Testing For Saccharides

Question 1

How do you test for starch?

Answer 1

Do the iodine test
-blue/black= starch is present

Question 2

How to test for reducing sugars?

Answer 2

The Benedict’s test;
- add 2cm of food sample (in liquid form)
- add 2 cm of Benedict’s reagent (equal volume)
- heat mixture in water bath for 5 mins

Question 3

Benedict’s test results

Answer 3

No reducing sugar= stays blue
Very low reducing sugar= green
Low reducing sugar= yellow
Medium reducing sugar= brown/orange
High reducing sugar= brick red

Question 4

What causes the change of colour during the Benedict’s test?

Answer 4

• all monosaccharides and some disaccharides are reducing sugars, which means they can donate electrons to another substance to reduce it
  -Benedict’s reagent is an alkaline solution of copper (ii) sulfate
• when a reducing sugar is heated with Benedict’s solution, the reducing sugar reduces the blue (Cu 2+) ions in the copper sulfate to produce a brick red precipice called copper oxide
• the more reducing sugar present, the less blue and more red the substance appears
• non-reducing sugar e.g. sucrose, can’t donate electrons

Question 5

What happens when copper (ii) sulfate accepts electrons from a reducing sugar?

Answer 5

It forms copper oxide which is red in colour

Question 6

How to detect a non- reducing sugar?

Answer 6

• if no colour change after Benedict’s test
• it MUST first be hydrolysed by adding 2 cm of the food sample to 2 cm of dilute HCl
• place in boiling water bath for 5 mins
• slowly add sodium hydrogen carbonate to neutralise the HCl in order for Benedict’s reagent to work
• re-test using Benedict’s reagent in water bath
• if a non-reducing sugar was present in the sample, then the colour will change from blue to orange/brown

Question 7

Reducing sugars reagent strips

Answer 7

• detect presence of reducing sugars
• often dipped in urine ad a colour appears on the test strip, which can then be compared to a colour key
  -useful in GP surgeries, glucose in urine can mean diabetes

Question 8

Examples of test strips

Answer 8

• diastix- no glucose- green to blue to red/brown
• clinistix- no glucose- green to blue to red
  OR
  no glucose- pink to dark purple

Question 9

Biosensors

Answer 9

• these devices measure the presence and concentration of molecules such as glucose
• the data is displayed on a screen either qualitatively as a colour or quantitatively as a number

Question 10

What is starch?

Answer 10

A granular storage molecule found in plants- a polysaccharide of alpha glucose monomers bonded together with glycosidic bonds
-When a plant photosynthesises and produces excess glucose, glucose is converted to starch, which is a long term storage molecule.
-It can be hydrolysed into maltose by the enzyme amylase, and then maltose can be hydrolysed into alpha glucose by the enzyme maltase. The glucose can then be used as a respiratory substrate
-It is made up of 2 different types of polysaccharides:
-Amylose
-Amylopectin

Question 11

Amylose structure ?

Answer 11

A polysaccharide made up of many alpha-glucose monomers which are joined together by only 1, 4 glycosidic bonds formed during condensation reactions (STRUCTURE)
-The glycosidic bonds all form between C1 of one alpha glucose, and C4 of the adjacent glucose (STRUCTURE)
-This forms a helical shape (STRUCTURE)
-Which is stabilised by hydrogen bonding within the molecule (STRUCTURE)

Question 12

How is its structure linked to its function? Amylose

Answer 12

The helical shape so it’s more compact so more energy can be stored in a small space
-The large helical molecule so it’s less soluble than glucose, so it doesn’t affect the water potential of the cell and therefore doesn’t causes osmosis to happen
-A polymer of glucose, which can be hydrolysed by enzymes to provides the respiratory substrate

Question 13

Amylopectin structure?

Answer 13

A polysaccharide made up of many alpha-glucose monomers which are joined together by 1, 4 AND 1, 6 glycosidic bonds formed during condensation reactions (STRUCTURE)
-The glycosidic bonds can form between C1 of one alpha glucose, and C4 of the adjacent glucose to form a chain (STRUCTURE)
-And they can also form between C1 of one alpha glucose, and C6 of another glucose to form branched side chains (STRUCTURE)

Question 14

How is it’s structure linked to it’s function?

Answer 14

-A polymer of glucose, which can be hydrolysed by enzymes to provides the respiratory substrate
-Branched chains due to 1-6 glycosidic bonds- so there are many sites for enzyme activity, so the molecule can be broken down and glucose removed more quickly for respiration.
-Therefore, amylopectin provides a rapid supply of energy
-Large molecule so it is insoluble, so it doesn’t affect the water potential of cells and causes osmosis to happen

Question 15

Glycogen what is it?

