AQA A-level Biology: Microscopy, Magnification and Cell Fractionation (7402) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions are aligned to section 3.2.1.3, Methods of studying cells, in the AQA AS and A-level Biology specification (A-level 7402), for first teaching 2015, exams June 2016 onwards. Nothing in this section is marked as A-level only. Read the study guide first, or use the revision notes to recall the method. Course hub: AQA A-level Biology. Printable checklist. Not sure where to start? Use the free diagnostics.

Questions

1. Define the term resolution. Explain why a transmission electron microscope can show more detail than an optical microscope. [3]

2. An SEM image of a fern spore is printed so that the spore is 54 mm across. The real diameter of the spore is 30 µm.

(a) Calculate the magnification of the image. Show your working. [3] (b) A bacterium on the same spore has a length of 3 µm. By how many orders of magnitude is the spore larger than the bacterium? [1]

3. A student sets up a microscope and compares the eyepiece scale with a stage micrometer whose divisions are each 10 µm. She finds that 50 eyepiece divisions line up with 15 stage divisions.

(a) Calculate the real length of one eyepiece division. [2] (b) A cell measures 8.5 eyepiece divisions. Calculate its length in µm. [1] (c) Explain why she must calibrate again if she changes the objective lens. [1]

4. Transmission and scanning electron microscopes both have limitations.

(a) State two differences between the images produced by a TEM and an SEM. [2] (b) State two limitations that apply to both types of electron microscope. [2]

5. A student writes: “A TEM is the best choice for watching living cells divide, because it gives a three-dimensional image.” Identify two errors in the statement and correct each one. [4]

6. A technician has leaves and needs a sample of chloroplasts.

(a) Describe how cell fractionation can be used to obtain the chloroplasts. Give the reason for each condition you state. [6] (b) Name the organelle in the first pellet formed from the leaf homogenate and explain why it forms before the chloroplast pellet. [2]

7. A leaf homogenate is spun at increasing speeds. Four pellets, A to D, are tested. The table shows the results.

Pellet	DNA	Chlorophyll	Oxygen released in light	Oxygen used with respiratory substrate	Hydrolytic enzymes
A	present	absent	no	no	no
B	absent	present	yes	no	no
C	absent	absent	no	yes	no
D	absent	absent	no	no	yes

(a) Identify the organelles in pellets A and B. [2] (b) Identify the organelles in pellets C and D. [2] (c) State which pellet formed first and explain why. [2]

8. Early electron micrographs of cells showed structures that scientists could not at first identify as real parts of cells.

(a) Explain what is meant by an artefact. [1] (b) A structure appears in TEM images of cells prepared by one fixing method but not in cells prepared by a different method. Suggest two ways a scientist could judge whether it is an artefact. [2] (c) Suggest why scientists took a considerable time to agree which structures were organelles. [2]

9. A TEM micrograph of a bacterium has a scale bar 24 mm long labelled 0.5 µm. The bacterium measures 60 mm on the micrograph.

(a) Work out the magnification. [2] (b) Find the actual length of the bacterium, in µm. [2]

10. For each task, name the most suitable type of microscope (optical, TEM or SEM) and give one reason.

(a) Watching a swimming single-celled organism from pond water. [2] (b) Examining the three-dimensional surface of an insect’s eye. [2] (c) Showing the internal membranes of a mitochondrion. [2]

11. A student examines onion epidermal cells with an optical microscope. A stage micrometer with divisions of 10 µm shows that 40 eyepiece divisions line up with 16 stage divisions.

(a) Calculate the real length of one eyepiece division. [2] (b) One cell is 25 eyepiece divisions long. Calculate its length in µm. [1] (c) The cell is drawn 80 mm long on the student’s diagram. What is the magnification of the diagram? [2] (d) On the drawing, the cell width is 28 mm. Calculate the real width of the cell in µm. [2] (e) The student wants to see whether the cells contain starch grains. Name the solution used and state the colour seen. [1] (f) State one feature of a good scientific drawing. [1]

Answers

1. Resolution is the ability to distinguish between two points that are close together, so that they are seen as separate [1]. The wavelength of an electron beam is far shorter than that of visible light [1], so a finer separation between points can still be seen as two, which gives more detail [1]. [3] Examiner insight: Answers that explain the better detail in terms of resolution and wavelength tend to gain credit, whereas answers that say only “it magnifies more” do not address the question.

2. (a) Image size = 54 mm = 54 × 1000 = 54 000 µm [1]. Magnification = 54 000 ÷ 30 [1]. ×1800 [1]. (b) 30 ÷ 3 = 10, so one order of magnitude [1]. [4] Examiner insight: Show the unit conversion as its own line. A clear conversion makes your method easy to follow and lets you spot a slip in the division, whereas a bare answer cannot be checked.

3. (a) 15 × 10 = 150 µm for 50 divisions [1]. 150 ÷ 50 = 3 µm per division [1]. (b) 8.5 × 3 = 25.5 µm [1]. (c) The objective changes the magnification, so each eyepiece division then covers a different length of the specimen [1]. [4] Examiner insight: Part (c) is stronger when it links the change of objective to the length covered by a division, rather than saying only “to be accurate”.

