Edexcel IGCSE Biology Biological Molecules PDF

Head to www.savemyexams.com for more awesome resources Edexcel IGCSE Biology Your notes Biological Molecules Contents 2.7 Chemical Elements in Biological Molecules 2.8 Structure o...

Head to www.savemyexams.com for more awesome resources Edexcel IGCSE Biology Your notes Biological Molecules Contents 2.7 Chemical Elements in Biological Molecules 2.8 Structure of Carbohydrates, Proteins & Lipids 2.9 Practical: Food Tests 2.10 Role of Enzymes 2.11 Temperature & Enzyme Function 2.12 Practical: Investigating Temperature & Enzyme Activity 2.13 pH & Enzyme Function 2.14B Practical: Investigating pH & Enzyme Activity Page 1 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.7 Chemical Elements in Biological Molecules Your notes Chemical Elements Most of the molecules in living organisms fall into three categories: carbohydrates, proteins and lipids These all contain carbon and so are described as organic molecules Page 2 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.8 Structure of Carbohydrates, Proteins & Lipids Your notes Large Molecules are Made from Smaller Molecules Carbohydrates Carbohydrates contain the elements carbon, hydrogen and oxygen A monosaccharide is a simple sugar e.g. glucose (C6H12O6) or fructose Glucose molecules contain lots of energy which can be released in respiration by breaking the bonds between the carbon atoms A disaccharide is made when two monosaccharides join together Maltose is formed from two glucose molecules Sucrose is formed from one glucose and one fructose molecule A polysaccharide is formed when lots of monosaccharides join together Starch, glycogen or cellulose are all formed when lots of glucose molecules join together Polysaccharides are insoluble and therefore useful as storage molecules Glycogen, cellulose and starch are all made from glucose molecules Fats Most fats (lipids) in the body are made up of triglycerides Their basic unit is one glycerol molecule chemically bonded to three fatty acid chains Page 3 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The fatty acids vary in size and structure Lipids are divided into fats (solids at room temperature) and oils (liquids at room temperature) Your notes The structure of a triglyceride Proteins Proteins are formed from long chains of amino acids There are 20 different amino acids When amino acids are joined together a protein is formed Amino acids can be arranged in any order, resulting in hundreds of thousands of different proteins Examples of proteins include enzymes, haemoglobin, ligaments and keratin Page 4 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Amino acids join together to form proteins Protein shape Different proteins have different amino acid sequences resulting in them being different shapes Even a small difference in the amino acid sequence will result in a completely different protein being formed The different sequences of amino acids cause the polypeptide chains to fold in different ways and this gives rise to the different shapes of proteins In this way, every protein has a unique 3-D shape that enables it to carry out its function The shape of a protein determines its function For example: Enzymes have a specifically shaped active site - this is where a specific substrate molecule fits in order for a reaction to take place If the shape of the active site does not match the shape of the molecule that fits into it, the reaction will not take place Antibodies are proteins produced by certain types of white blood cells that attach to antigens on the surface of pathogens The shape of the antibody must match the shape of the antigen so that it can attach to it and signal it for destruction Page 5 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Every enzyme has a different shaped active site-specific to one substrate Exam Tip You should be able to explain the importance of sugars, amino acids, fatty acids and glycerol in the synthesis and breakdown of carbohydrates, proteins and lipids. There will be many examples of each of these molecules throughout the course. Page 6 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.9 Practical: Food Tests Your notes Practical: Food Tests Preparing a sample Before you can carry out any of the food tests described below, you may need to prepare a food sample first (especially for solid foods to be tested) To do this: Break up the food using a pestle and mortar Transfer to a test tube and add distilled water Mix the food with the water by stirring with a glass rod Filter the mixture using a funnel and filter paper, collecting the solution Proceed with the food tests Test for glucose (a reducing sugar) Add Benedict's solution to the sample solution in a test tube Heat in a boiling water bath for 5 minutes Take the test tube out of the water bath and observe the colour A positive test will show a colour change from blue to orange / brick red Page 7 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The Benedict's test for glucose Test for starch using iodine We can use iodine to test for the presence or absence of starch in a food sample Add drops of iodine solution to the food sample A positive test will show a colour change from orange-brown to blue-black Page 8 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes In the presence of starch, iodine will turn from brown to blue-black Protein food test Add drops of Biuret solution to the food sample A positive test will show a colour change from blue to violet / purple Page 9 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The Biuret test for protein Test for lipids Mix the food sample with 4cm3 of ethanol and shake Allow time for the sample to dissolve in the ethanol Strain the ethanol solution into another test tube Add the ethanol solution to an equal volume of cold distilled water (4cm3) A positive test will show a cloudy emulsion forming Page 10 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The ethanol test for lipids Food Test Results Table Page 11 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Important hazards Whilst carrying out this practical you should try to identify the main hazards and be thinking of ways to reduce harm Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles Iodine is also an irritant to the eyes Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately Ethanol is highly flammable; keep it away from any Bunsen burner The Bunsen burner itself is a hazard due to the open