Things to take note of for Food Test

Things to take note:

For Benedict's Test,

• It is a quantitative test
• All monosaccharides and disaccharides are reducing sugars, except sucrose. 
• To record results: must write precipitate.  e.g. A green/ red/ orange precipitate was obtained. 
• Must add equal amounts of Benedict's solution. 
• Must be placed in a water bath. Note: water bath must be boiling before the test tubes are placed in it. 

For Iodine Test, 

• Never say the iodine turns blue black --> mixture turns blue black
• starch present: blue black
• starch absent: brown

positive results for B, negative for A

Biuret Test, 

• Must add equal amount of sodium hydroxide
• protein present: turned from blue to purple/ violet
• protein absent: mixture remained blue

Ethanol Emulsion Test, 

• Test tubes must be dry
• 1. Add alcohol, if lipid is present, it will dissolve
• 2. Add water, white emulsion observed 
• --> exergonic reaction -- heat is given off

negative results for both A and B

Have to remember the procedures and the reactions. 

Practical lesson #2: Leek cells Lab Report

Homeostasis

 Practical 1 - Estimation of the osmotic concentration of Leek cells
29th January 2014

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

Materials:

1. Scalpel
2. White tile (for cutting)
3. 5 Petri dishes
4. Forceps
5. Distilled water
6. 1%, 2%, 4%, 7%, 10% salt solution
7. Chinese leeks (Allium tubersosum)
8. Stop watch
9. Vernier calipers
10. Thread
11. Labels
12. Electronic balance

Procedure:

You will be investigating the effects of different concentration of salt solution on leek stem. The effects observed in the stem can be recorded quantitatively and used to approximate the osmotic concentration of the leek stem. 

1. Cut the stalk of the Chinese leek to baton a length of 3.0cm.
2. Cut the strip longitudinally as shown in the diagram below to obtain 4 equal quarters. Repeat till you have battened 6 strips.
3. Using the mass of the leek stem to estimate the osmotic concentration
• Wight the individual strips and record the masses.
• Place one strip in a Petri dish of distilled water, one each in 1%, 2%, 4%, 7% and 10% salt solution. 
• After 20-22 minutes, remove the strips from the solution and dab and the strips dry with paper towel. Weigh and record the mass of the individual strips in a table and use the use the data to plot a graph. 

Results: 

Effect of concentration of salt solution (%) on the percentage change in mass (%) of the leek stems

Concentration of salt solution (%)	Initial mass of leek (g)	Final mass of leek  (g)	Difference in mass (g)	Percentage change in mass (%)
0	0.30	0.37	+ 0.07	+ 23.3
1	0.32	0.35	+ 0.03	+ 9.38
2	0.26	0.27	+ 0.01	+ 3.85
4	0.28	0.29	+ 0.01	+ 3.57
7	0.29	0.30	+ 0.01	+ 3.45
10	0.33	0.32	- 0.01	- 3.03

Discussion questions:

• At which concentrations are the salt solution isotonic to the leek cells? Explain how you arrived at your answer. 

Based on the data I have gathered, the salt solution is isotonic to the leek cells at slightly less than 10% concentration. This is because the overall percentage change in mass of the leeks is the least when placed in the 10% salt solution. If a solution is isotonic to the leek cells, then there will not be an overall net movement through the partially permeable membrane of the cells, as their water potential are the same. Since the leek strip in the 10% salt solution experience the least mass change, which means that there is little movement of the water movement leaving the cells into the solution through the partially permeable membranes through osmosis. This accounts for the least percent change in mass, which is a negative -3.03%, which means that the water potential of the surrounding solution that that of the leek cells which is why water left the cell, resulting in a decrease in mass. Hence, the osmotic concentration of the leek cells is slight below 10%.

• Explain the gain or loss in mass of the stems

The leek strips in all the petri dishes except for the one in the 10% salt solution experienced a gain in mass. This is because they were placed in a hypotonic solution, where the water potential of the surrounding solutions is higher than that of the leek cells. This results in an overall net movement of water molecules entering the leek cells through a partially permeable membrane, through the process of osmosis. The water gain caused the gain in mass of the stems. 

