waec model questions vol1 2024 biology | Practical

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Question 1 View Details
You are given a prepared slide of an unknown plant leaf epidermal tissue and a compound microscope equipped with a 10× ocular lens and a 40× objective lens.
Question Parts
(a)
Describe the step‑by‑step procedure for preparing a temporary wet mount of the leaf epidermis for observation under the microscope.
(b)
Under the microscope you measure the average length of five guard cells as 12 µm, the average width as 4 µm and, from the literature, you assume the thickness (height) to be 2 µm. Treating a guard cell as a rectangular prism, calculate its approximate surface area. Show all working.
(c)
Identify two possible sources of error that could affect the accuracy of the guard‑cell measurements and suggest one practical way to minimise each error.
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Question 2 View Details
You are investigating mitosis in the root tip of an onion (Allium cepa). Five separate slides have been prepared, each stained with 1% aceto‑orcein and examined under a microscope with a 40× objective. The numbers of cells observed in each stage are recorded in the table below.
Question Parts
(a)
For each slide, calculate the mitotic index (MI) expressed as a percentage, using the formula MI = (M / T) × 100, where M is the total number of cells in mitosis (prophase + metaphase + anaphase + telophase) and T is the total number of cells counted on the slide.
(b)
Determine the average mitotic index for the five slides.
(c)
Based on the average mitotic index you have obtained, discuss what it reveals about the growth activity of the onion root tip.
(d)
Identify two procedural precautions that must be observed during slide preparation to avoid artefacts that could lead to misidentification of mitotic stages.
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Question 3 View Details
A student is required to determine the percentage carbohydrate in a 2.00 g sample of a commercial biscuit using the Benedict’s test. The student prepares a series of glucose standard solutions (0.2 g, 0.4 g, 0.6 g of glucose dissolved each in 100.0 mL water) and records the absorbance of the resulting coloured solutions at 540 nm after adding Benedict’s reagent and heating for 5 minutes. The absorbance readings obtained are: 0.2 g – 0.150, 0.4 g – 0.310, 0.6 g – 0.470. The biscuit extract is prepared by weighing 2.00 g of biscuit, adding 50.0 mL of distilled water, heating for 10 minutes, filtering and making the final volume up to 100.0 mL. The absorbance of the biscuit solution after the same treatment is 0.385.
Question Parts
(a)
State the purpose of preparing the glucose standard solutions and explain how they are used to determine the carbohydrate content of the biscuit.
(b)
Using the three standard absorbance values, construct the best‑fit straight‑line equation (assume a linear relationship) and then calculate the mass of glucose equivalent present in the biscuit extract.
(c)
From the mass of glucose equivalent obtained in part (b), calculate the percentage carbohydrate (by mass) in the original biscuit sample.
(d)
Identify two possible sources of systematic error in this experiment and suggest a way to minimise each.
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Question 4 View Details
A group of students investigates the effect of temperature on the rate of respiration in germinating mung beans using a closed‑system manometer. Ten healthy mung beans are placed in a 250 mL sealed container connected to a U‑tube manometer filled with water. The initial water column difference is recorded as 0 cm. The container is then placed in a water bath set at a chosen temperature and the change in the water column is noted every minute for 6 minutes. The experiment is repeated at 20 °C, 30 °C and 40 °C. The recorded total rise in the water column after 6 minutes at each temperature is: 20 °C – 2.4 cm, 30 °C – 4.8 cm, 40 °C – 6.0 cm. The density of water is 1.00 g cm⁻³ and 1 L of O₂ at STP occupies 22.4 L.
Question Parts
(a)
Explain why the rise in the water column of the manometer indicates the rate of respiration of the beans.
(b)
Calculate the volume of CO₂ (in mL) produced by the beans at each temperature after the 6‑minute period. (Assume the change in water column directly reflects the change in gas volume.)
(c)
Determine the respiration rate (mL CO₂ min⁻¹) at each temperature.
(d)
Using the respiration rates at 20 °C (R₁) and 30 °C (R₂), calculate the Q₁₀ value for this temperature interval. State what the Q₁₀ value indicates about the temperature sensitivity of respiration in mung beans.
(e)
Suggest two experimental improvements that would increase the reliability of the respiration rate measurements.
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