CP - CC2 - Preparation and scientific drawing of a slide of onion cells including calibration of actual size and magnification of drawing

Outline the stages of this practical (4)

making a wet mount on a microscope slide, look at onion cells using each objective lens, identify features of the cell visible under a light microscope, calculate the size of a cell using the eye piece graticule

What apparatus are required? (10)

microscope fitted with eyepiece graticule, microscope slide, cover slip, onion, paper towel, scalpel, white tile, mounted needle, iodine solution, forceps

Step 1

Place two drops of water onto a microscope slide

Step 2

Take a small piece of onion and using forceps peel off the membrane from the underside

Step 3

Lay a piece of the membrane flat on the surface of the slide

What is important to take care of in step 3?

that the piece of membrane is a single layer and not folded back on itself

Step 4

Add three drops of iodine solution

Step 5

Place one edge of a coverslip onto the slide and lower it gently using a mounted needle

What it’s important to avoid in step 5?

air bubbles, that would obstruct the view of the cells

Step 6

Gently press the coverslip down using a paper towel

Step 7

use the x4 objective lens position the slide and focus on the section

Step 8

Swing the x10 objective lens into place and move the slide carefully until a clear area of cells are observed

How should you draw the cells?

at least three, in the correct proportion, indicating the length of one cell in eyepiece units on the drawing

Which objective lens should you use to identify and label structures in the cells?

x40 objective lens

Step 9

Calculate the actual size of one of the cells and the magnification of the drawing

Outline the hazard, risk and control measure of the scalpel in this experiment (3) (use ‘due to’ and ‘while’ in risk)

Scalpel blade is sharp, cuts skin due to using scalpel while cutting onion, cut away from body onto a white tile

Outline the hazard, risk and control measure of the iodine used in this experiment (3)(use ‘due to’ and ‘while’ in risk)

Iodine in potassium iodide solution is an irritant, could transfer from skin to eyes due to using iodine while staining the onion sample, wear safety glasses

Outline the hazard, risk and control measure of the glass cover slip used in this experiment (3)(use ‘due to’ and ‘while’ in risk)

broken glass is sharp, cover slips could shatter and cut skin due to pressing coverslip while placing on the onion sample, place a paper towel over the coverslip before pressing down on it

What is the definition of magnification?

The number of times an image is bigger than the object from which it is derived

What is the definition of resolution?

The smallest distance that can be distinguished as two separate points in a microscope

Identify the structures that could be visible in an onion, using a light microscope (4)

cell wall, nucleus, vacuole, cytoplasm

Why do you use the x4 objective lens to locate the cells?(2)

it is the lowest power objective lens, that provides the widest field of view

Which lens is the highest power used in A-level Biology labs and why? (2)

100x objective lens with the 10x eyepiece lens gives maximum magnification, to view individual cells and their organelles

When moving from low to high power, how should you focus your microscope?

only use the fine focussing knob

Outline the hazard, risk and control measure of using the microscope in this experiment (3)(use ‘due to’ and ‘while’ in risk)

microscope lamp is hot, could burn skin due to holding it while trying to move the microscope, leave lamp to cool before moving

How many eyepiece units are in the eyepiece graticule?

100

Should you work in fractions or whole numbers of an eyepiece or stage micrometer unit?

whole numbers

Why is it necessary to calibrate a microscope?

to measure the size of a structure on a microscope slide

Describe the length of the eyepiece graticule (2)

it is graduated 1-10 with 10 subdivisions between each number, so it has 100 epu

When a stage micrometer is a slide with a line 1mm long on it, what is each stage micrometer unit equal to in mm and micrometres?

0.01 mm, 10 micrometres

When a stage micrometer is a slide with a line 10mm long on it, what is each stage micrometer unit equal to in mm and micrometres?

0.1 mm, 100 micrometres

How do you calibrate the microscope? (3)(part 1)

line up the zero of the eyepiece graticule and the zero of the stage micrometer, make sure the scales are parallel, look at the scales and see where they are in line again

How do you calibrate the microscope if 1 stage micrometer unit = 0.01 mm? (2)

use the x40 objective lens, 20 stage micrometer units make up 80 eyepiece units

How do you calibrate the microscope if 1 stage micrometer unit = 0.1 mm? (2)

use the x40 objective lens, 2 stage micrometer units would make up 80 eyepiece units