neco 2022 biology | Practical

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Question 1 View Details
A student uses a light microscope to examine a prepared slide of cheek epithelial cells. The microscope has an ocular lens of 10× magnification and an objective lens of 40×. The ocular micrometer scale has 100 divisions. To calibrate the micrometer, the student first observes a stage micrometer slide where each division corresponds to 0.01 mm. Under the 40× objective, the student records that 25 divisions of the ocular micrometer span a distance of 0.25 mm on the stage micrometer.
Question Parts
(a)
Determine the value of one division of the ocular micrometer in micrometres (µm) at the 40× objective.
(b)
Using the calibrated ocular micrometer, the student measures the diameter of a typical cheek epithelial cell as 12 ocular divisions. Calculate the actual diameter of the cell in µm.
(c)
If the student switches to a 100× oil immersion objective (keeping the same ocular lens), what will be the new value of one ocular division in µm? (Assume the same calibration factor applies proportionally.)
(d)
Discuss two sources of error that could affect the accuracy of the cell size measurement.
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Question 2 View Details
A biology student investigates the effect of a growth regulator (GA3) on the mitotic activity in onion (Allium cepa) root tip cells. Two sets of root tips are prepared: a control (no treatment) and a treated set soaked in 0.5 mg/L GA3 for 24 h. For each set, a squash preparation is made and 5 fields are examined under a microscope. The following counts are obtained: Control: total cells counted = 500; cells in prophase = 45; metaphase = 30; anaphase = 20; telophase = 15. Treated: total cells counted = 480; cells in prophase = 30; metaphase = 50; anaphase = 25; telophase = 20.
Question Parts
(a)
Calculate the mitotic index (MI) for each set, expressed as a percentage to one decimal place.
(b)
Determine the percentage increase or decrease in MI of the treated set relative to the control.
(c)
Based on the changes in the distribution of cells among the phases, infer which phase is most affected by GA3 and provide a brief explanation.
(d)
Suggest one precaution the student should observe when preparing the squash to avoid artefacts that could affect the counts.
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Question 3 View Details
A group of students carried out an investigation to determine the carbohydrate content of three food samples (A: white rice, B: banana, C: boiled potatoes) using the iodine colour test and a spectrophotometer. They prepared standard glucose solutions of known concentrations (0, 2, 4, 6 and 8 g per 100 mL) and recorded the absorbance at 620 nm as follows: 0.00, 0.12, 0.24, 0.36 and 0.48 respectively. The extracts of the food samples gave absorbance readings of 0.30 (A), 0.18 (B) and 0.42 (C). For each extract, 10 mL of the solution was placed in the cuvette for measurement. The extracts were prepared by weighing 10 g of each food, adding 100 mL of distilled water, heating, filtering and making up to 100 mL.
Question Parts
(a)
Construct a calibration curve using the standard glucose data and obtain the linear equation relating absorbance (A) to concentration (C) in g per 100 mL. (i) Determine the slope and intercept of the best‑fit line.
(b)
Using the equation from part (a), calculate the concentration of carbohydrate in each food extract (in g per 100 mL). Then determine the amount of carbohydrate (in grams) present in 100 g of each food sample, assuming the extraction procedure described above. (i) Show the calculation for sample A. (ii) Give the final values for samples B and C.
(c)
Identify two possible sources of error in this method and suggest one way to minimise each error.
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Question 4 View Details
Students investigated the effect of temperature on the rate of respiration in germinating peas using a closed respirometer. Five grams of germinated peas were placed in a sealed 250 mL respirometer containing 5 mL of 0.1 M KOH solution to absorb CO₂. The respirometer was immersed in water baths at 15 °C, 25 °C and 35 °C. The volume of water displaced (which equals the volume of O₂ consumed) was recorded every 5 minutes for a total of 30 minutes. The recorded displacements are:\n\n15 °C: 0, 0.8, 1.5, 2.2, 3.0, 3.7, 4.5 mL\n25 °C: 0, 1.2, 2.5, 3.9, 5.4, 6.9, 8.5 mL\n35 °C: 0, 1.8, 3.7, 5.6, 7.8, 10.0, 12.5 mL\n\n(Measurements are taken at 0, 5, 10, 15, 20, 25 and 30 min).
Question Parts
(a)
Using the data for the first 20 minutes (0–20 min), determine the average rate of respiration (ml O₂ min⁻¹) at each temperature. (i) Show the calculation for 25 °C.
(b)
Calculate the Q₁₀ temperature coefficient for the interval 15 °C to 35 °C using the formula Q₁₀ = (R₂/R₁)^{10/(T₂‑T₁)} where R₁ and R₂ are the average rates at the lower and higher temperatures respectively.
(c)
Discuss two physiological reasons why the rate of respiration increases with temperature up to an optimum, and two reasons why it may decline when temperature exceeds the optimum.
(d)
Suggest one modification to the experimental setup that would improve the accuracy of the measured respiration rates.
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