Cell Theory & Microscopy

Question 1 (12 marks)

(a) Actual size = image size ÷ magnification. Image length $= 48\,\text{mm} = 48000\,\mu\text{m}$. (1 — convert) $\text{Actual} = \frac{48000\,\mu\text{m}}{400} = 120\,\mu\text{m}$ (1 — divide, 1 — correct value + unit)

(b) $120\,\mu\text{m} = 120\times1000 = 120000\,\text{nm}$ (1) $120\,\mu\text{m} = 120/1000 = 0.12\,\text{mm}$ (1)

(c) Two ribosomes are $25\,\text{nm}$ across, so adjacent ribosomes are separated by a distance smaller than the microscope's resolution of $200\,\text{nm}$. (1) Any two points closer than the resolution limit appear as one. (1) Therefore the light microscope cannot resolve two adjacent ribosomes as separate. (1)

(d) The claim is incorrect. (1) Magnification only enlarges the image; it does not improve resolution. (1) Beyond the resolution limit, increasing magnification produces "empty magnification" — a larger but blurred image with no extra detail. (1) Since ribosome separation ($<200\,\text{nm}$) is below the light microscope's limit, no magnification will make them clearer; an electron microscope is required. (1)

Question 2 (10 marks)

(a) Magnification = measured scale-bar length ÷ actual length it represents. $16\,\text{mm} = 16000\,\mu\text{m}$. (1) $M = \frac{16000\,\mu\text{m}}{2\,\mu\text{m}} = 8000\;(\times8000)$ (1 — division, 1 — value)

(b) Actual = image ÷ magnification. Chloroplast image $=40\,\text{mm}=40000\,\mu\text{m}$. (1) $\frac{40000}{8000} = 5\,\mu\text{m}$ (1 — method, 1 — value + unit) (Alternatively by ratio: $40\,\text{mm}$ is $40/16 = 2.5\times$ the bar, so $2.5\times2 = 5\,\mu\text{m}$.)

(c) Most likely TEM. (1) Evidence 1: resolution — thylakoids $0.5\,\mu\text{m} = 500\,\text{nm}$ apart are resolved, which is within a light microscope limit; but resolving internal membrane fine detail and the high magnification ($\times8000$) point to electron microscopy. (1) Evidence 2: internal/cross-sectional membrane detail (thylakoid stacks) indicates a thin section viewed by transmission, characteristic of TEM (SEM gives surface-only 3-D images). (2)

Question 3 (10 marks)

(a) X = Robert Hooke (cork "boxes"/cells). (1) Y = Antonie van Leeuwenhoek (animalcules from teeth scrapings). (1) Z = Rudolf Virchow (cells from pre-existing cells). (1)

(b) Supports the tenet: all cells arise from pre-existing cells (by division) — omnis cellula e cellula. (2)

(c) Hooke observed only the empty cell walls of dead cork; he could not see living contents or establish that cells are the basic unit of all life. (2) Leeuwenhoek saw living single-celled organisms but had no framework linking them to the structure of larger organisms. (1) Neither could demonstrate that all organisms are composed of cells (Schleiden/Schwann's generalisation) nor how cells originate (Virchow). (2) Their microscopes and single observations established existence, not the unifying principles.

Question 4 (10 marks)

(a) Steps: place a drop of water on a clean slide (1); add/transfer the specimen into the drop (1); lower a coverslip at an angle onto the drop (1). Purpose of the angle: to allow water to spread and to exclude/prevent trapping of air bubbles which would obscure the view. (1)

(b) Unstained cells are largely transparent and let light pass, giving little contrast. (1) Methylene blue is a basic dye that binds to acidic components (DNA) in the nucleus. (1) The stained nucleus absorbs light, appearing dark against the pale cytoplasm, increasing contrast and visibility. (1)

(c) Any TWO of: SEM operates under vacuum so liquid water would evaporate/boil and disrupt cells (1); the specimen must be dry and coated with metal (conductive), not wet (1); electrons cannot pass through/around water and a glass slide + coverslip. (1) (Max 3)

Question 5 (8 marks)

(a) Eye: $100\,\mu\text{m} = 100000\,\text{nm}$ (1); Light microscope: $200\,\text{nm}$ (already) (1); TEM: $0.5\,\text{nm}$ (already) (1).

(b) Virus $80\,\text{nm}$: smaller than light microscope limit ($200\,\text{nm}$) so only the TEM ($0.5\,\text{nm}$) can resolve it. (1.5) Mitochondrion $3\,\mu\text{m} = 3000\,\text{nm}$: larger than the light microscope limit, so both light microscope and TEM can resolve it (not the naked eye). (1.5)

(c) Electrons have a much shorter wavelength than visible light, and resolution improves as wavelength decreases. (2)

[
  {"claim":"Q1a actual cell length is 120 micrometres","code":"image_um=48*1000; mag=400; result=(image_um/mag==120)"},
  {"claim":"Q1b 120 um equals 120000 nm and 0.12 mm","code":"result=(120*1000==120000) and (120/1000==0.12)"},
  {"claim":"Q2a magnification of micrograph is 8000","code":"bar_um=16*1000; actual=2; result=(bar_um/actual==8000)"},
  {"claim":"Q2b chloroplast actual length is 5 micrometres","code":"image_um=40*1000; mag=8000; result=(image_um/mag==5)"},
  {"claim":"Q5a eye resolving power 100 um equals 100000 nm","code":"result=(100*1000==100000)"}
]