ijmb model questions vol1 2021 biology | Practical

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Question 1 View Details
A prepared slide of human cheek epithelial cells is examined under a light microscope. The ocular micrometer has been calibrated with a stage micrometer so that one division equals 0.01 mm. At a total magnification of 400×, the average cell diameter measured from ten cells is 0.45 mm on the ocular scale.
Question Parts
(a)
Calculate the actual average cell diameter in micrometres (µm).
(b)
Assuming the cells are spherical, calculate the average cell volume (µm³) and surface area (µm²). Use π = 3.14.
(c)
Determine the surface‑area‑to‑volume ratio and comment briefly on its significance for diffusion across the cell membrane.
(d)
List two possible sources of error in the diameter measurement and suggest one way to minimise each error.
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Question 2 View Details
A fresh root tip of an onion (Allium cepa) is harvested, fixed in Carnoy's solution, stained with aceto‑orcein and examined under a compound microscope at a total magnification of 1000×. Five fields are selected, each containing approximately 200 cells. The numbers of cells observed in each stage of mitosis are recorded as shown in the table below. | Field | Prophase | Metaphase | Anaphase | Telophase | Interphase | |-------|----------|-----------|----------|-----------|------------| | 1 | 30 | 20 | 10 | 5 | 135 | | 2 | 28 | 22 | 12 | 6 | 132 | | 3 | 32 | 18 | 9 | 4 | 137 | | 4 | 31 | 21 | 11 | 5 | 132 | | 5 | 29 | 19 | 10 | 6 | 136 |
Question Parts
(a)
Calculate the total number of cells examined and the total number of cells in each of the five categories (prophase, metaphase, anaphase, telophase, interphase).
(b)
Determine the mitotic index (percentage of cells in mitosis) for the whole sample.
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Question 3 View Details
A student is required to determine the percentage (w/v) of reducing sugar (glucose) in a commercial fruit‑juice sample using the Benedict’s test and a series of standard glucose solutions.
Question Parts
(a)
Construct a calibration curve from the data given and determine the concentration of glucose (in g per 100 mL) present in the juice sample.
(b)
Calculate the percentage (w/v) of reducing sugar in the juice sample.
(c)
List two possible sources of systematic error in this experiment and give one practical way to minimise each.
(d)
Briefly explain why the Benedict’s test detects reducing sugars but not non‑reducing sugars such as sucrose.
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Question 4 View Details
A student investigates the effect of temperature on the rate of respiration in germinating wheat grains using a closed respirometer containing potassium hydroxide to absorb the CO₂ evolved.
Question Parts
(a)
Using the data below, calculate the respiration rate in millilitres of CO₂ per gram of wheat per minute for each temperature. Temperature (°C) : 15, 25, 35, 45, 55 Volume of gas collected after 30 min (mL) : 12.0, 18.5, 25.0, 28.0, 27.5 Mass of wheat grains in each respirometer = 5 g.
(b)
State the temperature at which the respiration rate is maximal and give the corresponding rate.
(c)
Explain why the respiration rate declines at 55 °C despite the temperature increase.
(d)
Suggest one procedural modification that could improve the accuracy of the respiration measurements and justify your suggestion.
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