waec model questions vol1 2023 biology | Practical

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Question 1 View Details
You are provided with a fresh onion bulb and a compound light microscope equipped with a 10× ocular lens and a 40× objective lens. Using standard laboratory techniques, prepare a temporary slide of an onion epidermal peel and observe the cells under the microscope.
Question Parts
(a)
List at least five distinct cell structures that can be clearly observed in the prepared slide.
(b)
Draw a labelled diagram of a typical onion epidermal cell as seen under the microscope, indicating the structures named in part (a).
(c)
State the main function of each structure listed in part (a).
(d)
Identify two possible sources of error that could affect the clarity of the observed structures and suggest a method to minimise each error.
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Question 2 View Details
A healthy mung bean (Vigna radiata) seedling is used to study mitotic activity in the root tip. The root tip is fixed in Carnoy's solution, stained with 1% aceto‑orcein, and a squash preparation is made. Under the microscope, 500 cells are examined and the number of cells in each stage of mitosis is recorded in the table below. Stage | Number of cells --- | --- Prophase | 120 Metaphase | 80 Anaphase | 50 Telophase | 30 Interphase | 220
Question Parts
(a)
Outline the step‑by‑step procedure for preparing the root‑tip squash slide, stating the purpose of each major step.
(b)
Calculate the mitotic index of the root‑tip sample. (Mitotic index = (Number of cells in mitosis / Total cells observed) × 100).
(c)
Which phase of mitosis is most frequently observed in this sample? Explain how you determined this from the data.
(d)
The mitotic index obtained is lower than that typically recorded for a rapidly growing shoot tip (15‑20 %). Discuss two biological or experimental reasons that could account for the lower index observed here.
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Question 3 View Details
A student is determining the carbohydrate content of a commercial cereal sample using colour reactions. The procedure is: 1. Accurately weigh 2.00 g of powdered cereal into a beaker. 2. Add 50 mL of distilled water, heat gently and stir until the material is completely dissolved. 3. Filter the solution and collect the clear filtrate. 4. Take 10 mL of the filtrate, add 2 mL of iodine solution and record the colour observed. 5. Take a separate 5 mL aliquot of the filtrate, add 5 mL of Benedict’s solution, heat in a water‑bath for 5 minutes and compare the colour of the resulting solution with the following standard glucose solutions (prepared in 100 mL distilled water): • 0.2 g glucose per 100 mL – colour faint yellow • 0.4 g glucose per 100 mL – colour orange‑red • 0.6 g glucose per 100 mL – colour brick‑red • 0.8 g glucose per 100 mL – colour deep red The student’s sample gives the same colour as the 0.4 g / 100 mL standard. Assume that all the carbohydrate present in the cereal is glucose‑equivalent and that the iodine test is only qualitative. Answer the following questions.
Question Parts
(a)
State the purpose of each of the following reagents in the experiment: (i) Iodine solution (ii) Benedict’s solution
(b)
Calculate the percentage (w/w) of carbohydrate in the cereal sample based on the colour comparison described above.
(c)
The true carbohydrate content of the cereal, determined by a reference method, is 18 % (w/w). Calculate the percentage error in the student's result and give two possible reasons for this discrepancy.
(d)
State two precautions that should be observed during the experiment to improve the reliability of the result.
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Question 4 View Details
A student investigates the effect of temperature on the rate of respiration in germinating mung beans using a closed‑system respirometer (gas syringe). The procedure is: 1. Ten germinated mung beans are placed in a 100 mL airtight container. 2. The container is connected to a gas syringe that records the volume of CO₂ released. 3. The whole assembly is immersed in a water bath set at one of three temperatures: 25 °C, 35 °C or 45 °C. 4. After a 10‑minute interval the volume of gas displaced in the syringe is noted. The recorded volumes of CO₂ are: • 25 °C → 2.3 mL • 35 °C → 3.9 mL • 45 °C → 6.1 mL Assume the gas behaves ideally and that the temperature of the gas equals the bath temperature. Answer the following questions.
Question Parts
(a)
Calculate the rate of respiration (in mL CO₂ min⁻¹) at each temperature.
(b)
Using the rates at 25 °C (R₁) and 45 °C (R₂), calculate the Q₁₀ temperature coefficient for the interval 25 °C → 45 °C. Use the formula Q₁₀ = (R₂/R₁)^{10/(T₂‑T₁)} where temperatures are in °C.
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