WJEC/Eduqas A Level Biology A Student Guide to Practical Work

• • • Practical tip • • • A low power plan shows only the layers of tissues, not individual cells. Only a high power drawing shows individual cells. • • • Practical tip • • • The epidermis of a leaf and the endodermis of an artery are very thin compared with the mesophyll and muscular layer, respectively. They should still be drawn with two lines, as they have a signi cant thickness (the thickness of one cell, as opposed to the thickness of a membrane) but those lines must be very close together. Drawing a low power plan A low power plan shows layers of tissues but does not show individual cells so you are likely to use only the ×4 objective lens. It is a tissue plan, with lines showing where the tissues are and their relative sizes. Suitable specimens include VS leaf, TS artery or TS root. If you draw a section through a leaf, most of your drawing will look like parallel lines at different distances from each other, although, depending on your specimen, you may see some vascular tissue to include. Similarly, if you draw a section through an artery, it may just appear to be concentric rings. The important point is that the proportions are correct. Your leaf section will look like this: Not like this: To show that your proportions are correct, you have to indicate the number of eyepiece units two of the tissue layers take. Rotate the eyepiece so that the graticule is aligned accurately for you to measure. In the leaf, it may be that the palisade layer is 1.2 epu thick and the spongy layer is 2.4 epu thick. Then you must make the spongy layer twice as thick in your drawing. You have to indicate where you have made the measurement and what the measurements are. Your examiner will check that the numbers you have written correspond with the thickness of the layers in the drawing. There are two ways to write the actual numbers: ▪ Write 1.2 epu on the scale line itself. ▪ Put A at one end of the line and B at the other, then in a key somewhere else on the page write A–B = 1.2 epu. Put the lines to show where you have measured on the drawing itself, not to one side. Draw a little bar at each end, to show the limits of your measurement. Position it exactly. 1.2 epu 2.4 epu Grade boost If your diagram has a feature with a precise position, make your measurements from there, e.g. the tip of a vascular bundle. That way, anyone looking at your diagram can see exactly where the measurements were made. Grade boost Somewhere on the page, have a key to say ‘epu = eyepiece units’. 14 Biology: A Student Guide to Practical Work