B1 L4Magnification and cell size

• 01.1 one or two lines correct(1)
• 01.1 all three lines correct: 1 cm → 10 mm; 1 mm → 1000 µm; 1 µm → 1000 nm(1)
• 01.2 a ribosome, 25 nm wide(1)

01.1: 3 correct lines = 2 marks; 1 or 2 correct lines = 1 mark (three correct: tick both boxes). If more than one line is drawn from a unit, none of its lines scores.
01.2: if more than one box is ticked, give 0 marks.

Model answer01.1 1 cm → 10 mm; 1 mm → 1000 µm; 1 µm → 1000 nm. 01.2 a ribosome, 25 nm wide.

• 02.1 22 ÷ 400(1)
• 02.1 = 0.055 (mm)(1)
• 02.1 0.055 × 1000 = 55 (µm)(1)
• 02.2 The drawing is 400 times bigger than the real cell.(1)
• 02.3 5.5 × 10⁻⁵ (m)(1)

02.1: correct answer with no working = 3 marks. allow 22 000 µm ÷ 400 = 55 µm (units changed first) for 3 marks. 0.055 mm = 2 marks; 0.055 with the unit µm = 2 marks (the unit was not changed). do not accept 8800 (22 × 400: multiplied).
02.2: if more than one box is ticked, give 0 marks.
02.3: allow their 02.1 answer correctly written in metres in standard form. do not accept 55 × 10⁻⁶ m or 0.000 055 m (not standard form). do not accept 5.5 × 10⁵ m.

Model answer02.1 22 ÷ 400 = 0.055 mm; 0.055 × 1000 = 55 µm. 02.2 The drawing is 400 times bigger than the real cell. 02.3 55 µm = 0.000 055 m = 5.5 × 10⁻⁵ m.

• 45 ÷ 30 (= 1.5)(1)
• 1.5 × 2 = 3 (µm)(1)

correct answer with no working = 2 marks. allow 45 ÷ 15 = 3 µm (15 mm stands for 1 µm). do not accept 2 µm (the scale bar's label). ignore units.

Model answer45 ÷ 30 = 1.5; 1.5 × 2 µm = 3 µm.

• 04.1 1.8 ÷ 30 = 0.06 (mm)(1)
• 04.1 0.06 × 1000 = 60 (µm)(1)
• 04.2 at a higher magnification the field of view is smaller / narrower(1)
• 04.2 each cell looks bigger / takes up more of the field of view, so fewer cells fit across it(1)

04.1: correct answer with no working = 2 marks. allow 1800 µm ÷ 30 = 60 µm. 0.06 mm = 1 mark. do not accept 54 (1.8 × 30).
04.2: allow "the field of view is only about a quarter as wide (about 0.45 mm)" for the first mark. allow "only about 7 or 8 cells fit across" for the second mark. do not accept "the cells get bigger" (the real cells do not change; their image does).

Model answer04.1 1.8 ÷ 30 = 0.06 mm = 60 µm. 04.2 At × 400 the field of view is smaller: only about a quarter as wide, about 0.45 mm across. Each cell looks bigger and takes up more of the field of view, so fewer cells fit across it.

Level of response. Read your answer, choose the level that fits best, then pick your mark.

Indicative content

• measure the length of a plant cell in micrograph A with a ruler (in mm)
• measure the length of a bacterium in micrograph B with a ruler (in mm)
• real size = image size ÷ magnification
• plant cell: divide its image length by 200
• bacterium: divide its image length by 20 000
• change both real sizes into the same unit (such as µm: × 1000)
• divide the real size of the plant cell by the real size of the bacterium
• the answer is how many times bigger the plant cell is
• measure several cells of each and use the mean
• measure the same dimension each time (such as the longest length)
• give the answer as an order of magnitude (for example, about 100 times = 2 orders of magnitude)
• check the answer is sensible: a plant cell is about 10 to 100 µm, a bacterium about 1 to 5 µm

Your mark:

Model answer (Level 3)Use a ruler to measure the length of one plant cell in micrograph A, in mm. Work out its real length: real size = image size ÷ magnification, so divide the image length by 200. Do the same for one bacterium in micrograph B, dividing its image length by 20 000. Change both real lengths into µm by multiplying by 1000, so they are in the same unit. Then divide the real length of the plant cell by the real length of the bacterium. This tells you how many times bigger the plant cell is. For example, a plant cell image 20 mm long gives 20 ÷ 200 = 0.1 mm = 100 µm, and a bacterium image 40 mm long gives 40 ÷ 20 000 = 0.002 mm = 2 µm, so the plant cell is 100 ÷ 2 = 50 times longer. To make the answer more reliable, measure several cells of each type and use the mean. Check it is sensible: a plant cell should be about one to two orders of magnitude bigger than a bacterium.