Interleaved — Phase 2

Biology interleaved practice

printable — key stays hidden on paper

Instructions: Answer all problems. Show working for calculations and unit conversions. Where a diagram is described, reason from the given data. State units. Total: 50 marks.

1. A student measures a chloroplast on a micrograph. The image length is 30 mm and the scale bar labelled $10\ \mu m$ measures 5 mm on the same image. Calculate (a) the magnification of the image, and (b) the actual length of the chloroplast in micrometers. (5 marks)

2. Match each scientist to their single most important contribution: Hooke, Leeuwenhoek, Schleiden, Schwann, Virchow. (5 marks)

3. Explain the surface-area-to-volume ratio for a cube of side $2\ \mu m$ versus a cube of side $6\ \mu m$ . Calculate both ratios and explain what this predicts about which "cell" exchanges materials more efficiently. (6 marks)

4. A eukaryotic cell exports a digestive enzyme. List, in correct order, the four organelles/structures the protein passes through from synthesis to secretion, and state the role of each. (6 marks)

5. Convert the following: (a) $0.45\ \mu m$ to nm, (b) $2\,500\ nm$ to $\mu m$ , (c) $0.008\ mm$ to $\mu m$ . (3 marks)

6. State the three tenets of cell theory. (3 marks)

7. The endosymbiotic theory is supported by features shared between mitochondria/chloroplasts and bacteria. Give three pieces of evidence and explain why each supports the theory. (6 marks)

8. A researcher wants to view the 3D external surface texture of a pollen grain at high resolution. State which microscope she should use and justify by referring to the difference between magnification and resolution. (4 marks)

9. Compare a prokaryotic and a eukaryotic cell across: genetic material location, ribosome type, and presence of membrane-bound organelles. (6 marks)

10. Briefly outline the steps to prepare a wet mount slide of onion epidermis, and explain the purpose of adding iodine stain. (6 marks)

Answer keyMark scheme & solutions

1. (Subtopic 2.1.5 — magnification/actual size from scale bars, with 2.1.8 units)

Scale bar: 5 mm on image = $10\ \mu m$ real → magnification = image/actual = $5\,000\ \mu m / 10\ \mu m = 500\times$ .
(a) Magnification = ×500
(b) Actual length = image length ÷ magnification = $30\ mm / 500 = 0.06\ mm = 60\ \mu m$ .
(a) ×500, (b) 60 μm Method-choice why: students must first extract magnification from the scale bar, not assume it — the bar is the only calibration given.

2. (Subtopic 2.1.2)

Hooke — first described "cells" in cork (1665).
Leeuwenhoek — first observed living microorganisms ("animalcules").
Schleiden — all plants are made of cells.
Schwann — all animals are made of cells (co-founder of cell theory).
Virchow — all cells arise from pre-existing cells (omnis cellula e cellula).

3. (Subtopic 2.2.6 / 2.2.7)

Cube side 2: SA $=6(2^2)=24\ \mu m^2$ ; V $=2^3=8\ \mu m^3$ ; ratio $=24/8=3$ .
Cube side 6: SA $=6(6^2)=216\ \mu m^2$ ; V $=6^3=216\ \mu m^3$ ; ratio $=216/216=1$ .
The smaller cube (ratio 3) has more surface per unit volume, so exchanges materials more efficiently. This is why cells stay microscopic — a large cell cannot supply its volume across its relatively small surface.

4. (Subtopic 2.3.5, 2.3.3, 2.3.4 integrated secretory pathway)

Ribosome — synthesises the protein (translation).
Rough ER — folds/modifies protein; buds transport vesicles.
Golgi apparatus — further modifies, sorts, packages into secretory vesicles.
Plasma membrane — vesicle fuses (exocytosis), enzyme released. Method-choice why: tests ordering — a smooth-ER distractor doesn't belong in protein secretion.

5. (Subtopic 2.1.8)

(a) $0.45\ \mu m \times 1000 = 450\ nm$
(b) $2500\ nm \div 1000 = 2.5\ \mu m$
(c) $0.008\ mm \times 1000 = 8\ \mu m$

6. (Subtopic 2.1.1)

All living things are composed of one or more cells.
The cell is the basic unit of structure and function in life.
All cells arise from pre-existing cells.

7. (Subtopic 2.2.5)

Own circular DNA — like bacterial nucleoid; suggests a free-living ancestor.
70S ribosomes (bacterial-type, not eukaryotic 80S) — indicates prokaryotic origin.
Double membrane — inner from the engulfed prokaryote, outer from host vesicle.
(Also acceptable: binary-fission-like division.)

8. (Subtopic 2.1.3 / 2.1.4)

Use a Scanning Electron Microscope (SEM) — gives 3D surface images at high resolution.
Magnification = how many times larger the image appears; resolution = ability to distinguish two close points as separate. Electron microscopes have far higher resolution (shorter wavelength) than light microscopes, so fine surface detail stays sharp; a light microscope would blur at that scale even if magnified.

9. (Subtopic 2.2.1)

Feature	Prokaryote	Eukaryote
Genetic material	Free in cytoplasm (nucleoid, no membrane)	Enclosed in nucleus
Ribosomes	70S (smaller)	80S (larger)
Membrane-bound organelles	Absent	Present

10. (Subtopic 2.1.7 / 2.1.6)

Place a drop of water on a clean slide.
Peel a thin single layer of onion epidermis; place flat in the drop.
Lower a coverslip at ~45° using a mounting needle to avoid air bubbles.
Add iodine at coverslip edge; draw through with filter paper.
Purpose of iodine stain: increases contrast, staining cell walls/nuclei so structures otherwise transparent become visible.

[
  {"claim":"Magnification from scale bar = 500x (Q1)","code":"bar_img_mm=5; bar_real_um=10; mag=(bar_img_mm*1000)/bar_real_um; result=(mag==500)"},
  {"claim":"Chloroplast actual length = 60 um (Q1)","code":"mag=500; img_mm=30; actual_um=(img_mm*1000)/mag; result=(actual_um==60)"},
  {"claim":"SA:V ratios are 3 and 1 (Q3)","code":"r2=(6*2**2)/(2**3); r6=(6*6**2)/(6**3); result=(r2==3 and r6==1)"},
  {"claim":"Unit conversions correct (Q5)","code":"a=0.45*1000; b=2500/1000; c=0.008*1000; result=(a==450 and b==2.5 and c==8)"}
]