Fig. 2.1 is a drawing of two onion epidermal cells, A and B, viewed under a light microscope. Cell A is the cell shown; cell B was drawn from the same peel ...

(a) A red blood cell is highly specialised for carrying oxygen: it loses its nucleus to pack in more haemoglobin, and being an animal cell it never had a wall, a large sap vacuole or chloroplasts. So structures present in the onion epidermal cell but absent from a red blood cell include (any two) the cell wall, the large permanent vacuole and the nucleus. [2] (Chloroplasts appear in the key, but a colourless onion epidermal cell normally has none, so the three above are the safest answers - flag for teacher review.)

(b) Magnification has no units, so both measurements must be in the same unit. Convert the actual length to micrometres first: \( 88\ \text{mm} = 88\,000\ \mu\text{m} \).

\( \text{magnification} = \dfrac{\text{image length}}{\text{actual length}} = \dfrac{88\,000\ \mu\text{m}}{220\ \mu\text{m}} \) [1] \( = \times 400 \) [1]

(c) An eyepiece graticule is just a scale of equal divisions; you do not know the real size of one division until it is calibrated. So: (1) calibrate the graticule against a stage micrometer to find the true length represented by one graticule division [1]; (2) line the graticule scale across the width of cell A and count the number of divisions it covers [1]; (3) multiply the number of divisions by the value of one division to get the actual width [1].

(d) The drawn length can differ from the true living cell because (any two): the pencil line has thickness, so the ends are hard to place exactly; the cell was not lying perfectly flat under the cover slip; there was shrinkage during staining/preparation; or the ruler was read inaccurately. [2]