neco model questions vol1 2023 biology | Practical

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Question 1 View Details
A fresh onion (Allium cepa) leaf was prepared for microscopic observation using a slide‑preparation technique. The slide was examined under a compound microscope with a 10× ocular lens and a 40× objective lens. The ocular micrometer was calibrated as 0.01 mm per division. The student recorded the following measurements of five parenchyma cells (length × width in micrometer divisions): Cell 1: 45 × 30 Cell 2: 48 × 28 Cell 3: 44 × 32 Cell 4: 46 × 31 Cell 5: 47 × 29 The textbook states that the typical area of an onion epidermal cell is 0.14 mm².
Question Parts
(a)
Calculate the average length and the average width of the cells in millimetres (mm).
(b)
Assuming the cells are approximately rectangular, calculate the average cell area in mm² using the averages obtained in part (a).
(c)
Compare your calculated average area with the textbook value (0.14 mm²). State the difference and give two possible reasons for any discrepancy.
(d)
List two precautions that should be observed while preparing the slide to minimise distortion of cell dimensions.
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Question 2 View Details
A sample of root tip from a 5‑day‑old pea (Pisum sativum) plant was collected, fixed in Carnoy's solution, stained with aceto‑orcein, and a slide was prepared. Under a microscope (10× ocular, 40× objective) the student examined 200 cells and recorded the number of cells in each stage of mitosis as follows: Prophase: 30 cells Metaphase: 45 cells Anaphase: 20 cells Telophase: 15 cells Interphase: 90 cells
Question Parts
(a)
Calculate the mitotic index (percentage of cells undergoing mitosis) for the sample.
(b)
Determine the percentage of cells in each mitotic phase (prophase, metaphase, anaphase, telophase) relative to the total number of cells observed.
(c)
Based on the percentages obtained, infer which phase of mitosis is the longest in duration and explain why the distribution of cells in the different phases can be used to estimate relative durations.
(d)
Identify two possible sources of error that could affect the accuracy of the mitotic index obtained.
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Question 3 View Details
A group of students investigated the rate of starch hydrolysis by amylase in three different food samples (boiled potato, raw banana and cooked rice). 5 g of each sample was homogenised in 50 mL distilled water. 10 mL of the homogenate was mixed with 10 mL of a standard amylase solution (0.5 U mL⁻¹) and incubated at 37 °C. At 0, 5, 10 and 15 min aliquots (2 mL) were withdrawn, the reaction was stopped by adding 1 mL of 0.5 M HCl and the remaining starch was quantified by the iodine colour‑reaction. Absorbance (A) of each aliquot was measured at 620 nm. The data obtained are shown in the table below. | Sample | Time (min) | Absorbance (A) | |-----------------|------------|----------------| | Boiled potato | 0 | 0.800 | | | 5 | 0.500 | | | 10 | 0.300 | | | 15 | 0.150 | | Raw banana | 0 | 0.750 | | | 5 | 0.620 | | | 10 | 0.520 | | | 15 | 0.460 | | Cooked rice | 0 | 0.820 | | | 5 | 0.600 | | | 10 | 0.380 | | | 15 | 0.200 | Assume that absorbance is directly proportional to the concentration of starch present and that the absorbance at time 0 represents 100 % starch remaining for each sample.
Question Parts
(a)
Calculate the percentage of starch hydrolysed after 15 min for each food sample.
(b)
Determine the average rate of starch hydrolysis (percentage per minute) over the 0‑15 min interval for each sample.
(c)
Which food sample shows the highest amylase activity and why?
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Question 4 View Details
A practical investigation was carried out to study the effect of temperature on the rate of cellular respiration in baker's yeast. 5 g of dry yeast was suspended in 100 mL of 0.5 % glucose solution. The suspension was divided equally into three conical flasks and placed in water baths set at 20 °C, 30 °C and 40 °C respectively. Each flask was connected to an inverted graduated cylinder (capacity 250 mL) to collect the CO₂ released. The volume of gas collected was recorded every 5 min for a total period of 20 min. The data obtained are shown below. | Time (min) | CO₂ volume (mL) – 20 °C | CO₂ volume (mL) – 30 °C | CO₂ volume (mL) – 40 °C | |------------|------------------------|------------------------|------------------------| | 0 | 0 | 0 | 0 | | 5 | 12 | 28 | 45 | | 10 | 25 | 55 | 85 | | 15 | 35 | 78 | 115 | | 20 | 42 | 95 | 130 | Assume the gas behaves ideally and that the temperature of the gas in the cylinder remains equal to the water‑bath temperature.
Question Parts
(a)
Calculate the average rate of CO₂ evolution (in mL min⁻¹) for each temperature over the 0‑20 min period.
(b)
State the temperature at which the respiration rate is highest and give a brief explanation.
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