waec model questions vol1 2019 biology | Practical

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Question 1 View Details
You are given a prepared slide of an unknown eukaryotic cell. The ocular micrometer of the microscope reads 25 divisions for the diameter of the nucleus. The ocular micrometer has been calibrated such that 1 ocular division = 0.5 µm when the microscope is set at 40× objective and 10× eyepiece (total magnification 400×).
Question Parts
(a)
Calculate the actual diameter of the nucleus in micrometres (µm).
(b)
Draw a labelled diagram of the cell as seen under the microscope, showing at least the nucleus, cytoplasm, cell membrane and one visible vacuole.
(c)
State the principal function of each structure you have labelled in part (b).
(d)
Identify two possible sources of error that could affect the measurement obtained in part (a) and suggest one way to minimise each error.
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Question 2 View Details
A fresh onion (Allium cepa) root tip was harvested, fixed in Carnoy’s solution, stained with aceto‑orcein and a permanent slide prepared. Under a microscope set at 1000× total magnification you examined five fields and recorded the following numbers of cells in each stage of mitosis: Prophase – 120 cells, Metaphase – 80 cells, Anaphase – 60 cells, Telophase – 40 cells, Interphase – 200 cells. A total of 500 cells were counted.
Question Parts
(a)
Calculate the mitotic index (percentage of cells undergoing mitosis).
(b)
Determine the percentage of cells in each individual stage of mitosis (prophase, metaphase, anaphase, telophase).
(c)
Which mitotic stage shows the highest proportion of cells and what does this indicate about the relative duration of that stage in onion root‑tip cells?
(d)
Suggest two practical measures that could improve the accuracy of the mitotic index you have calculated.
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Question 3 View Details
A group of students carried out a qualitative and quantitative analysis of starch in three food samples (Sample X, Sample Y and Sample Z) using the iodine test. The procedure involved extracting the soluble starch from 2.00 g of each sample, reacting the extract with iodine solution, and comparing the colour intensity with a standard colour chart. The colour chart relates intensity rating (0‑5) to starch concentration in the extract as follows: 0 = 0 mg, 1 = 40 mg, 2 = 80 mg, 3 = 120 mg, 4 = 160 mg, 5 = 200 mg. The intensity ratings obtained were: Sample X – 3, Sample Y – 5, Sample Z – 2.
Question Parts
(a)
Calculate the percentage (w/w) of starch in each sample.
(b)
Rank the samples from highest to lowest starch content.
(c)
Identify two possible sources of error that could affect the accuracy of the percentage values obtained.
(d)
Suggest one modification to the procedure that could reduce each of the errors identified in part (c).
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Question 4 View Details
Students investigated the effect of temperature on the rate of respiration in germinating mung bean seeds using a closed respirometer. Five seeds were placed in a sealed 250 ml flask containing a drop of water. The flask was connected via a graduated tube to a water‑filled inverted burette to collect the volume of CO₂ produced (shown as a decrease in water level). The water bath temperature was maintained at either 20 °C (control), 30 °C or 40 °C. The volume of gas collected was recorded at 0, 5, 10 and 15 min for each temperature as shown in the table. | Temperature (°C) | 0 min (ml) | 5 min (ml) | 10 min (ml) | 15 min (ml) | |------------------|------------|------------|-------------|-------------| | 20 | 0 | 4.2 | 8.1 | 12.0 | | 30 | 0 | 6.5 | 13.0 | 19.5 | | 40 | 0 | 9.0 | 18.5 | 27.5 | Assume the volume change is directly proportional to the amount of CO₂ produced.
Question Parts
(a)
For each temperature, calculate the average rate of respiration (ml min⁻¹) over the 15 min period.
(b)
Using the rates obtained, calculate the Q₁₀ temperature coefficient for the interval 20 °C → 30 °C and for 30 °C → 40 °C. (Q₁₀ = (R₂/R₁)^(10/(T₂‑T₁)))
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