Answer 15

A granular storage molecule found in animals, fungi and bacteria, but NOT plants (although the main storage molecule in animals are fats)
-A polysaccharide of alpha glucose monomers bonded together with glycosidic bonds
-It is stored as small granules in the muscle and liver cells of animals
-When the rate of respiration increases, glycogen is hydrolysed to form alpha-glucose molecules which is the respiratory substrate for respiration
-Animals have a higher rate of metabolism than plants as they do things such as muscle contraction, therefore they need a long-term storage molecule that can be quickly hydrolysed to provide the respiratory substrate

Question 16

Glycogen structure?

Answer 16

A polysaccharide made up of many alpha-glucose monomers which are joined together by 1, 4 AND 1, 6 glycosidic bonds formed during condensation reactions (STRUCTURE)
-The glycosidic bonds can form between C1 of one alpha glucose, and C4 of the adjacent glucose to form a chain (STRUCTURE)
-And they can also form between C1 of one alpha glucose, and C6 of another glucose to form branched side chains (STRUCTURE)

Question 17

How is it’s structure linked to it’s function? Glycogen

Answer 17

Polymer of alpha glucose, which can be hydrolysed by enzymes to provides respiratory substrate
-Structure is the same as amylopectin but is more coiled, therefore it is more compact and so it can store a lot of energy in a very small space
-Structure is the same as amylopectin but is more branched chains due to more glycosidic 1-6 bonds- so there are many sites for enzyme activity, so the molecule can be broken down and glucose removed more quickly for respiration.
-Therefore, glycogen provides a rapid supply of energy
-Large, insoluble molecule- so it doesn’t affect the water potential of the cell and therefore doesn’t’ causes osmosis to happen

Question 18

Cellulose what is it?

Answer 18

It is a fibrous molecule found in cell walls- it provides tensile strength to the cell walls to help the cell keep its shape and to withstand turgor pressure when water enters the cell by osmosis

Question 19

Cellulose structure?

Answer 19

Polysaccharide made up of many beta-glucose monomers which are joined together by glycosidic bonds formed during condensation reactions
-When 2 beta glucose molecules are next to each other, the OH on C1 of one of the beta glucose molecules is too far away from the OH on C4 of the adjacent molecule, so a condensation reaction cannot take place (see fig 5 and 6)
-To overcome this, alternate beta-glucose monomers are flipped 180o so the position of every other OH group faces in the opposite direction
-Long beta-glucose chains are straight/unbranched
-Many chains lie parallel to each other and are held together with many weak hydrogen bonds that form crosslinks between the alternate OH groups
-This forms thicker chains called microfibrils, and many microfibrils combine to form macrofibrils, which then also combine to form fibres

Question 20

How is it’s structure linked to it’s function?

Answer 20

A polysaccharide made up of beta-glucose monomers – forms long, straight, unbranched chains for high tensile strength
-Alternate beta-glucose molecules are inverted so position of OH groups are inverted- this allows weak hydrogen bonds to form between chains to make thicker microfibrils to help give high tensile strength to keep the cell’s shape and to withstand turgor pressure
-Many weak hydrogen bonds provides collective tensile strength to keep the cell’s shape and to withstand turgor pressure
-The fibres made of many macrofibrils, which are made of many microfibrils, are very strong
-Insoluble and unreactive

Question 21

Colorimeter

Answer 21

• A piece of equipment used to quantitatively measure the absorbance, or transmission, of light by a coloured solution
• the more concentrated a solution is, the more light will be absorbed and i.e. the less light will be transmitted
• this can be used to quantitatively calculate the concentration of reducing sugar present

Question 22

Overall principle of using a colorimeter

Answer 22

• zero the colorimeter using distilled water
• perform the Benedict’s test on a range of solutions of known concentrations of glucose
• filtered the solutions to remove the participate
• put glucose samples into cuvettes
• use the colorimeter to measure the % transmission of each solution, using the red filter (600nm)
• create a calibration curve with ‘concentration of glucose solution (mMol)’ on x axis, and ‘% transmission’ on y axis
• take an unknown sample of glucose concentration, and use the same colorimeter to measure the % transmission
• find the % transmission on the y axis, and move to the left until you reach the calibration curve, and then go downwards to reach the concentration of glucose on the x axis

Question 23

Series dilutions

Answer 23

• a dilution in chemistry is a process that reduces the concentration of a substance in a solution
• a serial dilution is the repeated dilution of a solution to amplify the dilution factor quickly and accurately. It’s commonly performed in experiments requiring highly dilute solutions with great accuracy, such as those involving concentration curves on logarithmic scale or involving experiments to determine density of bacteria

Question 24

Serial dilution with a dilution factor of 2

Answer 24

• you take a series of test tubes and add a specific volume of water to them e.g. 2cm3
• you the take the same volume e.g. 4 cm3 of a solution of known concentration and add it to the first test tube and shale it to mix it
• this test tube will now contain 8 cm3 of solution that is half as concentrated as the known stock solution
• then take the same volume e.g. 4 cm3 of the solution in the solution that is half as concentrated, and add it to the second test tube
• so if the stock solution of 40mM, the first test tube will be 20 mM and the next test tube 10 mM, the next 5 mM and the next 2.5 mM etc