neco 2019 biology | Practical

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Question 1 View Details
You are given a prepared slide of an onion (Allium cepa) epidermal cell that has been stained with iodine. Using a light microscope, observe the cell and answer the following tasks.
Question Parts
(a)
Draw a clear, labelled diagram of the observed cell. Include at least five distinct cellular structures.
(b)
Identify each labelled structure and state its primary function in the plant cell.
(c)
Explain why the cell wall appears as a thick outer line while the plasma membrane is not distinctly visible in the preparation.
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Question 2 View Details
A fresh root tip of an onion (Allium cepa) is prepared as a squash slide and stained with acetocarmine. Under the microscope you examine 500 cells and record the number of cells in each stage of mitosis as follows: Prophase – 120 cells, Metaphase – 80 cells, Anaphase – 60 cells, Telophase – 40 cells, Interphase – 200 cells.
Question Parts
(a)
Calculate the mitotic index of the sample.
(b)
Determine the percentage of cells in each of the four mitotic phases (Prophase, Metaphase, Anaphase, Telophase).
(c)
From the percentages obtained, discuss which phase of mitosis appears to occupy the longest duration in the cell cycle and give a brief reason for this observation.
(d)
Suggest two possible sources of error in this experiment and explain how each could affect the calculated mitotic index.
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Question 3 View Details
A student carried out a titration to determine the vitamin C (ascorbic acid) content of freshly squeezed orange juice using 2,6‑dichlorophenolindophenol (DCPIP) solution (0.001 M). The procedure was as follows: 1. 50 mL of orange juice was placed in a conical flask. 2. A few drops of starch indicator were added. 3. The juice was titrated against the DCPIP solution until the blue‑black colour disappeared. 4. A blank titration was performed with 50 mL of distilled water under identical conditions. The volumes of DCPIP used are recorded in the table below. | Trial | Volume of DCPIP (mL) | |-------|----------------------| | 1 | 12.3 | | 2 | 12.6 | | 3 | 12.1 | The blank titration gave an average volume of 0.2 mL. The molar mass of vitamin C is 176.12 g mol⁻¹. Answer the following:
Question Parts
(a)
Calculate the average volume of DCPIP required for the orange juice sample after correcting for the blank.
(b)
Using the corrected volume, determine the mass of vitamin C present in the 50 mL orange juice sample (express your answer in milligrams, correct to two decimal places).
(c)
Express the vitamin C concentration as milligrams per 100 mL of juice.
(d)
State the chemical principle on which this titration is based and mention two possible sources of error in the experiment, suggesting one improvement for each.
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Question 4 View Details
A student investigated the rate of respiration in germinating beans using a closed respirometer (BOD bottle). The set‑up was as follows: - Five freshly germinated beans (total fresh mass = 1.20 g) were placed in the bottle. - The bottle was filled with water and a water column of 0 cm was marked on the graduated tube attached to the bottle. - The cross‑sectional area of the tube is 3.14 cm² (radius = 1 cm). - The bottle was kept at a constant temperature of 25 °C for 30 minutes. - After 30 minutes the water level had risen to 2.5 cm. A control experiment (no beans) under identical conditions gave a rise of 0.2 cm. The student repeated the experiment at 35 °C and observed a water‑column rise of 3.4 cm (control at 35 °C gave 0.2 cm). Answer the following questions:
Question Parts
(a)
Calculate the volume of CO₂ produced by the beans at 25 °C (in millilitres), correcting for the control.
(b)
Determine the respiration rate expressed as millilitres of CO₂ per gram of bean per minute at 25 °C.
(c)
Repeat the calculation of the respiration rate for the experiment conducted at 35 °C.
(d)
Explain why the respiration rate increased with temperature, citing the underlying biochemical principle.
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