International A Level Biology / Cell structure
Core practical 5: the graticule and drawing animal cells revision notes
Edexcel International A Level Biology (YBI11), chapter 7: Cell structure, specification point 3.8. Free lesson.
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This practical uses a graticule to measure cheek cells and a microscope drawing to show their visible features. You need to calibrate the graticule for each objective and give your drawing a clear scale.
How to prepare and view cheek cells
Cheek cells are easy to collect and are usually seen as separate, flat cells. If you already have a prepared slide, start by using the microscope. Smear cheek cells from a clean cotton bud onto a slide, then put the bud into disinfectant. Add methylene blue and lower the coverslip at an angle to reduce air bubbles. Blot away excess stain. Find the cells on low power, then switch to high power and use the fine focus. Draw a few cells. Measure their widths in eyepiece units, convert using the calibration for that objective, and find a mean from several cells.
What to show in a cheek-cell drawing
Under a light microscope, cheek cells have only a few visible features. Methylene blue stains the nucleus darker than the pale-blue cytoplasm. Draw only what you can see, not structures remembered from other diagrams. Show the irregular, sometimes folded cell surface membrane. Cheek cells are flattened. Show the pale-blue cytoplasm and the small, roughly central, darker-blue nucleus. Do not draw a cell wall, rough ER or mitochondria. These are not visible here.
How to make a drawing that earns marks
The quality of the drawing matters. Keep the drawing simple and accurate, with labels that clearly identify each structure. Use a sharp pencil and one smooth, unbroken line for each edge. Do not use sketchy, doubled or broken lines. Do not shade or colour any part of the drawing. Keep the nucleus the right size and position compared with the cell. Use ruled label lines that touch the structure they name. Keep labels outside the drawing and do not use arrowheads. Give the drawing a scale by stating its magnification or adding a scale bar.
How to calibrate the eyepiece graticule
An eyepiece graticule is a scale inside the eyepiece. Its divisions have no real units until you calibrate them with a stage micrometer, a slide with a known scale. Line up the scales at zero, then count their divisions to where they meet again. In this calibration, 100 eyepiece units line up with 25 stage-micrometer divisions. Each stage-micrometer division is 10 µm, so 100 eyepiece units cover 25 × 10 = 250 µm. One eyepiece unit is therefore 250 ÷ 100 = 2.5 µm on the ×40 objective. Calibrate again for every objective. The graticule looks the same, but a stronger objective makes the…
How to measure cells and scale a drawing
Once calibrated, multiply a cell's eyepiece-unit width by the value for that objective to get its actual width. For a drawing, first use the same units for the drawing and the real cell. A cheek cell is typically about 60 µm across. Drawing magnification = drawing size ÷ actual size. Scale-bar length = magnification × the actual length represented.