Microscopy and measuring cells: exam questions

Answer: B

Answer: B (45 mm = 45 000 µm; 45 000 ÷ 900 = 50 µm)

• magnification: how many times larger the image is than the real object / image size ÷ actual size (1)
• resolution: the smallest distance between two points at which they can be seen as separate (1)

• TEM has a higher resolution / shows more detail (1)
• TEM uses electrons, which have a shorter wavelength than light (1)
• TEM can show ultrastructure / organelles such as ribosomes (1)
• light microscope can be used to view living cells, TEM only dead specimens (in a vacuum) (1)
• light microscope image is in colour (if stained), TEM is black and white (1)
• light microscope specimens are easier / quicker to prepare, TEM may produce artefacts (1)
• Max 4

• 40 mm = 40 000 µm (1)
• 40 000 ÷ 25 000 (1)
• 1.6 µm (1)
• correct answer with no working gains full marks; do not accept 1.6 mm or 0.0016 µm

• most cell structures are transparent / have little contrast (1)
• stain binds to structures and absorbs light, making them visible / increases contrast (1)
• electron microscopes use electrons, not light, so stains must scatter / absorb electrons (1)
• heavy metal salts such as lead or uranium are used (1)
• Max 3

• use a stage micrometer of known scale (1)
• line up the graticule scale with the micrometer scale (1)
• count graticule divisions that match a known number of micrometer divisions (1)
• calculate the length of one graticule division (in µm) (1)
• calibrate separately for each objective lens (1)
• measure several cells in graticule divisions, multiply by the calibration value and calculate the mean (1)
• Max 4

• sharp pencil and clear, continuous lines (1)
• no shading or colouring (1)
• labels drawn with ruled lines that touch the structure (1)
• drawing large enough, including a title and magnification / scale bar (1)
• Max 3

• use only her own cells / do not share swabs (1)
• dispose of the swab / slide in disinfectant (1)
• wear eye protection / take care with glass (1)
• Max 2