Required Practical 1: Microscopy

Question 1

Equipment list (8)

Answer 1

1. Onion
2. Knife
3. White tile
4. Microscope slide
5. Cover slip
6. Forceps
7. Iodine
8. Microscope

Question 2

Method (9)

Answer 2

1. Collect a small piece of onion
2. Using forceps, peel a thin skin or membrane away from the inner side of a layer.
3. Lay this skin flag on a microscope slide.
4. Put 2-3 drops of iodine onto the onion skin.
5. Using a mounted needle, gently lower a cover slip into the slide. **Make sure there are no trapped air bubbles.*,
6. Use a cloth to mop up any iodine.
7. Use the x40 magnification to focus the onion cells. Turn the course focus dial until the slide is almost in focus, then use the fine focus dial to make the image clear.
8. Now use x100 and x400 to view individual cells.
9. Draw and label 4-5 cells, as viewed under high power.

Question 3

Identify hazards for this practical and explain why they are dangerous and how to reduce these risks. (6)

Answer 3

1. Knife
   • could cut yourself
   • carry knife safely; use knife on the white tile
2. Iodine/ methylene blue
   • irritant
   • wear goggles when using stain

Question 4

State a possible source of error in this method and how to improve the method to reduce this. (2)

Answer 4

1. Might confuse air bubbles for structured within the cell
   • lower cover slip slowly to reduce the chances of bubbles.

Question 5

Explain how an image from an electron microscope would look different. (3)

Answer 5

1. More detail visible
2. Can see smaller structures (e.g., ribosomes, mitochondria)
3. Could be a 3D image