4. (a) TEM gives a 2D image of a thin section, whereas SEM gives a 3D image of a surface [1]. TEM shows internal structure, whereas SEM shows the surface only [1]. (Other valid differences: TEM has the higher resolution; SEM can use thicker specimens.) (b) The specimen is in a vacuum, so it cannot be alive [1]. The image is black and white [1]. (Another valid limitation: preparation is complex and can produce artefacts.) [4] Examiner insight: Two points that are really one idea, such as “dead” and “in a vacuum”, show less range than two separate ideas, so pick limitations that are clearly different.

5. Error 1: a TEM cannot examine living cells, because the specimen is dead and in a vacuum [1]; an optical microscope is the type that can show living cells dividing [1]. Error 2: a TEM gives a 2D image of a thin slice, not a 3D image [1]; an SEM gives a 3D image of a surface [1]. [4] Examiner insight: A correction is needed for each error. Spotting a mistake without the right replacement usually leaves marks unclaimed.

6. (a) Homogenise (break up) the leaf tissue in a blender [1]. Keep the solution cold, to reduce enzyme activity so enzymes released do not digest the organelles [1]. Use an isotonic solution, so there is no net movement of water by osmosis and the chloroplasts do not burst or shrink [1]. Use a buffer, to keep the pH constant so enzymes do not denature [1]. Filter to remove cell debris, then centrifuge at low speed and remove the pellet of heavier organelles, keeping the supernatant [1]. Spin the supernatant at a higher speed; the chloroplasts form the pellet [1]. (b) Nuclei [1]. They are larger and denser, so they are thrown down at a lower speed than chloroplasts [1]. [8] Examiner insight: Each condition needs its reason. Naming “cold, isotonic, buffered” alone is a list, and the reasons are what show understanding.

7. (a) A is nuclei, because it contains DNA and no chlorophyll [1]. B is chloroplasts, because it has chlorophyll and releases oxygen in light [1]. (b) C is mitochondria, because it uses oxygen with a respiratory substrate [1]. D is lysosomes, because they contain hydrolytic enzymes [1]. (c) Pellet A (nuclei) formed first [1], because nuclei are the largest and densest of these organelles, so they sediment at the lowest speed [1]. [6] Examiner insight: Link each identification to the evidence in the table. Naming an organelle without the supporting data is a weaker answer.

8. (a) A feature seen in the image that is not part of the living cell, produced by preparation or observation [1]. (b) Prepare the cells by several other methods and see whether it persists [1]. Look with a different type of microscope, or have another research group repeat the work [1]. (c) New techniques altered the specimen, so it was hard to tell real structures from effects of preparation [1]. Structures had to be confirmed by independent methods before they were accepted [1]. [5] Examiner insight: Part (b) asks for ways of judging, so state an action and what you would conclude, for example “if it is seen after every method, it is more likely to be real”.

9. (a) Magnification = 24 mm ÷ 0.5 µm = 24 000 µm ÷ 0.5 µm [1] = ×48 000 [1]. (b) Real length = 60 mm ÷ 24 mm × 0.5 µm, or 60 000 ÷ 48 000 [1] = 1.25 µm [1]. [4] Examiner insight: With a scale bar, you can either calculate magnification first or compare the lengths directly. Either route works, but keep units consistent.

10. (a) Optical microscope [1], because living organisms can be examined [1]. (b) SEM [1], because it gives a 3D image of a surface [1]. (c) TEM [1], because it has the highest resolution and shows internal structure [1]. [6] Examiner insight: A reason must match the choice. “Optical, because it is better” does not explain anything.

11. (a) 16 × 10 = 160 µm [1]. 160 ÷ 40 = 4 µm [1]. (b) 25 × 4 = 100 µm [1]. (c) 80 mm = 80 000 µm [1]. 80 000 ÷ 100 = ×800 [1]. (d) Real width = 28 000 ÷ 800 [1] = 35 µm [1]. (e) Iodine in potassium iodide solution; starch grains turn blue-black [1]. (f) Any one from: clear single lines, no shading, no colouring, label lines drawn with a ruler that do not cross, drawn large with correct proportions [1]. [9] Examiner insight: Later parts build on earlier ones, so show each stage clearly; a clear method is easier to credit than a bare number. Write units on sizes but not on magnification.

Where marks are usually lost

• Using “magnification” instead of “resolution” when asked why an electron microscope shows more detail.
• Leaving image and object in different units, so the magnification is out by a factor of 1000.
• Not explaining the reason for cold, isotonic and buffered conditions.
• Saying a TEM gives a 3D image or can examine living cells.
• Giving a calibration without showing how stage divisions were converted to micrometres.
• Calling any unfamiliar structure an organelle without considering preparation artefacts.
• Putting the organelles in the wrong order of sedimentation, or forgetting that chloroplasts occur only in plant tissue.
• Putting a unit on magnification, or leaving a unit off a size.

Next steps

• Recall the method: revision notes
• Re-read the explanations: study guide
• Course hub: AQA A-level Biology
• Tick off topics: checklist
• Check your level: all free 10-minute diagnostics
• Book a free trial class: trial

Official syllabus

AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015, exams June 2016 onwards, Version 1.5, published by AQA. Section 3.2.1.3 Methods of studying cells.