neco 2018 biology | Practical

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Question 1 View Details
You are given a prepared slide of a human cheek epithelial cell stained with methylene blue. Using a compound microscope at 400× magnification you observe the cells. Record your observations and answer the following questions.
Question Parts
(a)
Identify three cellular structures that are clearly visible in the slide and state one function of each.
(b)
The following diameters (in µm) were measured for five cells: 18, 20, 22, 19, 21. (i) Calculate the average cell diameter. (ii) Assuming the cells are spherical, estimate the average cell volume. Use π = 3.14 and show the formula used.
(c)
Briefly discuss how the observed cellular structures support the function of cheek epithelium in the oral cavity.
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Question 2 View Details
A sample of an onion (Allium cepa) root tip was collected, fixed in Carnoy's solution, stained with 2% aceto‑orcein, and a slide was prepared. Under a light microscope at 1000× magnification you examined 200 cells and recorded the number of cells in each stage of mitosis as shown below: Prophase: 40 cells Metaphase: 30 cells Anaphase: 20 cells Telophase: 10 cells Interphase: 100 cells Answer the following questions based on this data.
Question Parts
(a)
Calculate the mitotic index of the root tip sample.
(b)
Determine the percentage of cells observed in each of the five phases (Prophase, Metaphase, Anaphase, Telophase, Interphase).
(c)
Based on the mitotic index obtained, comment on the growth activity of the onion root tip.
(d)
Identify one possible source of error that could affect the accuracy of the mitotic index and suggest a precaution to minimise it.
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Question 3 View Details
A student is investigating the vitamin C (ascorbic acid) content of fresh orange juice using the DCPIP (2,6-dichlorophenol‑indophenol) titration method. The student prepared a 0.0100 M DCPIP solution and recorded the following data:
Question Parts
(a)
Write the balanced redox equation for the reaction between ascorbic acid (C6H8O6) and DCPIP.
(b)
The titration of 10.0 mL of the orange juice required 12.5 mL of the DCPIP solution. A blank titration (10.0 mL of distilled water) required 1.2 mL of DCPIP. Calculate the concentration of vitamin C in the juice (in mol L⁻¹).
(c)
The weight of the orange sample used to obtain the 10.0 mL juice was 85.0 g. Determine the vitamin C content of the fruit as mg per 100 g.
(d)
State two possible sources of error in this experiment and suggest how each could be minimised.
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Question 4 View Details
A group of students set up a simple closed‑system respirometer to measure the rate of aerobic respiration in germinating beans. The respirometer consists of a sealed container connected to a graduated syringe that records the volume of gas displaced. The experiment was performed at 25 °C. The following data were obtained:
Question Parts
(a)
For the control (no inhibitor) the syringe showed a displacement of 4.8 mL of gas in 5 minutes. Calculate the rate of respiration in mL CO₂ min⁻¹.
(b)
When 0.5 mL of 0.1 M potassium cyanide (KCN) was added to a second identical set‑up, the gas displacement after 5 minutes was only 1.2 mL. Calculate the respiration rate for the inhibited sample and the percentage inhibition relative to the control.
(c)
Explain, in biochemical terms, why cyanide reduces the rate of respiration observed in the respirometer.
(d)
State two precautions that should be observed when performing this experiment and two sources of error that could affect the results.
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