AQA A-Level Biology Paper 1, June 2025: Question 4

✅ Mark Scheme Answers

Any three from:

• Capsule (or slime layer)
• Plasmid(s)
• Flagellum / flagella
• Also accepted: Pili / pilus OR photosynthetic lamellae / mesosomes

💡 Key Knowledge: Universal vs Variable

Make sure you clearly categorise bacterial cell components:

• In ALL bacteria: Murein cell wall, cell-surface membrane, 70S ribosomes, circular DNA (nucleoid / genophore), cytoplasm.
• In SOME bacteria: Plasmids (extra-chromosomal DNA loops), capsule (mucilage protecting against phagocytosis/desiccation), flagella (locomotion).

❌ Common Student Errors

• Writing universal features: Naming "cell wall", "ribosomes", or "circular DNA" immediately loses credit because all bacteria have them.
• Naming eukaryotic organelles: Mitochondria, nucleus, and chloroplasts are never found in prokaryotes.
• Vague terminology: Writing "DNA" or "cell membrane" without context.

🧠 Exam Technique: Command Words

• Notice the bold keywords: some versus not found in all.
• Do not write conflicting answers. Providing four answers when three are asked can risk negative marking if one contradicts the specification.

✅ Investigation 1 Correct Choice & Reasons

Type of microscope: Optical microscope / Light microscope (identification necessary to access reason marks)

Two reasons from:

1. Can view living cells / organisms / specimens.
2. Can detect colour / stain / dye (blue stain vs colourless).
3. Resolution of optical microscope (0.25 µm) is sufficient to view yeast cells (mean diameter 3.0 µm, as 3.0 µm > 0.25 µm).

💡 Why Optical for Investigation 1?

• Living material: Electron microscopy uses a high vacuum; water boils away, meaning specimens must be dead/dehydrated.
• Colour distinction: Electron beams produce greyscale density maps (false colour is digital). Visible light retains true stain colour (methylene blue viability staining).
• Size vs Resolution: Yeast (3.0 µm) is well above the 0.25 µm resolution limit of light.

✅ Investigation 2 Correct Choice & Reasons

Type of microscope: Scanning electron microscope (SEM) (must be named to access reason marks)

Reasons:

1. Produces a three-dimensional (3D) image / shows surface structures.
2. Has high enough resolution (0.02 µm) to resolve/view mitochondria (diameter 0.22 µm, since 0.22 µm > 0.02 µm).

📐 Step-by-Step Resolution Check

Always verify whether an object can be resolved by comparing diameters:

1. Condition for visibility: Structure size > Microscope resolution limit.
2. Mitochondria diameter: 0.22 µm.
3. Optical limit: 0.25 µm → 0.22 µm < 0.25 µm (Optical CANNOT resolve mitochondria).
4. SEM limit: 0.02 µm → 0.22 µm > 0.02 µm (SEM CAN resolve mitochondria).
5. Conclusion: SEM provides both the required resolution (< 0.22 µm) and the requested 3D view.

❌ Common Errors in 04.2

• Choosing TEM for Investigation 2: TEM provides 2D cross-sections of internal structures, NOT 3D surface images. The prompt explicitly says "three-dimensional structure".
• Forgetting units in comparisons: Stating "mitochondria is 0.22 and SEM is 0.02" without stating µm or explaining which value must be smaller.
• Confusing magnification and resolution: Stating SEM has "higher magnification" rather than citing resolution. Resolution determines the ability to distinguish two separate points.

🧠 Top-Scoring Examiner Tips

• Link data directly: Quote the data from Table 3 and Table 4 in your justification (e.g. "SEM resolution of 0.02 µm is smaller than the mitochondrion diameter of 0.22 µm").
• Key trigger words: When an exam question specifies "three-dimensional" or "surface", the answer is virtually always SEM. When it mentions "internal organelles / ultra-thin section", think TEM.