igcse model questions vol1 2024 biology | Practical

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Question 1 View Details
A student investigates the size and shape of epidermal cells of an onion (Allium cepa) leaf using a light microscope. The slide is prepared by peeling a thin layer of epidermis, staining with iodine solution and observing at 400× magnification. The student records the length and width of ten cells (in µm) as shown in the table below. | Cell | Length (µm) | Width (µm) | |------|-------------|------------| | 1 | 45 | 20 | | 2 | 48 | 22 | | 3 | 50 | 21 | | 4 | 46 | 19 | | 5 | 49 | 20 | | 6 | 47 | 21 | | 7 | 51 | 22 | | 8 | 44 | 20 | | 9 | 48 | 19 | |10 | 50 | 21 | Using this data, answer the following questions.
Question Parts
(a)
Calculate the mean length and the mean width of the ten cells. Give your answers to one decimal place (µm).
(b)
Assuming each cell is a rectangular prism whose thickness equals the mean width, estimate the average cell volume in µm³.
(c)
Calculate the surface area to volume ratio (SA:V) of the average cell. Express your answer in µm⁻¹ and give it to two decimal places.
(d)
Explain two reasons why a high surface‑area‑to‑volume ratio is advantageous for plant epidermal cells.
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Question 2 View Details
A student studies mitosis in the root tip of an onion (Allium cepa) using the squash technique. The root tip is stained with 1% aceto‑orcein and examined under 1000× magnification. The student records the number of cells in each stage of mitosis and the number of interphase cells in five separate fields of view, as shown in the table. | Field | Prophase | Metaphase | Anaphase | Telophase | Interphase | |-------|----------|-----------|----------|-----------|------------| | 1 | 8 | 6 | 4 | 2 | 30 | | 2 | 7 | 5 | 3 | 1 | 32 | | 3 | 9 | 7 | 5 | 3 | 28 | | 4 | 6 | 4 | 2 | 1 | 35 | | 5 | 8 | 6 | 4 | 2 | 31 | Using this data, answer the following questions.
Question Parts
(a)
Calculate the total number of cells observed and the total number of cells that were in any stage of mitosis.
(b)
Determine the mitotic index, expressed as a percentage, to one decimal place. (Mitotic index = (mitotic cells / total cells) × 100)
(c)
Identify which phase of mitosis was observed most frequently and state what percentage of the total mitotic cells this phase represents (to one decimal place).
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Question 3 View Details
A group of students carried out an experiment to determine the amount of vitamin C in orange juice using the 2,6‑dichlorophenolindophenol (DCPIP) titration method. The DCPIP solution was standardised to a concentration of 0.010 mol L⁻¹. A 10.0 mL sample of freshly squeezed orange juice was titrated, and the average volume of DCPIP required to reach the endpoint was 12.5 mL. A blank titration (juice replaced by distilled water) required 1.0 mL of DCPIP.
Question Parts
(a)
Calculate the mass of vitamin C (ascorbic acid) present in the 10.0 mL juice sample. (Molar mass of ascorbic acid = 176.12 g mol⁻¹; 1 mol DCPIP reacts with 1 mol vitamin C.)
(b)
Express the vitamin C content as milligrams per 100 mL of juice.
(c)
Identify two possible sources of systematic error in this experiment and suggest how each could be minimised.
(d)
Explain why the blank titration is necessary.
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Question 4 View Details
Students used a closed respirometer to investigate the effect of temperature on the rate of respiration in germinating peas. The respirometer consisted of a sealed bottle containing 5.0 g of moist peas and a graduated capillary tube filled with water. As CO₂ was produced, water was displaced in the capillary. The volume of water displaced was recorded over a period of 10 minutes at two constant temperatures. Data recorded: - At 25 °C: water displaced after 10 min = 1.2 mL. - At 35 °C: water displaced after 10 min = 2.0 mL. Assume temperature and pressure remain constant and that 1 mL of water displaced corresponds to 1 mL of CO₂ at STP. Molar volume of a gas at STP = 22.4 L mol⁻¹.
Question Parts
(a)
Calculate the rate of CO₂ production (in µmol min⁻¹) at each temperature.
(b)
Determine the Q₁₀ temperature coefficient for the respiration rate between 25 °C and 35 °C. Use the formula Q₁₀ = (R₂/R₁)^{10/(T₂‑T₁)}.
(c)
Discuss two physiological reasons why the rate of respiration in germinating peas increases with temperature.
(d)
Identify one experimental limitation that could affect the accuracy of the measured respiration rates and suggest a method to reduce its impact.
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