flame Page 12 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes Food tests: analysis Write a conclusion to state which food groups are present one of the food samples you tested and an explanation of how you know this. Conclusion: The apple contained both starch and sugar as it tested positive for both the iodine test (orange → blue - black) and the benedict's test (blue → orange). The apple did not contain protein or lipid (fat) as the biuret and emulsion tests were both negative. Applying CORMS to practical work When working with practical investigations, remember to consider your CORMS evaluation. Page 13 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes CORMS evaluation In this investigation, your evaluation should look something like this: C - We are changing the type of food in the sample O - This is not relevant to this investigation as we aren't using an organism R - We will repeat the investigation several times for each food sample to ensure a reliable result M1 - The presence of the specific biological molecule in each food type by noting the colour change M2 -....after testing with each specific testing agent S - We will control the volume of each testing agent used, the quantity of the food sample, the concentration of the testing agents, the temperature of the water bath for the Benedicts test. There may be other examples that you can think of Exam Tip When describing food tests in exam answers, make sure you give the starting colour of the solution and the colour it changes to for a positive result. Page 14 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.10 Role of Enzymes Your notes Enzymes as Biological Catalysts Enzymes Enzymes are proteins that act as biological catalysts to speed up the rate of a chemical reaction without being changed or used up in the reaction They are biological because they are made in living cells Enzymes are necessary to all living organisms as they maintain reaction speeds of all metabolic reactions at a rate that can sustain life For example, if we did not produce digestive enzymes, it would take around 2 - 3 weeks to digest one meal; with enzymes, it takes around 4 hours Often the products of one reaction are the reactants for another (and so on) The mechanism of enzyme action Enzymes are specific to one particular substrate(s) as the active site of the enzyme, where the substrate attaches, is a complementary shape to the substrate When the substrate moves into the enzyme’s active site they become known as the enzyme-substrate complex After the reaction has occurred, the products leave the enzyme’s active site as they no longer fit it and it is free to take up another substrate Step One: Enzymes and substrates randomly move about in solution Step Two: When an enzyme and its complementary substrate randomly collide an enzyme- substrate complex forms, and the reaction occurs Step Three: A product (or products) forms from the substrate(s) which are then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions Page 15 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 16 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes How enzymes work Page 17 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.11 Temperature & Enzyme Function Your notes Factors Affecting Enzyme Action: Temperature Enzymes are proteins and have a specific shape, determined by the amino acids that make the enzyme and held in place by bonds This is extremely important around the active site as the specific shape is what ensures the substrate will fit into the active site and enable the reaction to proceed Enzymes work fastest at their ‘optimum temperature’ In the human body, the optimum temperature is 37⁰C Heating to high temperatures (beyond the optimum) will break the bonds that hold the enzyme together and it will lose its shape This is known as denaturation Substrates cannot fit into denatured enzymes as the shape of their active site has been lost Denaturation is irreversible - once enzymes are denatured they cannot regain their proper shape and activity will stop Page 18 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Effect of temperature on enzyme activity Increasing the temperature towards the optimum increases the activity of enzymes as the more kinetic energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction Page 19 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources This means that low temperatures do not denature enzymes, they just make them work more slowly due to a lack of kinetic energy Your notes Graph showing the effect of temperature on the rate of enzyme activity Page 20 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.12 Practical: Investigating Temperature & Enzyme Activity Your notes Practical: Enzymes & Temperature Amylase is an enzyme that digests starch (a polysaccharide of glucose) into maltose (a disaccharide of glucose) The effect of temperature on the activity of amylase can be investigated Apparatus Spotting tile Measuring cylinder Test tube Syringe Pipette Stopwatch Water Thermometer Water bath Iodine Starch solution Amylase solution Method Add 5cm3 starch solution to a test tube and heat to a set temperature using beaker of water with a Bunsen burner Add a drop of Iodine to each of the wells of a spotting tile Use a syringe to add 2cm3 amylase to the starch solution and mix well Every minute, transfer a droplet of solution to a new well of iodine solution (which should turn blue- black) Repeat this transfer process until the iodine solution stops turning blue-black (this means the amylase has broken down all the starch) Record the time taken for the reaction to be completed Repeat the investigation for a range of temperatures (from 20°C to 60°C) Page 21 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 22 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Investigating the effect of temperature on enzyme activity Results and Analysis Amylase is an enzyme which breaks down starch The quicker the reaction is completed, the faster the enzyme is working This investigation shows: At the optimum temperature, the iodine stopped turning blue-black the fastest This is because the enzyme is working at its fastest rate and has digested all the starch At colder temperatures (below optimum), the iodine took a longer time to stop turning blue-black This is because the amylase enzyme is working slowly due to low kinetic energy and few collisions between the amylase and the starch At hotter temperatures (above optimum) the iodine turned blue-black throughout the whole investigation This is because the amylase enzyme has become denatured and so can no longer bind with the starch or break it down Limitations Page 23 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Note that there are several different ways in which the temperature could be controlled. The method described above is not very precise, an improvement would be to use water baths kept at each temperature Your notes The starch and amylase solutions that need to be used should be placed in a water bath and allowed to reach the temperature (using a thermometer to check) before being used A colorimeter can be used to measure the progress of the reaction more accurately A solution containing starch will be darker than a solution containing glucose (as a result of the colour change of iodine) The absorbance or transmission of light through the coloured solution can be measured using a colorimeter Applying CORMS to practical work When working with practical investigations, remember to consider your CORMS evaluation CORMS evaluation In this investigation, your evaluation should look something like this: C - We are changing the temperature in each repeat O - This is not relevant to this investigation as we aren't using an organism R - We will repeat the investigation several times to make sure our results are reliable M1 - We will measure the time taken Page 24 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources M2 - for the iodine to stop turning black S - We will control the concentration and volume of starch solution, iodine and amylase used in the investigation Your notes Exam Tip Describing and explaining experimental results for enzyme experiments is a common type of exam question so make sure you understand what is happening and can relate this to changes in the active site of the enzyme when it has denatured, or if it is a low temperature, relate it to the amount of kinetic energy the molecules have. Page 25 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.13 pH & Enzyme Function Your notes Factors Affecting Enzyme Action: pH The optimum pH for most enzymes is 7 Some enzymes that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) Some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9) If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be disrupted/destroyed This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop Effect of pH on enzyme activity Page 26 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Graph showing the effect of pH on the rate of activity for an enzyme from the duodenum Exam Tip Remember the terminology when writing about enzymes is very important. Make sure you refer to an enzyme becoming 'denatured' not 'dying'.Being able to describe AND explain the effect of each environmental condition on enzyme action is key.Practise describing and explaining using the graphs and then check your descriptions against your notes. Page 27 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.14B Practical: Investigating pH & Enzyme Activity Your notes Practical: Enzymes & pH Amylase is an enzyme that digests starch (a polysaccharide of glucose) into maltose (a disaccharide of glucose) The effect of different pH levels on the activity of amylase can be investigated Apparatus Spotting tile Measuring cylinder Test Tube Syringe Pipette Stopwatch Buffer solutions Iodine Starch solution Amylase solution Method Add a drop of iodine to each of the wells of a spotting tile Use a syringe to place 2 cm3 of amylase into a test tube Add 1cm3 of buffer solution (at pH 2) to the test tube using a syringe Use another test tube to add 2 cm3 of starch solution to the amylase and buffer solution, start the stopwatch whilst mixing using a pipette Every 10 seconds, transfer a droplet of the solution to a new well of iodine solution (which should turn blue-black) Repeat this transfer process every 10 seconds until the iodine solution stops turning blue-black (this means the amylase has broken down all the starch) Record the time taken for the reaction to be completed Repeat the investigation with buffers at different pH values (ranging from pH 3.0 to pH 7.0) Page 28 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 29 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Investigating the effect of pH on enzyme activity Results and Analysis Amylase is an enzyme which breaks down starch When the iodine solution remains orange-brown, all the starch has been digested This investigation shows: At the optimum pH, the iodine stopped turning blue-black and remained orange-brown within the shortest amount of time This is because the enzyme is working at its fastest rate and has digested all the starch At higher or lower pH's (above or below the optimum) the iodine took a longer time to stop turning blue-black or continued to turn blue-black for the entire investigation This is because on either side of the optimum pH, the enzymes are starting to become denatured and as a result are unable to bind with the starch or break it down Limitations The starch and amylase solutions that need to be used should be placed in a water bath at optimum temperature before being used A colorimeter can be used to measure the progress of the reaction more accurately by measuring the absorbance/transmission of light through the coloured solution A control of iodine solution would be used for comparison Page 30 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A graph showing the optimum pH for an enzyme from a region of the small intestine Applying CORMS to practical work When working with practical investigations, remember to consider your CORMS evaluation Page 31 of 32 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes CORMS Evaluation In this investigation, your evaluation should look something like this: C - We are changing the pH of the environment O - This is not relevant to this investigation as we aren't using an organism R - We will repeat the investigation several times to ensure reliability M1 - We will measure the time taken for M2 - the iodine to stop turning black S - We will control the concentration and volume of the amylase, iodine and starch solution used in the investigation Exam Tip When describing the effect of pH on enzyme activity, it is important to remember that any pH outside of the optimum can lead to the enzyme becoming permanently denatured. 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