waec model questions vol1 2020 biology | Practical

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Question 1 View Details
A prepared slide of human cheek epithelial cells is examined under a compound microscope. The ocular micrometer has 100 divisions and the eyepiece magnification is 10×. The objective used is 40×. The ocular micrometer has been calibrated such that one division corresponds to 1 µm at the total magnification used. The distance between two opposite points of a typical cell measured on the ocular micrometer is 45 divisions.
Question Parts
(a)
Calculate the actual diameter of the cell in micrometres (µm).
(b)
Draw a labelled diagram of the cell as seen under the microscope. Include the cell membrane, cytoplasm, nucleus and indicate the measured diameter with an arrow.
(c)
State two precautions that should be observed to avoid error when measuring cell size with an ocular micrometer.
(d)
Identify two organelles that can be reliably observed in this preparation and briefly state one function of each.
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Question 2 View Details
A sample of rapidly growing onion (Allium cepa) root tip is prepared for microscopic observation. The slide is examined using a 40× objective and a 10× eyepiece (total magnification 400×). In five randomly selected fields of view a total of 200 cells are counted. The numbers of cells observed in the four phases of mitosis are: Prophase – 30 cells, Metaphase – 20 cells, Anaphase – 10 cells, Telophase – 5 cells. The remaining cells are in interphase.
Question Parts
(a)
Calculate the mitotic index, expressed as a percentage of cells in mitosis.
(b)
Determine the percentage of cells in each individual phase of mitosis (relative to the total number of mitotic cells).
(c)
From the percentages obtained, which phase of mitosis appears to occupy the longest period of time? Give a brief explanation.
(d)
Suggest one possible source of error that could affect the accuracy of the mitotic index and propose a method to minimise this error.
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Question 3 View Details
You are required to determine the carbohydrate content of an unknown fruit sample using the Benedict’s test. The procedure you followed was: 1. Weighed 5.00 g of the fruit and macerated it in a mortar. 2. Added 50 mL of distilled water and boiled the mixture for 5 minutes to extract soluble sugars. 3. Cooled the extract and filtered it into a 100 mL volumetric flask and made up to the mark with distilled water. 4. To 10.0 mL aliquots of the extract, added Benedict’s reagent and heated in a boiling water bath for 5 minutes. 5. The colour of the precipitate was compared with a standard colour chart. Standard glucose solutions (prepared in 100 mL water) gave the following volumes of Benedict’s reagent that produced the same brick‑red colour as the unknown: | Standard glucose (g) | Volume of Benedict’s reagent (mL) | |----------------------|-----------------------------------| | 0.5 | 2.0 | | 1.0 | 4.5 | | 1.5 | 7.0 | For the fruit extract, 5.20 mL of Benedict’s reagent produced the same colour. Using the data above, answer the following:
Question Parts
(a)
Calculate the percentage (w/w) of carbohydrate in the fruit sample.
(b)
State two possible sources of error that could affect the accuracy of your result.
(c)
Suggest one practical improvement to increase the reliability of the carbohydrate determination.
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Question 4 View Details
A closed‑system respirometer was used to study the effect of temperature on the rate of respiration in germinating mung‑bean seeds. Two identical chambers each contained five healthy seeds. Chamber A (control) was kept at 25 °C, while Chamber B (experimental) was maintained at 35 °C. The volume of CO₂ produced was recorded every 10 minutes for 60 minutes. The data obtained are shown below. | Time (min) | CO₂ volume in Chamber A (mL) | CO₂ volume in Chamber B (mL) | |------------|------------------------------|------------------------------| | 0 | 0.0 | 0.0 | | 10 | 2.1 | 3.4 | | 20 | 4.3 | 6.9 | | 30 | 6.5 | 10.3 | | 40 | 8.6 | 13.8 | | 50 |10.8 | 17.2 | | 60 |13.0 | 20.7 | The fresh weight of the five seeds in each chamber was 2.5 g. Answer the following questions:
Question Parts
(a)
Calculate the average rate of CO₂ production (in mL min⁻¹) for each temperature over the whole 60‑minute period.
(b)
Using the rates obtained in part (a), calculate the Q₁₀ temperature coefficient for respiration between 25 °C and 35 °C. Use the formula Q₁₀ = (R₂ / R₁)^{10 / (T₂‑T₁)}.
(c)
Discuss the effect of the temperature increase on the rate of respiration and mention one limitation of the experimental set‑up.
(d)
Calculate the specific respiration rate (mL CO₂ min⁻¹ g⁻¹ fresh weight) for each temperature using the fresh weight of the seeds.
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