Biology

1. 1.

   Explain how rough endoplasmic reticulum and the Golgi apparatus cooperate in secretion of a protein.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Follow the protein through a membrane-bound route: synthesis, processing, transport and exocytosis. The Golgi sorts the product rather than translating its amino-acid sequence.

   Marking points

   • Bound ribosomes translate the protein into the RER.
   • Vesicles carry it to the Golgi for modification and sorting.
   • Secretory vesicles fuse with the cell membrane to release it.

   Examiner tip: Assign synthesis to ribosomes, not to the Golgi.

2. 2.

   Distinguish magnification from resolution in microscopy and explain why enlarging a blurred image need not reveal two adjacent structures.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. A large image can reproduce the same merged spots at a larger scale. Resolving them requires information captured by the imaging system, not simply a larger display.

   Marking points

   • Magnification is image size divided by actual size.
   • Resolution is the ability to distinguish close points as separate.
   • Enlargement without better resolution adds no separable detail.

   Examiner tip: Do not use 'clearer' as an undefined substitute for resolution.

3. 3.

   A cell appears 36 mm long at magnification x1200. Calculate its actual length in micrometres.

   [3 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Convert the image measurement first, then divide 36000 by 1200. Magnification is a dimensionless ratio, so the chosen length unit remains in the answer.

   Marking points

   • Actual length = image length/magnification.
   • 36 mm = 36000 micrometres.
   • Actual length = 30 micrometres.

   Examiner tip: Do not divide by 1000 when converting millimetres to micrometres.

4. 4.

   A micrograph shows circular profiles of mitochondria. Explain why this does not establish that the intact mitochondria are spherical.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Several three-dimensional structures can intersect a plane in the same circle. The inference is underdetermined unless sections at other positions/orientations supply additional shape information.

   Marking points

   • A thin section is a two-dimensional slice.
   • A cylindrical or elongated organelle can have a circular cross-section.
   • Serial sections or three-dimensional imaging are needed to infer whole shape.

   Examiner tip: Differentiate the observed section from the entire organelle.

5. 5.

   During cell fractionation, explain why tissue is homogenised in cold, isotonic, buffered solution before differential centrifugation.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The aim is to release structures while preserving their function. Mechanical disruption opens cells; the three solution conditions each control a different source of subsequent damage.

   Marking points

   • Homogenisation breaks cells to release organelles.
   • Cold reduces enzyme activity that could damage organelles.
   • Isotonic conditions minimise osmotic swelling or shrinkage.
   • Buffer limits pH changes that could alter proteins/organelles.

   Examiner tip: Isotonic does not mean no water movement; it means no net osmotic volume change.

6. 6.

   A cell image is enlarged digitally after a scale bar was added. Explain when the bar remains valid, why a printed magnification label may not, and how to check a measured cell length.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The ratio of cell pixels to bar pixels survives uniform resizing. A magnification number instead encodes a particular display-to-object size ratio and must change with display size.

   Marking points

   • The bar remains valid if it is resized by the same factor as the image.
   • A numerical magnification label becomes outdated when display size changes.
   • Measure cell and bar in the same displayed image.
   • Actual cell length = measured cell/bar ratio times the bar's stated actual length.

   Examiner tip: A separately resized or cropped-out scale bar cannot calibrate the image reliably.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.