igcse model questions vol1 2023 biology | Practical

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Question 1 View Details
You are provided with a clean microscope, an ocular micrometer (scale = 0.01 mm per division) and a fresh cheek cell smear. Using the microscope you are to determine the average diameter of the cheek cells.
Question Parts
(a)
Describe the complete procedure for preparing a fresh cheek cell smear that is suitable for measuring cell diameter with an ocular micrometer.
(b)
You measured the diameters of 25 cells and recorded the number of micrometer divisions that spanned each cell as follows: 5, 4, 5, 6, 5, 5, 4, 5, 5, 6, 5, 4, 5, 5, 5, 4, 6, 5, 5, 5, 4, 5, 5, 6, 5. Calculate the average cell diameter in micrometres (µm). Show all steps of your calculation.
(c)
The textbook value for the average diameter of human cheek cells is 50 µm. (i) Calculate the percentage error of your experimental result. (ii) Suggest two possible sources of error that could have affected your measurement.
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Question 2 View Details
You are to investigate cell division in the root tip of an onion (Allium cepa) using a light microscope.
Question Parts
(a)
Outline the complete procedure for preparing a good onion root‑tip slide that will allow observation of the different stages of mitosis.
(b)
From five fields of view you obtained the following counts: | Field | Total cells | Prophase | Metaphase | Anaphase | Telophase | |-------|-------------|----------|-----------|----------|-----------| | 1 | 120 | 10 | 8 | 4 | 2 | | 2 | 110 | 9 | 7 | 3 | 1 | | 3 | 130 | 12 | 9 | 5 | 2 | | 4 | 115 | 8 | 6 | 4 | 1 | | 5 | 125 | 11 | 8 | 5 | 2 | Calculate: (i) The mitotic index (percentage of cells in any stage of mitosis). (ii) The percentage of cells in each individual stage (prophase, metaphase, anaphase, telophase). Show all working.
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Question 3 View Details
A student is investigating the effect of heat on the vitamin C content of orange juice using an iodine titration. Two 2.00 g samples of fresh orange juice are prepared. Sample A is titrated immediately (raw) and sample B is heated in a water bath at 80 °C for 5 minutes before titration. The iodine solution used is 0.0100 M. The volumes of iodine solution required to reach the end‑point are: - Sample A (raw): 12.5 mL - Sample B (cooked): 8.0 mL Vitamin C (ascorbic acid) reacts with iodine on a 1:1 mole basis. Molar mass of vitamin C = 176.12 g mol⁻¹. Answer the following:
Question Parts
(a)
Calculate the amount of vitamin C (in mg) present in the raw 2.00 g sample.
(b)
Calculate the amount of vitamin C (in mg) present in the cooked 2.00 g sample.
(c)
Determine the percentage loss of vitamin C caused by heating.
(d)
Suggest one likely source of error in this experiment and how it could be minimised.
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Question 4 View Details
A group of students are measuring the rate of respiration in germinating beans using a gas‑pressure syringe. Five freshly soaked beans are placed in a sealed syringe at three different temperatures. The volume of gas (CO₂) collected over each 30‑minute period is recorded as follows: | Temperature (°C) | Volume of CO₂ (mL) per 30 min | |------------------|--------------------------------| | 20 | 0.45 | | 30 | 0.78 | | 40 | 0.30 | Assume the syringe is calibrated and the pressure remains constant throughout the experiment. Answer the following:
Question Parts
(a)
Calculate the rate of respiration (in mL CO₂ h⁻¹) at each temperature.
(b)
Using the rates at 20 °C and 30 °C, calculate the temperature coefficient Q₁₀ for respiration.
(c)
Explain why the rate of respiration falls at 40 °C compared with 30 °C.
(d)
Suggest one modification to the experimental set‑up that would improve the reliability of the respiration rates measured.
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