OCR A-Level Biology: Cell Structure and Microscopy
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Brown adipose ("brown fat") tissue generates heat in newborn mammals. Each brown-fat cell is packed with mitochondria, and a transmission electron microscope (TEM) shows that these mitochondria have an unusually large number of cristae.
Describe and explain how the ultrastructure of a mitochondrion is related to its function in releasing energy. In your answer, link at least two structural features to the production of ATP, and use the brown-fat cell to explain why a cell with a high energy demand contains many mitochondria with many cristae.
(6 marks)
A student used a light microscope fitted with an eyepiece graticule to measure the length of cheek epithelial cells. To calibrate the graticule at the ×40 objective, they viewed a stage micrometer on which 1 mm is divided into 100 equal divisions.
The student recorded the following alignment.
| Measurement | Value |
|---|---|
| Eyepiece-graticule divisions aligned | 80 |
| Stage-micrometer divisions covered by those 80 eyepiece divisions | 25 |
| Value of one stage-micrometer division | 10 µm |
| Length of a cheek cell (measured at ×40) | 45 eyepiece divisions |
(a) Using these data, calculate the actual length of the cheek cell in µm. Show your working. (4 marks)
(b) The student then switched to the ×10 objective to view a larger field. Explain why the calibration above cannot be used at ×10, and what they must do before measuring at this magnification. (2 marks)
The table summarises the typical performance of four microscopes available in a research laboratory.
| Microscope | Typical maximum resolution | Specimen condition | Image produced |
|---|---|---|---|
| Light (optical) | ~200 nm | Living or dead | 2D, colour |
| Laser scanning confocal | ~180 nm | Living or dead (fluorescently labelled) | 2D/3D optical sections, fluorescent colour |
| Scanning electron microscope (SEM) | ~5 nm | Dead, metal-coated | 3D-like surface, greyscale |
| Transmission electron microscope (TEM) | ~0.2 nm | Dead, ultra-thin section | 2D internal section, greyscale |
A team wants to film a living white blood cell as it engulfs labelled bacteria, following the movement of a fluorescently tagged protein inside the living cell over several minutes.
(a) Using the data, state and justify which microscope is most suitable for this investigation. (3 marks)
(b) Explain why the TEM, despite having by far the best resolution, is unsuitable for this particular study. (2 marks)
To separate organelles, a sample of liver tissue was homogenised in an ice-cold, isotonic, buffered solution and the homogenate was filtered. The filtrate was then spun in an ultracentrifuge at increasing speeds. After each spin the pellet (sediment) was removed and the remaining liquid (supernatant) was spun again at a higher speed. The pellets obtained are summarised below.
| Spin | Relative speed | Main organelle in the pellet |
|---|---|---|
| 1 | Low | Nuclei |
| 2 | Medium | Mitochondria |
| 3 | High | Ribosomes |
A student suggested that ribosomes settle out at the lowest speed because they are the most numerous organelle.
(a) Explain why nuclei form the pellet at the lowest speed, whereas ribosomes only settle at the highest speed. (3 marks)
(b) The student's suggestion is incorrect. Explain why the order of settling depends on a property other than how numerous the organelles are. (2 marks)
A plasma cell is a specialised white blood cell that secretes large quantities of antibody (a glycoprotein) into the blood. A transmission electron micrograph of a plasma cell shows that it contains an extensive network of rough endoplasmic reticulum, a large Golgi apparatus, and many mitochondria.
Suggest and explain how these three ultrastructural features adapt the plasma cell for its role in secreting antibody. (4 marks)
A bacterium (a prokaryotic cell) and a human liver cell (a eukaryotic cell) were examined under an electron microscope.
Describe three structural differences that could be used to identify the cell as prokaryotic rather than eukaryotic. (3 marks)