neco model questions vol1 2025 biology | Practical

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Question 1 View Details
A prepared slide of an onion (Allium cepa) epidermal peel is examined under a light microscope using a 40× objective and a 10× ocular lens. The ocular micrometer has been calibrated so that one division equals 0.01 mm. The slide contains several rectangular cells with clearly visible cell walls, nucleus and a large central vacuole. Answer the following:
Question Parts
(a)
Identify and label the following structures on the diagram of the observed cell: cell wall, cell membrane, nucleus, nucleolus, vacuole and cytoplasm.
(b)
Measure the longest diameter of five different cells using the ocular micrometer. The recorded readings (in divisions) are: 12, 13, 11, 14, 12. (i) Convert each reading to millimetres. (ii) Calculate the mean cell diameter. (iii) Assuming each cell is approximately spherical, compute the average surface area (SA) and volume (V) of a cell. Use SA = 4πr² and V = 4/3 πr³, where r is the radius in millimetres. Take π = 3.14. (iv) Determine the average surface‑area‑to‑volume ratio (SA:V) and comment on its biological significance for diffusion.
(c)
Explain why a high surface‑area‑to‑volume ratio is advantageous for plant epidermal cells that are involved in gas exchange.
(d)
Identify two possible sources of error in the measurement of cell diameter and suggest a method to minimise each error.
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Question 2 View Details
A fresh root tip of an onion (Allium cepa) is used to prepare a squash slide for observing mitosis. The root tip is 1 cm long, the meristematic zone is identified, and the tissue is fixed, stained with aceto‑orcein and examined under a 40× objective with a 10× ocular. Answer the following:
Question Parts
(a)
Outline the step‑by‑step procedure for preparing a good quality onion root tip squash slide suitable for counting mitotic figures.
(b)
From a random sample of 200 cells observed on the slide, the following distribution was recorded: Interphase – 120 cells Prophase – 30 cells Metaphase – 25 cells Anaphase – 15 cells Telophase – 10 cells (i) Calculate the mitotic index (MI) as a percentage. (ii) Determine the percentage of cells in each of the five phases.
(c)
Explain the biological significance of the mitotic index in assessing the growth activity of a tissue.
(d)
Identify two factors that could lead to an under‑estimation of the mitotic index and suggest how to avoid each factor.
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Question 3 View Details
A student is required to determine the percentage of carbohydrate (starch) in a 2.00 g sample of a commercial biscuit using the iodine colour‑intensity method. The procedure is as follows: 1. Weigh 2.00 g of biscuit and grind to a fine powder. 2. Add 100 mL of distilled water, stir for 5 min and filter to obtain a clear extract. 3. Prepare four standard starch solutions of known concentration: 0.5 % (w/v), 1.0 % (w/v), 1.5 % (w/v) and 2.0 % (w/v). Add a few drops of iodine solution to each and record the colour intensity on a scale of 1 (very pale) to 5 (deep blue). 4. Add iodine to the biscuit extract and record its colour intensity. The colour‑intensity observations are shown in the table below. | Solution | Starch concentration (% w/v) | Colour intensity (1–5) | |---------------------|------------------------------|------------------------| | Standard 1 | 0.5 | 2 | | Standard 2 | 1.0 | 3 | | Standard 3 | 1.5 | 4 | | Standard 4 | 2.0 | 5 | | Biscuit extract | – | 3.6 | Using the data above, answer the following:
Question Parts
(a)
Calculate the percentage (w/w) of carbohydrate (starch) present in the biscuit sample.
(b)
State two possible sources of error in this experiment and suggest a method to minimise each error.
(c)
Explain why iodine gives a characteristic colour change with starch but not with simple sugars.
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Question 4 View Details
A student investigates the effect of temperature on the rate of respiration in germinating wheat grains. In each trial 5.00 g of wheat grains are placed in a sealed 250 mL container containing a small amount of water. The container is connected to a graduated gas syringe that records the volume of CO₂ evolved. The experiment is run for 30 minutes at four different temperatures. The data obtained are shown below. | Temperature (°C) | CO₂ volume after 30 min (mL) | |------------------|------------------------------| | 15 | 12.0 | | 25 | 18.5 | | 35 | 24.0 | | 45 | 20.0 | Answer the following questions:
Question Parts
(a)
Calculate the rate of respiration expressed as mL of CO₂ produced per gram of wheat per minute for each temperature.
(b)
From the rates obtained, state the temperature at which the respiration rate is maximum and give a brief explanation for the observed trend with increasing temperature.
(c)
Suggest one practical method to improve the accuracy of the CO₂ volume measurements in this experiment.
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