1. State the function of cellulose
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, , hydrogen bonds, β- glucose, macrofibrils, pectins
State the function of cellulose

2. Cellulose structure Each cellulose molecule made
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 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

3. Homework: Hand in: Summary sheet on carbohydrates NEW:
Prepare a summary sheet on the Food tests for carbohydrates pages
Plot a calibration curve : use results given to you

4. 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

5. 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

6. 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

7. 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)
Benedict’s 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

8. When a reducing sugar is heated with Benedict’s solution (alkaline copper sulphate) the solution will change from blue to an orange red (Benedict’s 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 don’t produce quantifiable results. They tell us what is present but not how much

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

10. 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 Benedict’s)
In order to determine if sucrose is present in a sample or solution then the following procedure is performed:

11. 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) (Benedict’s needs to be in alkali medium)
Then carry out reducing sugar test
If brick red precipitate formed then sucrose was present in original sample

12. Benedict’s test: Semi quantifiable:
Using the Benedict’s test reveals the presence of reducing sugars, resulting in an orange-red precipitate
The more reducing sugar there is the more copper sulphate (Benedict’s 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 Benedict’s 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

13. 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

14. Less transmission=less sugar Medium transmission= medium sugar
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

15. Quantify: Calibration curve
Whilst using a colorimeter alone will provide a measure, it doesn’t 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 Benedict’s 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

16. How else could quantifiable results be obtained???
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?

17. 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

18. Task: in groups of 3-4 You need to complete the following:
Carry out Benedict's test for reducing sugar on one solution
Use the filtrate to use the colorimeter to quantitatively measure the concentration of reducing sugar present
Note your result onto the board
Carry out non reducing sugar test
Summarise your understanding of the tests
Complete exam questions

19. Biochemical food tests
Watch

20. Description of the test
Plenary
Complete the summary table on the practical biochemistry tests for the presence of carbohydrates
Test for
Description of the test
Expected result
Starch
Reducing sugars
Non-reducing sugars

21. Description of the test
Test for
Description of the test
Expected result
Starch
Add a few drops of iodine solution
Brown to blue-black
Reducing sugars
Add Benedict’s solution, heat to 80°C in a water bath
Blue to orange-red
Non-reducing sugars
(If reducing sugar test is negative) boil with hydrochloric acid, cool and neutralise with sodium carbonate solution, repeat Benedict’s test
Initially no change, repeated Benedict’s test will turn blue to orange-red

22. Benedicts test results
Plot onto a graph
Concentration of glucose /g dm³
% Transmission 10 98 5 78 2 36 1 12
0.1