This water gain also caused the stem to curve outwards and turn rigid and coarse. This is because the epidermis has a waxy cuticle and thick as cells there have a thicker cell wall as compared to those in the cortex, while the cortex of the stem is softer as the cells there have a thinner cell wall as compared to those in the epidermis. The waxy surface on the epidermis minimises water entry and loss, coupled with the fact that the epidermis is much thicker than the cortex, very little water is able to enter the cell through the epidermis when placed in a hypotonic solution. However, the cortex is quite the opposite, as it is softer and has thinner cell walls, more water is able to enter the cells easily, hence the inner layer of the stem gains much more water than the epidermis. As the inner cortex is also more flexible than the epidermis, the difference in flexibility and water gain, causes the stem to bend outwards. The water gain of the leek cells causes them to be turgid and rigid and coarse as they are expanded.

The strip in the 10% salt solution is the only one that decreased in mass. The solution the leek strip is placed is hypertonic to the leek cells, where the water potential of the surrounding solution is lower than that of the leek cells. This results in an overall net movement of water molecules leaving the leek cells and entering the surrounding salt solution through a partially permeable membrane, through the process of osmosis. This water loss caused the decrease in the mass of the stem. 

The water loss caused the stem to bend inwards in a hypertonic solution and turn soft and a little slimy, the opposite of what happens when placed in a hypotonic solution. This is also due to the difference in properties between the epidermis and the cortex. As mentioned, the waxy cuticle on the epidermis and its hard layer minimises water loss and makes it harder for water molecules to leave the cells. However, the cortex is much thinner and has no waxy layer, more water would be able to escape through the cortex than the epidermis, and because the cortex is more flexible than the epidermis, the stem would bend inwards. The water loss from the leek cells caused it to be plasmolysed and turn soft. 

• Instead of measuring the change in mass/ length of the stem, is there another variable you can measure as an indication to osmosis?

The concentration of salt in the solution can be used as an indication to osmosis. If there osmosis occurring, for example, water molecules leaving the cells and entering the surrounding salt solution, the surrounding salt solution would then be diluted to the increased amount of the solution. Vice versa, if there is water molecules entering the cells, then the amount of water in the salt solution decreases, and the solution would be more concentrated. This can be used an indication of water movement between the salt solution and the leek cells.

• Suggest a way to improve the experiment.
• Firstly, as the experiment with the petri dishes uncovered, one way to improve the experiment is to cover the petri dishes with a cover to prevent the evaporation of the water in the salt solution. This ensures a fair test, as the results will be reliable because the solutions will not be subjected to evaporation or contamination that might affect the results. 
• Secondly, the experiment could be repeated, this time with 3 leeks in one petri dish, and the average of the results could be obtained and calculated, so that the results would be reliable as the human errors would be reduced. 
• Thirdly, the experiment could be conducted with more intermediate concentrations of salt solutions (e.g. 3%, 5%, 6%,8%,9%), so more data could be gathered and when plotting the graph, the gradual change in mass can be observed more clearly.

Graph:

Effect of concentration of salt solution (%) on the percentage change in mass (%)

Photos:

Practical lesson #1: Beetroot Lab Report

Cell Homeostasis

Practical 1 - Effect of environmental conditions on beetroot
22nd January 2014

Aim: To investigate the factors affecting the cellular homeostasis of Beetroot cells

Materials:

1. Scalpel
2. Ruler
3. 5 test tubes & rack
4. Forceps
5. Distilled water
6. 25%, 50% alcohol
7. Beetroot (Beta vulgaris)
8. White tile (for cutting purposes)
9. 3 droppers (water, 25%, 50% alcohol)
10. Thermometer
11. Stop watch 
12. 1 beaker
13. Colourimeter

Procedure:

1. Set up a water bath by collecting hot water into a beaker provided in the lab. 
2. Use a ruler and scalpel to cut the cylinder of beetroot into 25 discs of 2mm each. 
3. Take 5 discs of beetroot and cut them into smaller pieces. 
4. Rinse the beetroot discs and pieces until the water is colourless.
5. Label and prepare 5 test tubes as follows:
   
   

   Tube	Content
   A	4 ml of water
   B	4 ml of 25% alcohol
   C	4 ml of 50% alcohol
   D	4 ml of hot water (90-100°C)
   E	4 ml of water with chopped beetroot

   
   
6. Place 5 discs of beetroot in tubes A-D and all the chopped beetroot in tube E using the forceps. 
7. Leave the tubes to stand in your test tube rack for 15 minutes.
8. Shake the tubes gently after 15 minutes and hold it against the white tile to note the colour. Record your observations in a table. 
9. Use the colourimeter to record the percentage of transmission of light
10. Decant a small amount of the liquid from each tube into a cuvette to measure the percentage of transmission. * Hold the cuvette only at the rough sides. Turn the cuvette so that the arrow is facing you when you insert it into the colourimeter.
11. Dispose of the content of the tubes after the experiment. 

