemic-index food (low in carbohydrates) - digest and absorb slowly and results in more controlled changes in blood sugar levels (i.e. whole grains, whole fruits, beans and most vegetables) Enzyme sucrase (as a catalyst) helps to separate the large sugar into smaller ones, which can penetrate the intestinal wall to enter the bloodstream 26 Carbohydrates and Diet Starch in foods serves as macronutrients; Cellulose in the more fibrous structure of plants like celery stalks provides us the dietary fibre But we can digest starch but not cellulose, why? CH2OH CH2OH CH2OH CH2OH H H H H H H H H OH OH OH OH O O O 𝛽-side OH OH OH OH CH2OH 𝛼-glycosidic linkage H H CH2OH OH OH OH CH2OH H OH CH2OH H OH CH2OH H OH O H 𝛼-side O H OH OH OH O H OH H H OH OH 𝛽-glycosidic linkage Enzymes in our bodies can only recognise the shape of the 𝛼-linkage of starch 27 Carbohydrates and Diet Glycogen Serves as a source of quick energy in animals Make up of 𝛼-glucose rings but highly branched and less units Deposit largely in liver and muscles, where can cleaved quickly to yield individual glucose molecules Cannot be stored up and used up as we sleep and be replenished the next day 28 Carbohydrates and Diet Lactose Dependance on the enzyme (lactase) to hydrolyse sugars The digestive systems of infants and children with plenty of lactase (40% of their calories from the lactose of their mothers’ milk) Gradually, adults lose the ability to produce lactase in large quantities (lactase-deficient) Unabsorbed lactose → fermentation to become lactic acid, CO2 and H2O → gastric distress 29 Proteins Proteins are polymers of amino acids A gene carries the blueprint for a specific protein and each protein serves a particular purpose Proteins are required in daily diet to provide amino acids for making muscles, hair, enzymes and many other cellular components 30 Amino acids An organic compound that contains both an amino (-NH2) group and a carboxyl (-COOH) group Standard amino acid is the 20 amino acids normally found in proteins Amino acids are grouped into four categories according to side-chain polarity Carboxylic Group Amino Group H H O N C C H R OH Side Chain Group General Structure of Amino acids 31 Amino acids Non-polar amino acid contains one amino group, one carboxyl group and a hydrophobic side chain Polar neutral amino acid contains one amino group , one carboxyl group and a hydrophilic side chain but neutral 32 Amino acids Polar acidic amino acid contains one amino group, two carboxyl groups and one of which being part of the side chain Polar basic amino acid contains two amino group, one carboxyl group and the second amino group being part of the side chain 33 Structures of Proteins Four protein structural levels, listed in order of increasing complexity, are primary structure, secondary structure, tertiary structure and quaternary structure 34 Structures of Proteins Primary structure Two amino acids join together with loss of a water molecule to form a peptide link The length of amino acid chain can vary from a few amino acids to many; two amino acids - dipeptide; three amino acids - tripeptide; a polypeptide is a long unbranched chain of amino acids H 2O H 2O H 2O H O H O H O H O H H H H N H C N H C N H C N H C H C O H C O H C O H C O R1 R2 R3 R4 H O R2 H O R4 H N H C C N H C C O H C N C C N C H H R1 H O R3 H O 35 Structures of Proteins Secondary structure Regions of the chain form organised structures like coils Tertiary structure In some types of proteins, the coils bend and twist to form complex, globular shapes Quaternary structure Two of more tertiary structures can bind together to form larger complexes 36 Fibrous and Globular Proteins Our hair, nails and muscles are tough and strong because of their fibrous protein structures The strength comes from the organisation of these protein molecules, lying in parallel strands, entwining much like strands of fibre twisted into strong rope 37 Fibrous and Globular Proteins The peptides of globular proteins bend and twist back to themselves, forming small spheres Move around easily in the blood and other fluids, and are available to do their works when needed Enzymes are globular proteins, as are the proteins of egg white 38 Protein Denaturation Cooking an egg disrupt the attractive force that hold the protein molecules in their native globular shapes Such protein is considered as denature, and form bonds to other protein molecules, which gives the characteristic white colour and texture It is not a reversible transformation