Practicals

Question 1

PAG 01: Cheek Cell Slides

Answer 1

1) Place a drop of methylene blue solution on the microscope slide.

2) Gently wipe the inside of your cheek with a cotton bud.

3) Wipe the cotton bud in methylene blue solution on the slide and place the cotton bud in the beaker of disinfectant.

4) Lay one edge of the cover slip on the slide, and use the mounted needle to gently lower the opposite edge of the cover slip onto the slide to avoid the formation of air bubbles.

5) Remove any excess stain by soaking it with paper towels.
6) Place the slide on the stage of the microscope.

7) Turn the eyepiece to select a low power objective (x10).

8) Set up the microscope- don’t look into the eyepiece lens yet. Instead, use the coarse adjustment knob to raise the stage until the cover slip just touches the objective.

9)Now look into the eyepiece and turn the coarse adjustment knob to move the stage away
until the image comes into focus (doing this helps avoid you breaking the slide).

10) Turn the nosepiece to select a high power objective.
11) Repeat the same process as above and then look into the eyepiece and turn the fine
adjustment knob until the image comes into focus.

12) View using high power (40X) on the light microscope. Identify the features of the cells
(nucleus, cell membrane, cytoplasm). Draw a cell diagram of 3-4 cells.

Question 2

PAG 01: Cheek Cell Slides- risks and precautions

Answer 2

Methylene blue is an irritant- avoid contact with skin by wearing safety goggles and gloves.

Broken or cracked slides constitute a broken glass hazard.

Cheek cells are a biohazard, dispose of cotton bud in disinfectant after use.

Question 3

Iodine test for starch

Answer 3

1. Put some of the food sample into a test tube.
2. Add a few drops of iodine solution to the food sample using a pipette.
3. If starch is present, the solution turns from brown to blue-black. Note any colour change in a
   table of results.

Question 4

Testing for sugars

Answer 4

1. Add an equal volume or excess of Benedict’s solution to the food sample in a test tube.
2. Place in a hot water bath for a few minutes.
3. If reducing sugar is present, a brick red precipitate is formed. If reducing sugar is absent, the solution remains blue. Note any colour change in a table of results.

Question 5

Emulsion test for lipids

Answer 5

1. Add a few cm3​ ​ of ethanol to the food sample.
2. Pour this mixture into a test tube of equal volumes of distilled water.
3. If lipids are present, a white emulsion is formed on the surface of the mixture.

Question 6

Testing for protein

Answer 6

1. Add a few drops of Biuret’s reagent (sodium hydroxide and copper (II) sulphate) to the food
   sample in a test tube.
2. Shake the solution to mix and wait for a few minutes.
3. If protein is present, the solution turns from blue to purple.

Question 7

Safety precautions for food tests

Answer 7

Tie hair back and wear safety goggles when performing the Benedict’s test using a Bunsen burner and hot water bath.

Handle Biuret solution with care as it contains copper sulphate (poisonous) and sodium hydroxide (corrosive). Wash immediately if it comes into contact with skin and wipe away any spills to surfaces.

Keep ethanol solution away from flames as ethanol is highly flammable.

Question 8

Source of error for food tests

Answer 8

Colour change of Benedict’s test and Biuret test may be subtle and difficult to judge if the concentration of the tested molecule is low.

Question 9

PAG 08: Osmosis

Answer 9

1. Label 7 beakers with the concentrations of sucrose: 0.0M, 0.1M, 0.2M, 0.3M, 0.4M, 0.5M and 0.6M.
2. Use a cork borer to form 7 potato cylinders and trim each to a length of 50 mm with a scalpel and ruler.
3. Assign one potato cylinder to each concentration of sucrose.
4. Weigh each potato cylinder and record the masses in a table (as seen below).
5. Use a measuring cylinder to transfer 50 cm3​ ​ of each solution to its corresponding beaker
   (use distilled water for 0.0M).
6. Place the potato disc into their corresponding beaker, making sure it is submerged.
7. Start timing for 30 minutes.
8. Remove the potato cylinders from the solution and blot dry with a paper towel.
9. Weigh each potato cylinder again and record the new mass in the table.
10. Calculate the percentage increase or decrease in mass.