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2.2.2Cell Structure

Microscopy and Drawing Scientific Diagrams — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 13 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.2.2 Microscopy and Drawing Scientific Diagrams — AQA A Level Biology 7402 notes: Use a light microscope correctly, calibrate an eyepiece graticule and produce accurate labelled biological drawings.To make a temporary mount of plant tissue (for example onion epidermis or a thin …

  • lowest-power objective lens
  • calibrated
  • stage micrometer
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3

Section 1 of 8Preparing a temporary slide

To make a temporary mount of plant tissue (for example onion epidermis or a thin section of a plant stem):

1
Place a drop of water or stain (e.g. iodine solution) on a clean slide.
2
Use forceps to peel a thin layer of tissue and lay it flat on the drop, so it is a single layer.
3
Lower a coverslip at an angle with a mounted needle to avoid trapping air bubbles.
4
Blot excess liquid with filter paper.

Thin sections let light pass through and avoid overlapping cells. Stains add contrast: methylene blue or iodine for nuclei, and eosin for cytoplasm. Always handle stains and sharp tools with care.

Section 2 of 8Using a light microscope

Always start with the lowest-power objective lens, to locate the specimen and to prevent the lens from hitting the slide.

1
Use the coarse focus to move the lens close, then focus using the coarse knob, then the fine adjustment.
2
Centre the area of interest before switching to a higher power objective, and then use only the fine focus.
3
Adjust the light and the diaphragm for good contrast.
4
The total magnification equals the magnification of the eyepiece multiplied by that of the objective: a x10 eyepiece with a x40 objective gives x400.

Section 3 of 8The eyepiece graticule and stage micrometer

The eyepiece graticule is a small scale in the eyepiece, marked in arbitrary units, that can be used to measure specimens. It must be calibrated for each objective lens using a stage micrometer, which is a slide with a scale in known units (usually 0.01 mm divisions, i.e. 10 µm).

Example: 100 eyepiece units cover 0.5 mm of the stage micrometer. One eyepiece unit = 0.5 mm / 100 = 0.005 mm = 5 µm. If a cell spans 14 units, its length is 14 x 5 = 70 µm. When the objective changes, the calibration must be repeated, because each objective gives a different value for one eyepiece unit.

Section 4 of 8Drawing biological specimens

1
Use a sharp pencil and unlined paper; draw with clear, continuous single lines, not sketchy or shaded.
2
The drawing should be large - occupying at least half the page - and in proportion.
3
Draw only what you see, not what you expect: do not include artefacts,
  • and draw only a few representative cells for a tissue plan, showing outlines only.
    • (no individual cells for low power plan drawings)
4
Label with straight, ruled lines that do not cross and do not have arrowheads, touching the structure; write labels horizontally away from the drawing.
5
Give a title and state the magnification or add a scale bar.

Calculate the magnification of a drawing as drawing size / actual size. Use the same method to compare drawings.

Section 5 of 8Common issues and tips

Problems include air bubbles (circular, dark-edged shapes), uneven staining, overlapping cells and specimens that are too thick.

1
If the image is blurry, check the lens is clean and you are using the fine focus.
2
Drawings of a high-power view show details of a few cells,
  • such as the nucleus, cytoplasm and cell wall;
    • a low-power plan drawing shows the different tissue layers and their proportions.
3
To measure a cell, measure several cells and calculate a mean to reduce the effect of variation.
4
To count cells in an area, use a sample of several fields of view.

Required practical skillsthe specification expects you to prepare and observe temporary mounts, to use the graticule to measure, and to draw.

Common issues and tips
Include the control of variables when measuring many cells, for instance sampling cells from random fields of view and using at least 10 cells to find a mean length.
A common exam question asks you to evaluate how reliable a measurement of cell size is; good answers mention the small size of the cells, the difficulty in lining up the scale, the resolution of the graticule,
  • and the variation between cells.

Safety pointshandle coverslips and slides carefully because glass breaks, take care with scalpels and razor blades by cutting away from the body, and treat stains with caution because iodine is an irritant.

Common issues and tips
Never use the mirror to reflect direct sunlight into the microscope, as this can damage the eyes.
After use, return the microscope to the lowest power objective and clean the lenses only with lens tissue.

Section 6 of 8Exam-style questions

  1. Describe how you would calibrate an eyepiece graticule. [3]3 marks

  2. Using a calibrated graticule, 40 eyepiece units equal 0.2 mm. A cell measures 18 units. Calculate the actual length of the cell in µm. [3]3 marks

  3. State three features of a good biological drawing. [3]3 marks

Section 7 of 8Key terms

TermMeaning
GraticuleA scale in the eyepiece used for measurement.
Stage micrometerA slide with a precise scale used for calibration.
CalibrationWorking out the real size represented by each unit of a scale.
Temporary mountA quickly made slide of a specimen in liquid under a coverslip.
Objective lensThe lens nearest the specimen.
Plan diagramA low-power drawing showing tissues and their proportions.
Field of viewThe area visible through the microscope.

Section 8 of 8Exam technique

1
Starting with the high-power objective lens.
2
Using the coarse focus on high power; only use fine focus.
3
Forgetting to recalibrate the graticule for a different objective.
4
Shading drawings, or drawing cells in the plan diagram.
5
Labelling with arrowheads or crossing label lines.

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