waec model questions vol1 2018 biology | Practical

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Question 1 View Details
A fresh transverse section of an onion (Allium cepa) epidermis was prepared using a hand‑section technique and observed under a compound microscope at 400× magnification. The average dimensions of five typical cells measured with an ocular micrometer are: length = 30 µm, width = 15 µm, thickness = 5 µm.
Question Parts
(a)
Calculate the surface area (SA) and volume (V) of a typical cell, assuming the cell is a rectangular prism. Show all steps.
(b)
Determine the surface‑area‑to‑volume ratio (SA/V) and explain why a high SA/V ratio is advantageous for plant epidermal cells.
(c)
Identify two cellular structures that can be observed in the preparation and state one function of each.
(d)
List two possible sources of experimental error in the measurement of cell dimensions and suggest a way to minimise each.
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Question 2 View Details
A root tip of an onion (Allium cepa) was harvested, fixed in Carnoy’s solution and a squash preparation was made for observation of mitotic figures under a compound microscope at 1000× magnification. From the slide, a total of 500 cells were examined and the following numbers of cells in each stage of mitosis were recorded:
Question Parts
(a)
Calculate the mitotic index (MI) as a percentage.
(b)
Which phase of mitosis is most frequently observed in the sample? Provide the percentage of cells in that phase.
(c)
Discuss what the obtained mitotic index indicates about the growth activity of the root tip.
(d)
Suggest one improvement to the preparation technique that could increase the reliability of the mitotic index determination.
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Question 3 View Details
A group of students investigated the effect of glucose concentration on the rate of respiration in yeast. Fresh baker's yeast (5 g) was added to 100 mL of glucose solutions of different concentrations (0%, 5%, 10% and 15% w/v) in separate sealed respirometers kept in a water bath at 30 °C. The volume of carbon dioxide collected in the graduated tube was recorded after 10 minutes for each concentration as shown in the table below. | Glucose concentration (%) | CO₂ collected (mL) | |----------------------------|--------------------| | 0 | 2.0 | | 5 | 6.0 | | 10 | 9.0 | | 15 | 8.0 | Using the data above, answer the following questions:
Question Parts
(a)
Calculate the rate of respiration (in mL min⁻¹) for each glucose concentration.
(b)
State which concentration gives the highest rate of respiration and give a biochemical explanation for the observed trend.
(c)
Identify two possible sources of experimental error and suggest one practical improvement to minimise error.
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Question 4 View Details
Students examined the effect of temperature on the rate of respiration in germinating beans using a closed respirometer. Ten germinating Phaseolus vulgaris beans were placed in the respirometer, which was filled with 50 mL of water and sealed. The change in gas volume (decrease due to CO₂ uptake) was recorded over 5 minutes at three different water‑bath temperatures as shown: | Temperature (°C) | Volume decrease (mL) in 5 min | |------------------|--------------------------------| | 15 | 4.0 | | 25 | 9.0 | | 35 | 15.0 | Assume the gas behaves ideally and that the volume change is solely due to CO₂ produced by respiration. Answer the following:
Question Parts
(a)
Determine the rate of respiration (in mL min⁻¹) at each temperature.
(b)
Calculate the temperature coefficient Q₁₀ for the intervals 15 °C–25 °C and 25 °C–35 °C using the formula Q₁₀ = (R₂/R₁)^{10/(T₂‑T₁)} where R is the respiration rate.
(c)
Explain why the respiration rate increases with temperature and why the increase is not proportional beyond a certain temperature.
(d)
Identify two possible sources of experimental error in this set‑up and suggest one method to minimise each error.
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