Biological molecules: practicals

Question 1

how do you prepare food to be tested?

Answer 1

grind the food with a small amount of distilled water using a mortar and pestle. Filter the mixture to remove any solid food particles.

Question 2

how do you test for starch?what is the positive and negative result?

Answer 2

place 3cm cubed of food solution in test tube, then add 1cm cubed of iodine solution. The positive result is a blue-black colour. Negative test result is orange colour

Question 3

how do you test for proteins?

Answer 3

place 3cm cubed of food solution in test tube,, then add 3cm cubed of dilute sodium hydroxide solution and mix. Next add 10 drops of dilute copper sulphate solution and mix. Positive result= purple/lilac, negative result=blue

Question 4

what is biurets solution made of?

Answer 4

sodium hydroxide solution and copper sulphate solution.

Question 5

what does biurets solution detect to produce a result?

Answer 5

peptide bonds

Question 6

how do you test for lipids?what is the positive result?

Answer 6

do not filter the food sample as lipids can attach to the filter paper. First add 3cm cubed of food solution to a test tube. Add 3cm cubed of ethanol and 3 cm cubed of water. Positive result= white, cloudy emulsion. Negative result= clear

Question 7

what are some hazards when dealing with food tests?

Answer 7

solutions may be irritant and ethanol is flammable

Question 8

what is the difference between a reducing sugar and non-reducing sugar?

Answer 8

reducing sugars can donate an electron to another molecule. All monosaccharides are reducing sugars but only some disaccharides are reducing sugars.

Question 9

how do you test for reducing sugars?

Answer 9

place 3cm cubed of food solution in a boiling tube and add 3cm cubed of Benedicts solution. Benedicts solution contains the Cu2+ ion which makes the solution blue. Place the boiling tube in the beaker of boiling water and wait for 5 minutes. Negative result= blue. Positive result= green, yellow, orange, red.

Question 10

why is the positive result for testing reducing sugars green, yellow, orange or red?

Answer 10

When a reducing sugar is present, an electron is added to Cu2+ ion forming Cu1+ ion which forms a red precipitate. The colours depend on how much reducing sugar there is.

Question 11

how do you test for non-reducing sugars?

Answer 11

carry out the Benedicts test on the unknown solution. Then take a fresh boiling tube and add 3cm cubed of unknown solution. Add 3cm cubed of dilute hydrochloric acid and gently boil the solution for 5 minutes. If a non-reducing sugar is present, then acid hydrolyses the glycosidic bonds, releasing monosaccharides. Next add 3cm cubed of dilute alkali e.g sodium hydroxide solution and use pH paper to check it is alkaline as the Benedicts test doesn’t work in acidic conditions. Add 3cm cubed of Benedictus solution and heat for 5 mins.

Question 12

Scientists call the Benedicts test semi-quantitative, what does semi-quantitative mean?

Answer 12

the test only gives an approximate idea of how much reducing sugar is present

Question 13

what does a colorimeter quantify in a benedicts test?

Answer 13

the blueness of the solution from the red precipitate formed from the Benedicts test.

Question 14

how do you prepare the sample before using it on the colorimeter?

Answer 14

filter off the red precipitate, leaving just the blue Benedicts solution.

Question 15

how does a colorimeter work when doing the benedicts test?

Answer 15

a lamp with a red filter over is shining on the Benedicts solution. Red light will be transmitted to the photoelectric cell which is a light detector

Question 16

how is the red light transmitted?

Answer 16

if the Benedicts solution is less blue/ more pale, less red light will be absorbed and more will be transmitted.

Question 17

if there is more glucose will the benedicts solution look more blue or pale

Answer 17

pale as benedicts solution is being reacted with the glucose.

Question 18

Overall, when using the colorimeter to test for glucose, the less red light absorbed by the solution…

Answer 18

the greater the amount of glucose.

Question 19

how do you find the concentration of glucose in an unknown sample?

Answer 19

compare the unknown solution results with known (known glucose conc) solutions from the colorimeter

Question 20

how do you prepare the known solutions in an experiment to find the glucose conc in an unknown solution?

Answer 20

place:
-5cm cubed of stock solution(known solution) cubed in test tube 1
-4cm cubed of stock solution with 1cm cubed of distilled water in test tube 2
-3cm cubed of stock solution with 2cm cubed of distilled water in test tube 3
-2cm cubed of stock solution with 3cm cubed of distilled water in test tube 4
-1cm cubed of stock solution with 4cm cubed of distilled water in test tube 5
-to test tube 6, add 5cm cubed of distilled water

Question 21

what do you do after preparing the known substances for the experiment to find the glucose conc of an unknown solution?

Answer 21

-add 5cm cubed of benedicts solution, always add the same amount of benedicts solution than to the amount in the test tube.
-place all boiling tubes in a boiling water bath for 5 min
-filter all solutions from red precipitate, leaving only benedicts solution

Question 22

how do you prepare the solutions for the colorimeter?

Answer 22

place all solutions in cuvettes which have two clear transparent sides and two transparent sides. Light passes through the transparent sides in the cuvette. Make sure to change the filter to red as red is most absorbed by blue. First put distilled water in the colorimeter and set it to zero as were telling the colorimeter that distilled water absorbs zero red light. Use the colorimeter to measure all absorbencies.

Question 23

how do you use the calibration curve to determine the conc of the unknown solution?

Answer 23

plot all the absorbencies on a graph and draw a line of best fit. The line is the absorption curve. Depending on the absorption of the unknown solution, use the absorption curve to determine the glucose concentration.

Question 24

what if the absorption unknown sample is too great to read off the calibration curve?

Answer 24

Dilute the solution and read the absorbance again

Question 25

how is thin layer chromatography used to separate and identify a mixture of amino acids in a solution?

Answer 25

The mobile phase involves an organic solvent. The mobile picks up the amino acids and moves through the stationary phase and the amino acids are separated.

Question 26

In the stationary phase, a thin layer of silica gel is applied to the rigid surface of the glass/ metal, why?

Answer 26

silica causes the amino acids to separate.

Question 27

what does the rate at which the different amino acids move depend on?

Answer 27

depends on the amino acid interactions (hydrogen bonds) with silica and their solubility in the mobile phase.

Question 28

Describe how to separate amino acids using thin layer chromatography?

Answer 28

1)wearing gloves, draw a pencil line on the chromatography plate about 2cm from the bottom
2)spot your amino acid solution on the pencil mark using a capillary tube
3)place the plate in a jar containing the organic solvent. The solvent should be below the pencil line. Close the jar, so the solvent doesn’t evaporate.
4)leave the plate until the solvent reaches about 2cm from the top. Then remove the plate and draw the solvent front using a pencil, let it dry.
5)spray the plate with ninhydrin spray in a fume cupboard. Colour will be produced.

Question 29

how do you calculate the rf value?

Answer 29

distance travelled by component divided by distance travelled by solvent.

Question 30

how do you calibrate a colorimeter? (1 marks)

Answer 30

set it to zero

Question 31

why is it important to calibrate the colorimeter?(1marks)

Answer 31

so all values are measured at the same standard

Question 32

Describe how to separate amino acids in collagen using chromatography?

Answer 32

-hydrolyse collagen to release amino acids
-place amino acid samples on pencil line of chromatography paper
-dry and repeat

Question 33

In chromatography, the darker the spot…

Answer 33

the higher the concentration