Starter - Mrs Jones A-Level Biology

Starter - Mrs Jones A-Level Biology

Starter Onto whiteboards put the following words into an order to help you describe cellulose, what else can you add? 60-70, Condensation reaction, 1-4 glycosidic bonds, microfibrils, 10 000, hydrogen bonds, - glucose, macrofibrils, pectins State the function of cellulose Cellulose structure

Each cellulose molecule made 10 000 b glucose monomers in straight chains Joined by condensation reactions Forming 1-4 glycosidic bonds H bonds form between the OH groups neighbouring cellulose chains About 60-70 cellulose molecules become cross linked (hydrogen bonds) to form bundles called microfibrils These are then held together by more hydrogen bonds=

macrofibrils Wound in helical arrangement around the cell Laid down in layers held together by matrix Matrix = polysaccharide glue: hemicelluloses and pectins Homework: Hand in: Summary sheet on carbohydrates NEW: Prepare a summary sheet on the Food tests for carbohydrates pages112-115 Plot a calibration curve : use results given to you Learning outcomes Some will: Describe how the concentration of

glucose in a solution may be estimated semi quantitatively and quantitatively Most will: Summarise the expected results for the chemical tests and apply their understanding to some exam questions All will: carry out chemical tests to identify the presence of carbohydrates. PLTS: To organise my time and resources to plan my actions effectively Practical biochemistry: Food tests Food tests are simple tests that show the presence of various biological molecules in a sample. These can be questioned on in the exams: you

need to know the method, results expected and how to interpret. We are going to carry out 3 food tests today and gather qualitative and quantitative results: Starch, reducing and non-reducing sugars REACTION BETWEEN STARCH AND IODINE SOLUTION: what happens? When iodine in potassium iodide solution is added to starch, the iodine molecules pack inside the amylose helix to give a blue-black colour from a yellow-orange colour When iodine reacts with the starch in this piece of potato, the blue-black colour develops Reducing sugars: Benedict's test

Testing for reducing sugars: A reducing sugar is a monosaccharide or a disaccharide. Reducing: A molecule of the sugar can react with other molecules giving electrons to them (OIL RIG) Benedicts solution is a turquoise/blue solution containing copper ions and sodium hydroxide; the copper ions exist as Cu2+in this reagent If a sugar is a reducing sugar then the Cu2+ ions are reduced to Cu+ which, in the presence of alkaline sodium hydroxide, form copper oxide Copper oxide is insoluble and precipitates out of the solution as a brick-red precipitate When a reducing sugar is heated with Benedicts solution

(alkaline copper sulphate) the solution will change from blue to an orange red (Benedicts test) The orange red is described as a precipitate because it has come out of solution and forms suspended dispersed solid particles The result is positive if there is a colour change and negative if not. The colour scale below is used as a results range for the test which is used to describe the amount of reducing sugar in a sample based on the strength of the colour and the colour change: (nothing) blue green yellow orange red (lots) BUT We call these tests semi qualitative because they dont produce quantifiable results. They tell us what is present but not how much

REDUCING SUGARS When Benedict's test is performed with the disaccharides maltose and sucrose, the following result is obtained Sucrose is a non-reducing sugar Sucrose Result Maltose is a reducing sugar Maltose

Result Non-reducing sugars Sucrose is a disaccharide formed by linking glucose and fructose with a glycosidic bond in a condensation reaction ( glycosic bond is different to maltose and so does not react with Benedicts) In order to determine if sucrose is present in a sample or solution then the following procedure is performed: The sample or solution under consideration is boiled for at least fifteen minutes in hydrochloric acid Boiling in acid breaks glycosidic bonds splits sucrose to give glucose and fructose (both monosaccharides)

This procedure is called acid hydrolysis The solution is then cooled and neutralised by adding drops of sodium hydrogencarbonate solution (alkali) (Benedicts needs to be in alkali medium) Then carry out reducing sugar test If brick red precipitate formed then sucrose was present in original sample Benedicts test: Semi quantifiable: Using the Benedicts test reveals the presence of reducing sugars, resulting in an orange-red precipitate The more reducing sugar there is the more copper sulphate (Benedicts solution) will have been used up and more precipitate formed. The precipitate can be filtered out and the

concentration of the remaining solution measured telling us how much Benedicts solution has been used up allowing an estimate of the concentration of reducing sugar in the original sample Could have a known standard solution which you could compare to Obtaining quantifiable results: Colorimeter : a device which shines a beam of light through a sample calculating percentage light transmission; The sample is placed into a cuvette which goes into the colorimeter, and then a photoelectric cell picks up on the amount of light transmitted, and the reading gives a measure of the amount of reducing

sugar, based on the principle that the more copper sulphate that has been used up the less light will be blocked out Sugar solution + Benedicts HEAT Precipitate in solution Filter precipitate out Filtrate DARK BLUE MEDIUM BLUE COLOURLESS

Transmission in colorimeter Less transmission=less sugar Medium transmission= medium sugar Highest transmission= Highest sugar Higher concentration of reducing sugar, the more blue Benedicts

solution will be used, more precipitate formed SO the filtrate will be clearer so more transmission Quantify: Calibration curve Whilst using a colorimeter alone will provide a measure, it doesnt specify an exact amount: in order to quantify the amount, a calibration curve must be made Take a range of known concentrations of reducing sugar, carry out a Benedicts test on each one, then filter out the solution; use a colorimeter to give readings of the amount of light passing through the solutions (transmitted) Plot the readings in a graph to show the amount of light getting through (transmission) against reducing sugar concentration Then you can take the reading of an unknown concentration use the graph to make a precise measurement

This is known as an assay. Look at the calibration curve on page 115 Colorimeters Can also measure absorption What would your results be if you were measuring absorption? Less absorbance= more sugar How else could quantifiable results be obtained??? THINK?SUGGEST? Quantifiable results: Filter the precipitate and weigh it Greater mass= more sugar present

Could compare to a standard curve Centrifuge: look at the size of the pellet produced/ colour of the liquid, to indicate the concentration Task: in groups of 3-4 You need to complete the following: 1. Carry out Benedict's test for reducing sugar on one solution 2. Use the filtrate to use the colorimeter to quantitatively measure the concentration of reducing sugar present 3. Note your result onto the board 4. Carry out non reducing sugar test 5. Summarise your understanding of the tests

6. Complete exam questions Biochemical food tests Watch Plenary Complete the summary table on the practical biochemistry tests for the presence of carbohydrates Test for Starch Reducing sugars Non-reducing sugars

Description of the test Expected result Test for Description of the test Expected result Starch Add a few drops of iodine solution

Add Benedicts solution, heat to 80C in a water bath Brown to blue-black (If reducing sugar test is negative) boil with hydrochloric acid, cool and neutralise with sodium carbonate solution, repeat Benedicts test Initially no change, repeated Benedicts test will turn blue to

orange-red Reducing sugars Non-reducing sugars Blue to orange-red Benedicts test results Plot onto a graph Concentration of glucose /g dm 10 5

2 1 0.1 % Transmission 98 78 36 12 2

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