ijmb model questions vol1 2020 biology | Practical

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Question 1 View Details
A fresh onion (Allium cepa) bulb is used to prepare a temporary squash of an epidermal peel for microscopic observation. The slide is examined under a low‑power objective (40×) and a high‑power objective (100×). The following measurements of five intact parenchyma cells are recorded using an ocular micrometer (1 mm = 100 divisions). The length of each cell (in divisions) is given in the table below. | Cell | Length (divisions) | |------|--------------------| | 1 | 48 | | 2 | 52 | | 3 | 47 | | 4 | 51 | | 5 | 49 | Assume that the width of a cell is half its length and the thickness is one‑fifth of its length. Answer the following questions.
Question Parts
(a)
Identify three cellular structures that can be clearly seen in the preparation and state one function for each.
(b)
Calculate the mean length of the five cells in micrometres. (1 division = 10 µm).
(c)
Using the mean length obtained in (b), estimate the average surface‑area‑to‑volume ratio (SA:V) of the cells. Treat each cell as a rectangular prism where width = 0.5 × length and thickness = 0.2 × length. Express the ratio as a simplified numerical value (units cancel).
(d)
State two possible sources of error in the measurements and suggest one way to minimise each.
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Question 2 View Details
A root tip of a young onion plant is harvested, fixed in Carnoy's solution and prepared as a squash slide for observation of mitosis under a microscope (100× oil immersion). While scanning the slide, the following counts are recorded from a total of 500 cells examined: - Prophase: 120 cells - Metaphase: 80 cells - Anaphase: 50 cells - Telophase: 30 cells - Interphase: 220 cells Answer the questions that follow.
Question Parts
(a)
Calculate the mitotic index (MI) of the root tip sample. Express your answer as a percentage to one decimal place.
(b)
Determine the percentage of cells in each of the five phases (prophase, metaphase, anaphase, telophase, interphase). Present the results in a table format, rounding to one decimal place.
(c)
Interpret the mitotic index obtained in (a). What does a MI of 56 % suggest about the growth activity of the root tip tissue?
(d)
List two precautions that should be observed during slide preparation to avoid artefacts that could affect the mitotic count.
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Question 3 View Details
A group of students carried out a titration to determine the ascorbic acid (vitamin C) content of a freshly squeezed orange juice sample using 2,6‑dichlorophenol‑indophenol (DCPIP) dye. The procedure was as follows: 1. 10.0 mL of the orange juice was mixed with 10 mL of 0.1 M oxalic acid solution (to maintain acidity). 2. The mixture was diluted to a final volume of 50 mL with distilled water. 3. 2.00 mL of the diluted solution was transferred to a conical flask and 2 drops of starch indicator were added. 4. The solution was titrated with 0.010 M DCPIP until the blue‑black colour of the starch‑iodine complex disappeared, indicating the endpoint. 5. The volume of DCPIP required to reach the endpoint was recorded as 12.5 mL. Using the data above, answer the following questions: a. Calculate the concentration of ascorbic acid in the original orange juice (expressed in mg L⁻¹). (Molar mass of ascorbic acid = 176.12 g mol⁻¹) b. The Recommended Dietary Allowance (RDA) for vitamin C for an adult male is 90 mg per day. Determine the percentage of the RDA supplied by 250 mL of this orange juice. c. Discuss two possible sources of systematic error in this titration and suggest one practical way to minimise each error.
Question Parts
(a)
Calculate the concentration of ascorbic acid in the original orange juice (expressed in mg L⁻¹). (Molar mass of ascorbic acid = 176.12 g mol⁻¹)
(b)
The Recommended Dietary Allowance (RDA) for vitamin C for an adult male is 90 mg per day. Determine the percentage of the RDA supplied by 250 mL of this orange juice.
(c)
Discuss two possible sources of systematic error in this titration and suggest one practical way to minimise each error.
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Question 4 View Details
A class investigated the effect of temperature on the rate of aerobic respiration in yeast (Saccharomyces cerevisiae) by measuring the volume of CO₂ produced in a sealed gas syringe. The experimental set‑up was as follows: - 5.0 g of active dry yeast was mixed with 50 mL of 5 % (w/v) glucose solution. - The mixture was divided equally into three 100 mL gas syringes, each fitted with a rubber stopper. - The syringes were placed in water baths maintained at 20 °C, 30 °C and 40 °C respectively. - After a 10‑minute incubation, the volume of gas displaced in each syringe was recorded as follows: • 20 °C : 12.3 mL • 30 °C : 27.8 mL • 40 °C : 45.6 mL Assume that the gas behaves ideally and that the pressure inside the syringe equals atmospheric pressure (1 atm). The temperature of the gas inside each syringe is the same as the water bath temperature (in Kelvin). The gas constant R = 0.0821 L atm mol⁻¹ K⁻¹. Answer the following: a. Calculate the rate of CO₂ production (in µmol min⁻¹) at each temperature. b. Determine the Q₁₀ temperature coefficient for the interval 20 °C to 30 °C and for 30 °C to 40 °C. (Q₁₀ = (rate₂ / rate₁)^(10/(T₂‑T₁))) c. Discuss which temperature appears optimal for yeast respiration in this experiment and give two physiological reasons why respiration declines at temperatures above the optimum.
Question Parts
(a)
Calculate the rate of CO₂ production (in µmol min⁻¹) at each temperature.
(b)
Determine the Q₁₀ temperature coefficient for the interval 20 °C to 30 °C and for 30 °C to 40 °C. (Q₁₀ = (rate₂ / rate₁)^(10/(T₂‑T₁)))
(c)
Discuss which temperature appears optimal for yeast respiration in this experiment and give two physiological reasons why respiration declines at temperatures above the optimum.
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