Practical 3 (Biomolecules) - Food Tests for Starch, Glucose, Lipids and Proteins

Today during Biology Practical, we conducted food tests on some solutions.

Aim of experiment: To conduct food tests on known and unknown food samples.

The food tests conducted on the unknown food samples were to find out the composition of these samples.

This is the summary of our results:

 Solution 1 is glucose solution, solution 2 is egg albumen (which is rich in proteins), solution 3 is starch suspension and solution 4 is oil. As shown in the table above, the Benedict Test is the test for reducing sugar, Iodine test is the test for starch, Biuret's Test is the test for proteins and lastly the ethanol emulsion test is the test for lipids. The tests conducted on solutions 1, 2, 3 and 4 proves the components in the solutions.

We were also given unknown solutions A and B, where we had to conduct all the tests on. Based on the table, it is apparent that solution A had large amounts of reducing sugar but no starch, protein and lipid present. The contents of solution B is opposite of those in solution A (ie. starch, protein and lipids were present but reducing sugar was absent).

These are the photos taken during the tests.

Benedict's test

 Set-up of water bath (heating the water to boiling point with a bunsen burner)

State of the solution B, A and 1 (from left to right) when nearing the end of the experiment 

Note: colour seen in this photo is not the final colour (ie. it was taken a little to early and we have not shaken the test-tubes to see the final colour)

As can be seen from the photo, solution B remained blue after the test which required us to add 2cm3 of Benedict's Solution into all 3 solutions and heat them up in a boiling water bath. Since solution B remained blue, it indicates that no reducing sugar. Solution A turned brick-red in colour by the end of the experiment which means that a large amount of reducing sugar is present in solution A. Lastly, solution 1, which was glucose solution turned orange in colour, indicating moderate amount of reducing sugar present in the solution. 

Iodine Test

From left to right: Solution B, A and 3 

Colour of solutions after the test 

As seen from the photo, solution B and 3 turned blue-black in colour after iodine was added. This indicates that both solution B and 3 had starch content. However, solution A remained yellow-brown in colour after iodine was added; hence, there is no starch in solution A. 

 Biuret's Test

 From left to right: Solution B, A and 2 after NaOH was added, but before CuSO4 was added

From left to right: Solution B, A and 2 after CuSO4 was added

As can be seen from the photo, solutions B and 2 turned violet in colour, indicating that protein is present. However, solution A remained clear, which means that protein is absent.

Ethanol Emulsion test 

From left to right: Solutions A, B and 4 after ethanol and water was added. 

From the photograph, it is apparent that solutions B and 4 had a white emulsion after ethanol and water was added (and the test tube was shaken); hence, lipid is present in solutions B and 4. However, solution A remained clear (did not have a white emulsion) after the test, meaning that lipid was absent in solution A. 

Moreover, we realised that solution 4 was warm after the test.

Practical Leek Stem Cells

Aim: To estimate the approximate osmotic concentration of the leek stem cells

I placed leek stems of equal thickness and length (3.0cm) into 1%, 2%, 4%, 7% and 10% salt solution. After 25 minutes, I recorded the length of the inner surface (cortex), as shown below.

Pictures:

Results:

Precautions I took:

- I ensured that the leek stems were all fully submerged in the salt solutions.
- I used the same volume (40cm3) of salt solutions for all set-ups.
- I placed the leek stems into the petri-dishes with salt solution at the same time.

Question to think about:
- When we cut the leek stem, why does it curve?
The inner cortex of the leek stem is thin, therefore allowing water to enter and escape easily. However, the outer epidermis of the leek strip is thick and rigid. The inner cortex is also more flexible and can expand or contract easily white the outer epidermis is rigid, waxy and more waterproof. Hence, due to the difference in flexibility of the cortex and the epidermis, the leek stem will bend outwards.

Beetroot Practical Experiment

For this Beetroot experiment, the aim was to find out the effect if different types of conditions the beetroot is placed in will affect the permeability of the cell membrane of the beetroot.

 

Apparatus used for the experiment 

There were 5 tubes in this experiment, A to E. 

Tube	Content
A (Control)	5 pieces of beetroot in 4 ml water
B	5 pieces of beetroot in 4 ml 25% alcohol
C	5 pieces of beetroot in 4ml 50% alcohol
D	5 pieces of beetroot in 4ml hot water
E	5 pieces of beetroot diced into smaller pieces in 4ml water

 

We put 5 small pieces of beetroot (approximately the same size) into each tube (with different contents as shown in the table above) for about 15 to 20 minutes.

From left to right: E, C, B, A

Tube D

 

After 15 to 20 minutes, we noted the colour change of the solution in the tubes. However, I acknowledge that we did not shake the tubes so that the red pigments were evenly spread out throughout the solution at this stage.

 

Colour of water at the end of experiment

 

 Colour of 25% alcohol solution at the end of experiment

 

 Colour of 50% alcohol solution at the end of experiment

 Colour of hot water at the end of experiment

 Colour of water at the end of experiment

(Note: The solutions turned red due to the red betacyanin pigments in the beetroot cell sap that diffused out the the cell and into the tube.) 

Next, we poured all the solutions into cuvettes after shaking. 

Final colour after experiment. From left to right: A, B, C, D, E.

 

At the end of the experiment, the water in tube A was very light pink, the 25% alcohol solution in B was light pink, the 50% alcohol solution in C was pink, the hot water in D was red and the water in E was reddish-pink.

Next, we took readings from the colourimeter.

Light transmission value of solutions shown on the Colourimeter

 

This shows that the darkness of the solution corresponds to the light transmission value. (ie. The darker the colour of the solution, the lower the light transmission value).

From the results, it is apparent that tube A had the highest light transmission value as no harm has been done to the cell wall, cell membrane and tonoplast of the vacuole. Hence, diffusion of the betalain pigments out of the beetroot cell was very low. In Tube B, the light transmission value was 86.71% as the 25% alcohol solution causes damage to the cell, causing the betalain pigments to be able to flow out of the cell more easily, thus turning the solution light pink in colour. As for tube C which contained 50% alcohol solution, the cell membrane and tonoplast were damaged even more than that in tube C due to the high alcohol solution, causing even more betalain pigments to diffuse out of the cell, causing the solution to turn pink in colour. The beetroot pieces were placed in hot water in tube D. High temperatures will cause the weakening in the forces holding the polypeptide chains in the cell membrane together, leading to the betalain pigments leaking out of the cell. This caused the hot water to turn red and hence, it has a very low light transmission value of 5.43%. Lastly, tube E with the beetroot cells of larger surface area would also have a lower light transmission value than the control (tube A) as the increased surface area will lead to a higher rate of diffusion of the betalain pigments out of the cell.