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UNIT 2 - NATURAL PRODUCTS CARBOHYDRATES Carbohydrates are one of the most important groups of compounds in the world. All living things require them for energy. The name ‘carbohydrate’ tells us the elements found in these compounds. ‘carbo’ = carbon ‘hydr’ = hydrogen ‘ate’ = oxygen Sugars are carbohydrates. Alkanols contain the 3 same elements but are not carbohydrates. In a carbohydrate the ratio of hydrogen:oxygen must be 2:1 (as in water, H2O). Photosynthesis (Making Carbohydrates) This is a process by which green plants make the carbohydrate glucose. This is required for energy for the plant to live and grow. Animals then feed on these plants to provide their own carbohydrate needs. The green pigment in plants is called chlorophyll. This traps the energy from the sun to catalyse the photosynthesis reaction. Equation: light carbon dioxide + water glucose + oxygen chlorophyll Formula equation: CO2 + H2O C6H12O6 + O2 The glucose is stored in green plants as starch. Reactions of Carbohydrates 1. Dehydration Heating sugar (sucrose) with concentrated sulphuric acid takes the water out of the sugar. What is left? Equation: sucrose C12H22O11 c.H2SO4 carbon + water C + H 2O 2. Respiration This is the process by which all living things get their energy. Carbohydrates are the main starting materials in this reaction. They are the ‘fuels’ we all need to survive. Getting energy from Carbohydrates What do we get when we burn sugar? Answer: carbon dioxide + water Equation: carbohydrate + oxygen carbon dioxide + water + energy This is also the equation for RESPIRATION. glucose + oxygen carbon dioxide + water Formula equation: C6H12O6 + O2 CO2 + H2O Note that this is the reverse of the equation for photosynthesis. These 2 opposite reactions form a major part of the carbon cycle. NB. Even though green plants photosynthesise during daylight, producing oxygen, they still require some oxygen to meet their own energy needs through respiration. Testing for Starch - Iodine In a dimple tile, add 2 drops of the solution being tested to a dimple. Then add 2 drops of iodine solution and note any changes to the colour. Carbohydrate Glucose Sucrose Starch Fructose Lactose Result The test for starch is that it turns iodine solution from brown to blue/black. Testing For Reducing Sugars Benedict’s Solution Method: 1. Pour 3ml of Benedict’s solution into 5 test tubes. 2. To each test tube add a spatula of one of the carbohydrates being tested. 3. Pour hot water into a 250ml beaker. 4. Place your test tubes in the hot water for 5min. Results: Carbohydrate Colour at start Colour at end Starch Glucose Sucrose Fructose Lactose Conclusion: Benedict’s reagent (solution) gives a positive test (blue orange or brick red) with: Glucose Maltose Lactose These sugars are known as reducing sugars. Sucrose and starch do not give a positive test. Sucrose is the only common carbohydrate which gives a negative result with both the Iodine and Benedict’s tests for carbohydrates. Testing Foods For Carbohydrates Food Does it have starch? Does it have reducing sugars? COMPARING STARCH AND GLUCOSE Tyndall’s Beam When you shine light on a glucose solution, the beam just passes through. However, when you shine light on a starch solution it appears to glow. This is because starch molecules are so big they reflect the light beam. Also, starch does not dissolve easily in water because starch molecules are large, non-polar and covalent while water is polar. Structures of Carbohydrates There are different types of carbohydrates (saccharides) depending on their size. monosaccharides disaccharides polysaccharides C6H12O6 fructose galactose glucose C12H22O11 lactose maltose sucrose (C6H10O5)n cellulose glycogen starch The simplest carbohydrates are the monosaccharides, containing just one sugar unit (formula C6H12O6). The polysaccharides contain over 300 of these sugar units joined together. Glucose Starch Glucose is turned into starch by condensation polymerisation. HO G OH HO G OH HO HO G O G O G G OH + 2H2O OH This is called CONDENSATION POLYMERISATION because for every monomer which joins the polymer a water molecule is produced. General equation: nC6H12O6 (C6H10O5)n + (n-1)H2O Hydrolysis of Starch Starch is broken up into its monomers in the lab and also during digestion in animals by hydrolysis reactions. 1. Digestion Starch is made and stored in plants such as potatoes and wheat. When we eat these foods we need to break down the starch into smaller molecules to allow the carbohydrates to pass through the wall of the gut into the blood. starch G G + H 2O G G + G+ glucose In digestion this reaction is catalysed by enzymes such as amylase (found in saliva). G This works best at 37°C and at near neutral pH. Extreme temperatures and pH can permanently change the shape of enzymes. This stops them working for good and they are said to be denatured. 