3.1.8Unit 3: Practical Skills in Biology I
Core Practical 5 – Light Microscopy — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 10 of 27 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Preparing a Slide
A temporary mount lets you view fresh cells quickly with a light microscope.
1
Cut a small piece of tissue and use forceps to peel off a very thin layer, so that light can pass through it.
2
Place the thin layer on a clean slide in a drop of water, which prevents the cells from drying out.
3
Add a drop of a suitable stain, such as iodine solution, to make the structures easier to see.
4
Lower a coverslip gently at an angle with a mounted needle to avoid trapping air bubbles.
5
Blot away any excess liquid with a paper towel before placing the slide on the stage.
Part 2 of 5Using the Microscope and Calibrating
Start at low power
- Start at low power Always begin with the lowest-power objective lens, which gives a wide field of view and helps prevent damage to the lens or slide.
Focusing
- Focusing Use the coarse focus to find the specimen and then the fine focus to sharpen the image before changing lens.
Eyepiece graticule
- Eyepiece graticule A small scale in the eyepiece has arbitrary divisions, and so it must be calibrated for each objective lens.
Stage micrometer
- Stage micrometer A slide with a scale of known size, often divided into 10 μm units, is used to calibrate the graticule.
Calibration
- Calibration Line up the two scales and count how many graticule divisions match a known number of stage divisions.
Measuring cells
- Measuring cells Measure a cell in graticule divisions and multiply by the calibration value to get the real size.
| Graticule divisions | Stage micrometer length | One graticule division |
|---|---|---|
| 40 | 250 μm | 6.25 μm |
Part 3 of 5Biological Drawing and Limitations
Biological Drawing and Limitations
Drawing rules
- Drawing rules Use a sharp pencil on plain paper, draw clear single lines, with no shading or sketching, and add a title and magnification.
Labels
- Labels Use ruled label lines that do not cross, ending at the structure they identify, with no arrowheads.
Size
- Size The drawing should fill at least half the space available and show the proportions of the structures accurately.
Limitations
- Limitations Tissue sections are cut at different planes, and preparation may change the appearance of cells.
Part 4 of 5Worked Example
Question
- On a ×40 objective, 40 eyepiece graticule divisions line up with 25 divisions of a stage micrometer, each 10 μm.
- A cell measures 12 graticule divisions.
- Calculate its length.
- Length of 25 stage divisions = 25 × 10 μm = 250 μm.
- One graticule division = 250 ÷ 40 = 6.25 μm.
- Cell length = 12 × 6.25 = 75.0 μm.
Answer The cell is 75 μm long.
Part 5 of 5Exam Checklist & Tips
3.1.8 – Key Points for the Exam
- Coverslip A thin piece of glass placed over a specimen on a slide.
- Eyepiece graticule A scale inside the eyepiece used to measure specimens.
- Stage micrometer A microscope slide with a precise scale, used to calibrate the graticule.
- Stain A dye that makes structures more visible.
Common Mistakes
- Using the high-power lens first, which can crush the slide and scratch the lens.
- Trapping air bubbles under the coverslip, which can be mistaken for cells.
- Drawing cells with shading or overlapping lines instead of clear single lines.
- Using the same graticule calibration for different objective lenses.
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