neco 2021 biology | Practical

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Question 1 View Details
A fresh sample of onion (Allium cepa) epidermal tissue is to be examined under a light microscope to study the cell structure.
Question Parts
(a)
Describe the complete procedure for preparing a temporary slide of the onion epidermis suitable for observation under the microscope.
(b)
The microscope is fitted with a 10× ocular lens and a 40× objective lens. The scale in the eyepiece reticle indicates that 1 mm on the slide corresponds to 0.025 mm in the actual specimen. If a distance of 2.5 cm is measured on the reticle, calculate the actual length represented in the specimen.
(c)
Identify three distinct cellular structures that can be clearly seen in the prepared slide and state one function for each.
(d)
The student notes that some cells appear to have a relatively thicker cell wall than others. Propose two plausible reasons for this observation, considering the nature of the tissue and the preparation technique.
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Question 2 View Details
A student is investigating the effect of a low concentration of colchicine (0.05%) on the mitotic index of onion (Allium cepa) root tip cells.
Question Parts
(a)
Outline the steps required to prepare a squashed slide of onion root tip for observing mitosis under a light microscope.
(b)
From several fields the student records the following numbers of cells: Prophase = 120, Metaphase = 150, Anaphase = 80, Telophase = 50, total cells observed = 500. Calculate (i) the mitotic index and (ii) the percentage of cells in each of the four phases.
(c)
Based on the calculated percentages, discuss the likely effect of colchicine on the cell cycle, indicating which phase is most affected and why.
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Question 3 View Details
A group of students were asked to determine the starch content of three food samples (Sample A – boiled potato, Sample B – white rice, Sample C – banana) using the iodine test. The procedure was as follows: 1. Prepare a 1% iodine solution. 2. Weigh 2.0 g of each food sample, grind to a fine paste and add 20 mL of distilled water. 3. Add 2 drops of iodine solution to each test tube and observe the colour change. 4. To quantify the starch, a spectrophotometer was used at 620 nm. A standard curve was prepared using starch solutions of known concentration as shown below. Standard Curve Data (Absorbance at 620 nm): Concentration (mg mL⁻¹) : 0  2  4  6 Absorbance : 0.00 0.15 0.30 0.45 Absorbance readings for the unknown samples were recorded as follows: Sample A : 0.38 Sample B : 0.10 Sample C : 0.28 Using the information above, answer the following questions:
Question Parts
(a)
Record the observed colour change for each sample after adding iodine and state whether starch is present.
(b)
Using the standard curve, calculate the starch concentration (mg mL⁻¹) in each sample. Show all steps.
(c)
Answer the following:
() Identify which sample has the highest starch content and justify your answer.
() List two possible sources of error and suggest how to minimise them.
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Question 4 View Details
A practical investigation was carried out to study the effect of temperature on the rate of respiration in germinating beans. The set‑up consisted of a sealed 50 mL gas syringe attached to a container holding 10 germinating beans. The volume of CO₂ produced was recorded at 5‑minute intervals for 20 minutes at three different temperatures. Data recorded: Time (min) | CO₂ volume (mL) at 20 °C | CO₂ volume (mL) at 30 °C | CO₂ volume (mL) at 40 °C -----------|---------------------------|---------------------------|--------------------------- 0 | 0.0 | 0.0 | 0.0 5 | 0.8 | 1.2 | 1.6 10 | 1.6 | 2.4 | 3.2 15 | 2.4 | 3.6 | 4.8 20 | 3.2 | 4.8 | 6.4 Using the data above, answer the following questions:
Question Parts
(a)
Plot CO₂ volume against time for each temperature and determine the rate of respiration (mL CO₂ min⁻¹) from the linear portion of each graph.
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