2. In the laboratory In the lab starch can be broken up into its monomers by heating it with an acid. Testing with iodine shows starch is no longer present, while testing with Benedict’s solution shows the formation of reducing sugars (maltose and glucose). Note that the hydrolysis of starch is the opposite reaction of the formation of starch. condensation polymerisation glucose starch + water acid or enzyme hydrolysis Proteins All proteins contain the elements: Carbon Hydrogen Oxygen Nitrogen Proteins are essential for us as they make up: • • • • • Muscle fibres Skin Hair Nails Enzymes Proteins are natural polymers made up from smaller molecules called amino acids. The number of amino acids joined can range from just 51 (insulin) to hundreds (haemoglobin) to thousands (urease). These amino acids contain 2 functional groups: 1. an amine group, -NH2. 2. a carboxyl group, -COOH. X = H, alkyl group, or a group containing OH, S, P or N. Natural amino acids must have the 2 functional groups attached to the same carbon atom. These are called -amino acids and there are about 20 of them. Glycine and alanine are the 2 simplest. Amino Acids in Nature 1. 2. 3. 26 amino acids are used to make proteins. Proteins are made of many amino acids linked in a specific order. Some amino acids cannot be synthesised in the body so have to be taken in through our diet. Making Proteins Condensation Polymerisation of Amino acids As with polyamides, proteins are made by condensation reactions between amine groups (-NH2) and carboxyl groups (-COOH). This produces the same linking group of atoms. However, in proteins this is known as a peptide link rather than an amide link. Hydrolysis of Proteins As with all condensation polymers proteins can be hydrolysed to give their monomers, the amino acids. In the body this is carried out by enzymes. The amino acids can pass through the gut wall into the bloodstream. They are then taken to cells which build them back up into the proteins needed by the body. [Diagram] Hydrolysis is done in the lab by heating with fairly concentrated hydrochloric acid under ‘reflux’. Once hydrolysed the amino acids can be separated and identified using chromatography. Fats and Oils These are high energy food compounds. They produce about twice as much energy as an equal mass of carbohydrate. Fats tend to come from land animals while oils come from plants and marine animals. Both are broken down to give glycerol and 3 fatty acids but fats are solids at room temperature while oils are liquids. So there must be differences in their structure to cause the different physical states. The main difference is the higher level of unsaturation in oils. Oils have much more C=C double bonds than fats. This means they would decolourise more bromine solution than an equal mass of fat. Consequences of Unsaturation The shape of both oil and fat molecules is roughly that of a tuning fork. If there are no double bonds the molecules can fit into one another. This creates an ordered structure with lots of Van der Waals forces between parallel chains and neighbouring molecules. However, as double bonds appear, the chains cannot pack as closely together. This means less Van der Waals forces between molecules and so, lower melting and boiling points. Hence, the saturated fats are solids and the unsaturated oils are liquids at room temperature. Hardening of Oils If the cause of oils being liquids is the high level of unsaturation it seems reasonable to think that making them more saturated would make them more solid, or harden them. Oils are hardened by hydrogenation – addition of hydrogen across the C=C double bonds. Margarines are made by partial hydrogenation of oils over a nickel catalyst. This is how alternatives to butter are produced from vegetable oils. The Structure of Fats and Oils Fats and oils are triesters, formed from a triol, glycerol, and 3 long-chain carboxylic acids, called fatty acids. glycerol It is in the acid chains that we find any double bonds. Some of the most common fatty acids are: Palmitic acid Stearic acid Oleic acid Linoleic acid CH3(CH2)14COOH CH3(CH2)16COOH CH3(CH2)7CH=CH(CH2)7COOH CH3(CH2)3(CH2CH=CH)2(CH2)7COOH Fats and oils can come from 3 of the same fatty acids, but more commonly a combination of 3 fatty acids. The 3 acids form ester linkages with the glycerol by condensation reactions. Example Hydrolysis of Fats and Oils (Digestion) Esters can undergo a hydrolysis reaction to give an alcohol and a carboxylic acid. Fat and oil molecules can be hydrolysed to give glycerol and 3 fatty acids. This can be done by treatment with superheated steam. In the lab it is normally done using aqueous acid or alkali. Fats and Health The intake of saturated fat in the diet is linked to heart disease in many countries, including Scotland. Mediterranean countries tend to have more unsaturated oils in their diet and the incidence of heart disease in these countries is much lower.