Discussion questions:

• Why was it necessary to wash the beetroot slices thoroughly before using them int this exploration?

The coloured pigments that give the beetroot its red colour are located in the vacuoles of the cells, called betalain. When the beetroot is cut into discs or chopped (for tube E), many of the cells are cut mechanically, and these betalain are released from the vacuole, which is why the cutting tile would have red stains, caused by the pigments leaking out of the beetroot. These pigments need to be removed by rinsing them off as the experiment results will get affected. The change in colour of the surrounding solution must be due to the pigments leaving the cell rather than from the pigments that have already previously leaked out due to the cutting, as if the discs were not rinsed, the final colour of the solution would be darker than expected, which would lead to unreliable results.

• Identify the independent and dependent variables in this experiment. Which was the control set-up?

Independent variable(s): 1. Concentration of alcohol in solution 

                                       2. Temperature 

                                       3. Exposed surface area of beetroot

Dependent variable(s): 1. Percentage of light transmitted 

                                     2. Final colour of liquid 

Control set-up: Test tube A

• Construct a suitable table, with appropriate headings and units, to tabulate your data. 

Test tube	Contents	Description of liquid	Percentage transmission of light (%)
A	4 ml of water with 5 discs of beetroot	From colourless to pale pink	71.04
B	4 ml of 25% alcohol with 5 discs of beetroot	From colourless to pink	69.27
C	4 ml of 50% alcohol with 5 discs of beetroot	From colourless to dark pink	40.37
D	4 ml of hot water (90-100°C) with 5 discs of beetroot	From colourless to very dark pink	2.74
E	4 ml of water with chopped beetroot	From colourless to pink	58.66

• Explain, with reference to the tabulate data, the effect of different solutions in tubes A-C on the readings obtained in the experiment. You should make references to the knowledge you acquired from the lessons on cellular homeostasis.

Tube A (4 ml of water with 5 discs of beetroot):
The beetroots in tube contains a higher concentration of betalain pigments as compared to the surrounding diffusion. Hence the betalain will move down a concentration gradient, from the beetroot and diffuse through the tonoplast and cell membrane into the surrounding solution, causing the water in tube A to turn from colourless to pale pink, with a transparency of 71.04%.

Tube B/C (4 ml of 25%/ 50% alcohol with 5 discs of beetroot):
With alcohol in the solution, the alcohol, a fat solvent, interacts with the phospholipid bilayer and the breaks down the structure by dissolving the fats and proteins, leaving the cell membrane denatured. This leaves gaps in the membrane, making the membrane porous and the alcohol can then enter the cell easily through the cell. Similarly, the structure of the tonoplast is similar to the cell membrane, hence the membrane will also get denatured and porous, and the betalain then leave the cell quickly through these gaps in the membranes. The higher the concentration of alcohol, the greater the amount of damage done onto the membrane, the greater amount of betalain escaping the cell, finally resulting in a lesser percentage of transmission of light. The solution in tube B turned from colourless to a pink colour while the solution in tube C turned from colourless to a dark pink colour.Since tube C has higher concentration of alcohol in its solution than tube B, C has a lower transparency of light of 40.37% than tube B with a higher transparency of light of 60.27%. 

• Suggest an explanation for the observations of tube D & E.

Tube D (4 ml of hot water with 5 discs of beetroot):

The higher the temperature of the water, the greater the amount of betalain diffusing out of the beetroot, the lower of the transparency of the solution. When the beetroot is heated up by the hot water, the membrane gets disrupted and the phospholipids become more fluid due to the increasing vibrations of surrounding molecules caused by the gain in heat. When the proteins in the phospholipid bilayer is heated, the proteins (formed of coiled and folded strings of amino acids) will untangle and break apart due to the vibrations of the surrounding molecules. The proteins will span the membrane, forming holes that destroys the structure, and this repeats itself on the tonoplast, and the pigments can then spill out of the cell easily and causes the solution in tube D to turn from colourless to a very dark pink colour, with a low transparency of 2.74%. 

Tube E ( 4 ml of water with chopped beetroot):

The beetroot are chopped into smaller pieces instead of discs, which gives them a greater exposed surface area. The greater the exposed surface area, the greater the amount of diffusion, the lower the transparency of the solution. Hence, more betalain can diffuse out of the exposed ares of the chopped betalain, which makes the solution turn from colourless to a pink colour, with a transparency of 58.66%.

Photos of experiment:

Instructions on the board

Cuvettes A-E, according to chronological order starting from the left.