waec model questions vol1 2022 biology | Practical

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Question 1 View Details
You are given a prepared slide of human cheek epithelial cells stained with methylene blue. Using a light microscope with a 10× ocular lens and a 40× objective lens, you are to determine the average cell diameter and calculate the surface‑area‑to‑volume ratio of a typical cell.
Question Parts
(a)
Record the diameter (in µm) of ten different cells as measured with the ocular micrometer. Use the data provided in the table below. | Cell | Diameter (µm) | |------|---------------| | 1 | 12 | | 2 | 13 | | 3 | 11 | | 4 | 14 | | 5 | 13 | | 6 | 12 | | 7 | 15 | | 8 | 13 | | 9 | 12 | |10 | 14 |
(b)
Calculate the mean diameter of the cells (to one decimal place).
(c)
Assuming the cells are spherical, calculate: i. The average surface area (µm²). ii. The average volume (µm³).
(d)
Determine the surface‑area‑to‑volume ratio of a typical cell (expressed as µm⁻¹, to two decimal places).
(e)
Mention any two possible sources of error that could affect the accuracy of your measurements.
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Question 2 View Details
You are to investigate cell division in the root tip of an onion (Allium cepa). Prepare a slide, stain with 2% aceto‑orcein, and examine the meristematic region under a light microscope (10× ocular, 40× objective). Record the number of cells observed in each phase of mitosis in five randomly selected fields.
Question Parts
(a)
Record the numbers of cells observed in prophase, metaphase, anaphase, telophase and interphase for each of the five fields as shown in the table. | Field | Prophase | Metaphase | Anaphase | Telophase | Interphase | |-------|----------|-----------|----------|-----------|------------| | 1 | 8 | 5 | 4 | 3 | 30 | | 2 | 7 | 6 | 5 | 2 | 28 | | 3 | 9 | 4 | 3 | 4 | 32 | | 4 | 6 | 7 | 5 | 3 | 27 | | 5 | 8 | 5 | 4 | 2 | 29 |
(b)
Calculate the total number of cells observed in all five fields.
(c)
Determine the mitotic index (percentage) for the sample. Show your working and give the answer to one decimal place.
(d)
Based on the mitotic index obtained, state whether the onion root tip is undergoing active cell division.
(e)
Suggest any two factors that could lead to an under‑estimation of the mitotic index in this experiment.
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Question 3 View Details
A group of students carried out an experiment to investigate the effect of sucrose concentration on the rate of osmosis in potato tuber discs. Uniform discs (diameter 1 cm, thickness ≈ 0.5 cm) were cut from a fresh potato and each disc was weighed to the nearest 0.01 g (initial mass). The discs were then placed in 50 mL of sucrose solutions of different concentrations (0%, 5%, 10% and 15% w/v) for 30 minutes at room temperature (≈25 °C). After the exposure, each disc was removed, surface water gently blotted with filter paper and weighed again (final mass). The data obtained are shown in the table below. | Concentration (%) | Initial mass (g) | Final mass (g) | |-------------------|------------------|----------------| | 0 | 2.45 | 2.78 | | 5 | 2.46 | 2.71 | | 10 | 2.44 | 2.62 | | 15 | 2.47 | 2.55 | Using the data above, answer the following questions:
Question Parts
(a)
Calculate the percentage change in mass of the potato disc for each sucrose concentration. Show your working.
(b)
Draw a simple graph of percentage change in mass (y‑axis) against sucrose concentration (x‑axis) and describe the trend observed.
(c)
Identify the sucrose concentration at which the potato disc shows the maximum percentage mass gain and give a brief physiological explanation for this observation.
(d)
Suggest two possible sources of experimental error in this investigation and propose a method to minimise each error.
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Question 4 View Details
To study the effect of temperature on the rate of respiration, students used a closed respirometer in which the volume of carbon dioxide (CO₂) produced by germinating beans displaces water. Three identical respirometers were placed in water baths maintained at 15 °C, 25 °C and 35 °C respectively. Ten healthy beans were placed in each respirometer, the system was sealed and the displaced water volume was recorded every 5 minutes for a total period of 30 minutes. The data obtained are shown below. | Time (min) | CO₂ volume (mL) – 15 °C | CO₂ volume (mL) – 25 °C | CO₂ volume (mL) – 35 °C | |------------|--------------------------|--------------------------|--------------------------| | 0 | 0.0 | 0.0 | 0.0 | | 5 | 0.8 | 1.4 | 2.1 | |10 | 1.6 | 2.9 | 4.3 | |15 | 2.4 | 4.4 | 6.5 | |20 | 3.2 | 5.9 | 8.6 | |25 | 4.0 | 7.3 |10.8 | |30 | 4.8 | 8.8 |12.9 | Answer the following:
Question Parts
(a)
Determine the average rate of respiration (in mL CO₂ min⁻¹) for each temperature over the 30‑minute period.
(b)
Calculate the Q₁₀ temperature coefficient for the intervals 15 °C → 25 °C and 25 °C → 35 °C. Use the formula Q₁₀ = (R₂/R₁)^{10/(T₂‑T₁)} where R is the respiration rate and T is temperature in °C.
(c)
Provide a concise physiological explanation for the observed increase in respiration rate with temperature and why the Q₁₀ value decreases at the higher temperature range.
(d)
Identify two limitations of the experimental set‑up and suggest improvements to obtain more reliable data.
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