Prokaryotic vs Eukaryotic Cells

Question 1 (10 marks)

(a) Prokaryotic (1). Justifications (any two, 1 each): no membrane-bound nucleus / DNA is a single circular molecule (not linear chromosomes in a nucleus); 70S ribosomes are characteristic of prokaryotes (eukaryotes have 80S cytoplasmic ribosomes); lacks membrane-bound organelles such as mitochondria. (3) Why: identification must be tied to defining prokaryotic features, not generic statements.

(b) Penicillins inhibit the synthesis/cross-linking of peptidoglycan in the bacterial cell wall (1). Since this organism's wall lacks peptidoglycan, penicillin would have no target and would be ineffective (1). (2) (This mirrors real Archaea/atypical bacteria.)

(c) Flagellum (1). Prokaryotic flagellum: made of the protein flagellin, rotates like a rigid propeller driven by a proton-motive-force motor, and is NOT enclosed by the cell membrane (1). Eukaryotic flagellum: has a 9+2 microtubule arrangement, is membrane-bound, and beats/whips rather than rotating (1). (3)

(d) 70S ribosomes differ from the human cell's 80S cytoplasmic ribosomes (1). This difference allows antibiotics to target bacterial protein synthesis selectively, and confirms the organism is not eukaryotic (1). (2)

Question 2 (12 marks)

(a) Any three, 2 marks each (1 for evidence, 1 for reasoning):

• Own circular DNA independent of the nucleus — resembles the bacterial genome, suggesting an ancestral free-living cell.
• 70S ribosomes (bacterial type) rather than 80S — mitochondria synthesise some of their own proteins using bacterial machinery.
• Double membrane — the inner membrane is thought to be the original bacterial membrane, the outer from the host's engulfing vesicle.
• Binary-fission-like self-replication — divide independently of the host cell, like bacteria. (Max 6.)

(b) Under endosymbiosis, chloroplasts arose from engulfed photosynthetic prokaryotes (cyanobacteria) in a separate, later event than the mitochondrial ancestor (aerobic bacterium) (1). Only the lineage of cells that acquired the cyanobacterial endosymbiont became photosynthetic; this lineage gave rise to plants/algae (1). Animals descended from a lineage that never acquired the chloroplast, so they retain mitochondria but lack chloroplasts (1). (3)

(c) Unlikely to be endosymbiotic (1). A single membrane argues against engulfment (which predicts a double membrane) (1), and host-identical ribosomes indicate it uses the host's protein-synthesis machinery rather than its own bacterial-type ribosomes — so it lacks the hallmarks of a former free-living prokaryote (1). (3)

Question 3 (12 marks)

(a) Surface area of cube $= 6L^2$; Volume $= L^3$ (1). SA:V $= \dfrac{6L^2}{L^3} = \dfrac{6}{L}$ (2). (3)

(b)

• $L=2$: $6/2 = 3\ \mu m^{-1}$ (1)
• $L=8$: $6/8 = 0.75\ \mu m^{-1}$ (1)
• The $2\,\mu m$ cube has the higher ratio and is better supplied by diffusion (1). (3)

(c) Set $6/L = 1.5 \Rightarrow L = 6/1.5 = 4\ \mu m$ (2). Maximum side length $= 4\ \mu m$ (1). (3)

(d) Any valid adaptation (1) + explanation (2), e.g. microvilli / membrane folds increase surface area without greatly increasing volume, restoring a high SA:V so exchange keeps pace with demand. (Alternatively: elongated/flattened shape, or many small compartments.) (3)

Question 4 (10 marks)

(a) (Identity 1, justification 1 each = 6)

• X = plant cell — has cellulose wall + chloroplasts + large central vacuole (any one).
• Y = animal cell — eukaryotic (nucleus present) but no wall, no chloroplasts, no large vacuole.
• Z = bacterial cell — no nucleus and peptidoglycan wall; small (2 µm). (6)

(b) Any two of: nucleus/nuclear envelope, mitochondria, endoplasmic reticulum, Golgi apparatus, 80S ribosomes, membrane-bound organelles generally. (2)

(c) Cell Z (bacterial) (1). It is smallest (2 µm) so has the highest SA:V ratio → most efficient nutrient/waste exchange → supports rapid metabolism and division (1). (2)

Question 5 (6 marks)

(a) Any two, 2 marks each: peptidoglycan cell wall (absent in human cells); 70S ribosomes (human cytoplasmic ribosomes are 80S); bacterial DNA gyrase / topoisomerase enzymes; bacterial folic-acid synthesis pathway (humans obtain folate from diet); circular DNA replication machinery. (4)

(b) Example (2): disabling peptidoglycan synthesis → wall cannot be built/maintained → cell cannot resist internal osmotic pressure → water enters and the cell lyses (bursts) and dies. (Accept: blocking 70S ribosomes halts protein synthesis → bacterium cannot grow/replicate.) (2)

[
  {"claim": "SA:V ratio of a cube equals 6/L", "code": "L=symbols('L',positive=True); SA=6*L**2; V=L**3; result = simplify(SA/V - 6/L)==0"},
  {"claim": "Cube of side 2 um gives SA:V = 3 /um", "code": "result = Rational(6,2)==3"},
  {"claim": "Cube of side 8 um gives SA:V = 0.75 /um", "code": "result = Rational(6,8)==Rational(3,4)"},
  {"claim": "Maximum side length for SA:V of 1.5 is 4 um", "code": "L=symbols('L',positive=True); sol=solve(Eq(6/L, Rational(3,2)), L); result = sol[0]==4"}
]