neco 2024 biology | Practical

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Question 1 View Details
You are given a prepared slide of an onion epidermal cell stained with iodine. Using a light microscope, you are to observe and record observations of the cell.
Question Parts
(a)
State the steps you would follow to focus the slide and obtain a clear image of the cell at 400× magnification.
(b)
Identify three cell organelles that can be observed in the slide and describe their appearance.
(c)
Measure the length of a typical epidermal cell using the ocular micrometer (scale 0.01 mm per division). If the cell spans 45 divisions, calculate its length in micrometres. Show your calculation.
(d)
List two possible sources of error in the measurement and suggest a way to minimise each.
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Question 2 View Details
You are to investigate mitosis in the root tip of an onion using a prepared slide.
Question Parts
(a)
Describe the procedure to prepare a fresh onion root tip slide for observing mitosis, including steps to obtain a good spread of cells.
(b)
Using the microscope, you count the number of cells in each phase of mitosis in 5 fields of view. The data obtained are: Prophase 12, Metaphase 8, Anaphase 5, Telophase 5, Interphase 70. Calculate (i) the mitotic index (percentage of cells in mitosis) and (ii) the percentage of cells in each mitotic phase relative to the total number of cells observed. Show all calculations.
(c)
Based on the percentages obtained, interpret which phase is most prolonged in the cell cycle and explain why this observation might occur.
(d)
Identify two factors that could affect the accuracy of the mitotic index obtained and propose how to control each factor.
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Question 3 View Details
A group of students investigated the effect of temperature on the rate of starch digestion by amylase. The experiment was set up as follows: - 10 mL of 1% starch solution was mixed with 5 mL of amylase solution (enzyme concentration kept constant). - The mixture was incubated for 10 minutes at three different temperatures: 20°C, 30°C, and 40°C. - After incubation, 2 mL of the mixture was withdrawn and the iodine test was performed. The intensity of the blue‑black colour was measured using a spectrophotometer (wavelength 620 nm) and recorded as absorbance values. The absorbance recorded for each temperature is shown in the table below. | Temperature (°C) | Absorbance (A) | |------------------|----------------| | 20 | 0.80 | | 30 | 0.45 | | 40 | 0.20 | A blank containing only distilled water gave an absorbance of 0.00. A standard solution of 1% starch gave an absorbance of 1.00, which is taken as 100 % starch concentration. Using the data above, answer the following questions:
Question Parts
(a)
Calculate the percentage of starch hydrolysed after 10 minutes at each temperature.
(b)
Identify the temperature at which amylase shows its optimum activity based on the results and give a brief explanation.
(c)
Suggest two possible sources of experimental error that could affect the accuracy of the percentage hydrolysis values.
(d)
Propose one modification to the procedure that would improve the reliability of the results.
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Question 4 View Details
Students carried out an experiment to compare the rate of respiration in germinating beans kept in light and in darkness. The set‑up was as follows: - 50 mL of dry beans were placed in two identical airtight containers, one kept in a well‑lit laboratory bench (Light) and the other wrapped in aluminium foil (Dark). - Each container was fitted with a gas syringe that measured the volume of CO₂ produced over a period of 30 minutes. - The initial reading of each syringe was zero. After 30 minutes the readings were: • Light container: 12.5 mL of CO₂ • Dark container: 7.8 mL of CO₂ - The temperature in the laboratory was constant at 25°C and atmospheric pressure was 1 atm. Assume the volume of CO₂ measured is at STP (0°C, 1 atm) for calculation purposes. Answer the following questions:
Question Parts
(a)
Calculate the rate of CO₂ production (in mL min⁻¹) for the beans in the Light and Dark conditions.
(b)
Determine the percentage increase in the rate of respiration in the Light condition compared with the Dark condition.
(c)
Provide a scientific explanation for why the rate of respiration is higher in the Light condition.
(d)
Identify two possible sources of error that could affect the measured volumes of CO₂.
(e)
Design a control experiment that would help you confirm that the observed difference is due to light and not another variable.
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