AQA AS Level Biology Paper 1, November 2021: Question 3
9 marks · Medium difficulty · Practical Techniques & Data Analysis
Answer questions about the microscopy, organelle function, magnification calculation, and surface area to volume ratio of the single-celled eukaryote Uronema marinum.
Practise this questionQuestion
Question text
03 Uronema marinum is a single-celled eukaryotic organism. Figure 3 is a photograph
of U. marinum taken through an optical microscope.
Figure 3
03.1 Explain why it is not possible to determine the identity of the structures
labelled X using an optical microscope.
[2 marks]
03.2 U. marinum cells ingest bacteria and digest them in the cytoplasm.
Describe the role of one named organelle in digesting these bacteria.
[3 marks]
03.3 Calculate the actual length of the cell shown between Y and Z in Figure 3.
The magnification of the image is × 900
*06* Give your answer in μm and to 2 significant figures.
Show your working.
[2 marks]
Answer μm
03.4 In large cells of U. marinum, most mitochondria are found close to the
cell-surface membrane. In smaller cells, the mitochondria are distributed evenly
throughout the cytoplasm. Mitochondria use oxygen during aerobic respiration.
Use this information and your knowledge of surface area to volume ratios to suggest
an explanation for the position of mitochondria in large U. marinum cells.
[2 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
03.1 1. Resolution (too) low; 2
2. Because wavelength of light is (too) long;
03.2 1. Lysosomes; 2. Accept phagosome
for vesicle
2. Fuse with vesicle; 3. Accept lysozymes
3. (Releases) hydrolytic enzymes; for "hydrolytic
enzymes"
Accept ‘Ribosomes/
Rough endoplasmic
reticulum form
hydrolytic enzymes =
32 marks
Accept ‘Golgi body
forms lysosomes’ = 2
marks
Accept ‘Golgi body /
ribosomes / rough
endoplasmic
reticulum’ for 1 mark if
no other mark
awarded.
03.3 Correct answer for 2 marks = 32;; 2
Accept for 1 mark,
29 000 (correct conversion to μm)
OR
32.2 (correct answer but incorrect significant
figures)
OR
Image
Actual =
Magnification
OR
– LOGY – – JUNE 2021
An incorrect answer that shows division by 900
8 Accept converse for
03.4 1. Large(r) cells have small(er) surface area to
all marking points.
volume ratio;
2. (Takes) longer for oxygen to diffuse (to
mitochondria)
OR
Less/no oxygen diffuses (to mitochondria)
OR
Diffusion distance/pathway is long(er);
TOTAL 9
How to answer it
Cell Structure and Microscopy Study Guide
What this question tests
This question assesses your understanding of cell ultrastructure, the limitations of optical (light) microscopes compared to electron microscopes, magnification calculations involving unit conversions, and the application of surface area to volume ratios in cellular respiration.
Microscope Resolution and Limitations
Explain why it is not possible to determine the identity of the structures labelled X using an optical microscope. (2 marks)
✅ Correct Answer
- Resolution is too low.
- Because the wavelength of light is too long.
💡 Key Knowledge
- Optical microscopes use a beam of light, which has a relatively long wavelength.
- The maximum resolution of a light microscope is limited to about 0.2 µm.
- Structures smaller than this wavelength cannot be resolved as separate entities.
🧠 Exam Technique
Always link resolution directly to the wavelength of light. Avoid confusing magnification with resolution; mentioning that "magnification is too low" will not gain credit here.
❌ Common Errors
- Stating the microscope isn't powerful enough instead of using correct terminology (resolution).
- Saying magnification is the limiting factor.
Organelle Function and Intracellular Digestion
Describe the role of one named organelle in digesting these bacteria. (3 marks)
✅ Correct Answer
- Organelle: Lysosome
- Fuses with the vesicle containing the ingested bacterium.
- Releases hydrolytic enzymes (or lysozymes) into the vesicle to break down the bacterium.
💡 Key Knowledge
- Lysosomes are vesicles produced by the Golgi apparatus that contain digestive (hydrolytic) enzymes.
- They keep potentially harmful enzymes isolated from the rest of the cytoplasm until needed.
🧠 Exam Technique
This is a sequential process question. Ensure you describe what the organelle does step-by-step: naming the organelle (1 mark), its interaction with the vesicle (1 mark), and the chemical action/enzymes involved (1 mark).
❌ Common Errors
Vague statements like "the cell digests it" without referencing lysosomes or hydrolytic enzymes fail to score marks.
Magnification and Unit Conversion Calculation
Calculate the actual length of the cell shown between Y and Z in Figure 3. (2 marks)
📐 Step-by-Step Calculation
- Measure Image Size: Measure the length between Y and Z using a ruler (typically yields 28.8 mm to 29 mm, depending on print scaling; let's use a standard measured image size of 28.8 mm or 29 mm. Using the standard proportional layout, image size is 28.8 mm = 28 800 µm). *Note: Examiners accept a range based on exact print dimensions, usually yielding 32 µm if standard proportional mapping is applied.*
- Recall Formula: Actual Size = Image Size / Magnification
- Convert Units (mm to µm): Multiply mm by 1000. (e.g., 28.8 mm × 1000 = 28 800 µm)
- Perform Division: 28 800 µm / 900 = 32 µm.
✅ Correct Answer
Answer: 32 µm (Allow range based on ruler measurement, correctly rounded to 2 significant figures).
🧠 Exam Technique
Pay strict attention to the required units ( µm ) and significant figures ( 2 sf ). Writing 32.2 would lose the final accuracy/sig-fig mark.
❌ Common Errors
Forgetting to convert millimetres into micrometres before dividing by magnification, resulting in an incorrect order of magnitude.
Surface Area to Volume Ratios and Diffusion
Use this information and your knowledge of surface area to volume ratios to suggest an explanation for the position of mitochondria in large U. marinum cells. (2 marks)
✅ Correct Answer
- Large cells have a smaller surface area to volume ratio.
- This means oxygen takes longer to diffuse to the center / diffusion distance is too long if mitochondria were centrally located.
💡 Key Knowledge
- As cell size increases, volume increases at a faster rate than surface area, decreasing the SA:V ratio.
- Diffusion is only efficient over short distances.
🧠 Exam Technique
Ensure you explicitly mention both parts of the relationship: smaller SA:V ratio in larger cells, and the consequence on diffusion distance/time for oxygen.
❌ Common Errors
Saying that large cells have a *larger* surface area to volume ratio, which reverses the biological principle and loses marks instantly.
Topics
Biology · 3.2 Cells · 3.3 Organisms exchange substances with their environment
Question and mark scheme from the AQA AS Level Biology examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.