Cell Theory & Microscopy

Question 1 (8)

(a) (1 mark each, 3):

1. All living organisms are composed of one or more cells.
2. The cell is the basic (structural and functional) unit of life.
3. All cells arise from pre-existing cells (by division).

(b) (5):

• Schleiden (1838): concluded all plants are made of cells → supports tenet 1 for plants. (1)
• Schwann (1839): extended this to animals, generalising "all organisms are made of cells" and proposing the cell as the fundamental unit → tenets 1 & 2. (1)
• Virchow (1855): "Omnis cellula e cellula" — all cells come from pre-existing cells → tenet 3. (1)
• Logical gap (2): Schleiden and Schwann established what organisms are made of, but both flirted with spontaneous generation / crystallisation ideas about where new cells originate. Their work described composition but left cell origin unexplained. Virchow's contribution closed this by asserting continuity — cells only from cells — completing the theory and undermining spontaneous generation.

Question 2 (7)

(a) (3):

• Hooke (1665): first to observe and name "cells" (in cork), using a compound microscope. (1.5)
• Leeuwenhoek (1670s): first to observe living microorganisms ("animalcules"), bacteria, and blood cells using superior single-lens microscopes. (1.5)

(b) (4):

• Hooke saw only the empty cell walls of dead cork — box-like compartments, not living contents. (1)
• He had no evidence of cell function, of living material inside, or that all organisms share this structure. (1)
• To establish tenet 1 (all organisms made of cells) one needs observations across many living tissues/organisms, not a single dead plant material. (1)
• Establishing that cells are the unit of life requires seeing them alive and functioning — which Leeuwenhoek's animalcules and later systematic work (Schleiden/Schwann) provided. (1)

Question 3 (9)

(a) (4):

• Magnification (2): the number of times larger an image appears compared with the real object (a ratio of image size to actual size).
• Resolution (2): the smallest distance between two points at which they can still be seen as separate/distinct (the level of detail an instrument can distinguish).

(b) (5):

• Magnification only enlarges the image; it does not add information. (1)
• Once you magnify beyond the resolving limit, two points that the instrument cannot separate stay merged and simply appear bigger and blurrier — "empty magnification." (2)
• Resolution is limited by the wavelength of the illuminating radiation (roughly ~½ the wavelength). (1)
• Electrons have a far shorter wavelength than visible light, so electron microscopes resolve much finer detail (nm vs ~200 nm for light). (1)

Question 4 (10)

(a) (4):

• Convert scale bar drawn length: $20\ \text{mm} = 20000\ \mu\text{m}$. (1)
• It represents an actual $2\ \mu\text{m}$. (1)
• $\text{Magnification} = \dfrac{\text{image size}}{\text{actual size}} = \dfrac{20000\ \mu\text{m}}{2\ \mu\text{m}}$. (1)
• $= \mathbf{10000\times}$ (i.e. ×10,000). (1)

(b) (4):

• Image size of chloroplast $= 55\ \text{mm} = 55000\ \mu\text{m}$. (1)
• Actual length $=\dfrac{\text{image}}{\text{mag}}=\dfrac{55000}{10000}=\mathbf{5.5\ \mu m}$. (2)
• In nm: $5.5\ \mu\text{m} \times 1000 = \mathbf{5500\ nm}$. (1)

(c) (2):

• $\text{Magnification}=\dfrac{\text{Image size}}{\text{Actual size}}$ (1)
• $\Rightarrow \text{Actual size}=\dfrac{\text{Image size}}{\text{Magnification}}$ (1)

Question 5 (9)

(a) (5) — 1 mark per correct contrast, max 5:

Award: internal vs surface (1); 2D vs 3D (1); thin section vs thick/whole (1); electrons transmitted vs reflected (1); TEM higher resolution (1).

(b) (4):

• Most cell structures are colourless/transparent, giving little contrast. (1)
• Stains bind selectively to components, adding colour/contrast so structures become visible and distinguishable. (1)
• EM images use electrons, not visible light, so coloured dyes are useless — colour has no meaning. (1)
• Heavy-metal stains (e.g. osmium, lead, uranium) are electron-dense: they scatter electrons, creating contrast in the electron image. (1)

Question 6 (7)

(a) (4) — ordered steps (≈0.5–1 each, plus reasoning):

1. Place a drop of water on a clean glass slide. (1)
2. Using forceps, place the thin onion epidermis specimen flat in the water. (1)
3. (Optional) add a drop of stain (e.g. iodine). Lower the coverslip at an angle using a mounting needle. (1)
4. Reason: lowering at an angle lets water spread and excludes air bubbles, which would obscure viewing; blot excess water with tissue. (1)

(b) (3, 1 each):

• (i) $0.045\ \text{mm}\times1000 = \mathbf{45\ \mu m}$ (×1000)
• (ii) $850\ \text{nm}\div1000 = \mathbf{0.85\ \mu m}$ (÷1000)
• (iii) $7000\ \text{nm}\div10^{6} = \mathbf{0.007\ mm}$ (÷1,000,000)

[
  {"claim":"Q4a magnification = 20000/2 = 10000","code":"result = (Rational(20000,2) == 10000)"},
  {"claim":"Q4b actual chloroplast length = 5.5 um","code":"actual = Rational(55000,10000); result = (actual == Rational(11,2))"},
  {"claim":"Q4b 5.5 um = 5500 nm","code":"result = (Rational(11,2)*1000 == 5500)"},
  {"claim":"Q6b(i) 0.045 mm = 45 um","code":"result = (Rational(45,1000)*1000 == 45)"},
  {"claim":"Q6b(iii) 7000 nm = 0.007 mm","code":"result = (Rational(7000,10**6) == Rational(7,1000))"}
]