neco 2020 biology | Practical

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Question 1 View Details
A student is using a compound microscope to examine a fresh cheek cell preparation. The microscope is equipped with an ocular lens of 10\u00d7 magnification and an objective lens of 40\u00d7 magnification. The ocular micrometer has 100 divisions, each division representing 0.2\,mm on the reticle.
Question Parts
(a)
List three cellular structures that the student is likely to observe in the cheek cell preparation.
(b)
Explain why mitochondria are not visible under the conditions described.
(c)
If the cell nucleus occupies 25 divisions on the ocular micrometer, calculate the actual diameter of the nucleus in micrometres (\u03bcm). Show your working.
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Question 2 View Details
A biology student is investigating the stages of mitosis in an onion (Allium cepa) root tip. Fresh root tips are harvested, fixed in Carnoy’s solution, stained with aceto‑orcein and squashed on a slide. The student examines the slide under a microscope and records the number of cells in each phase of mitosis as shown in Table 1.\n\nTable 1 – Untreated root tip (total cells examined = 500)\nProphase: 120 cells\nMetaphase: 80 cells\nAnaphase: 50 cells\nTelophase: 30 cells\nInterphase: 220 cells\n\nThe same procedure is repeated after the seedlings have been treated with 0.1\% colchicine for 4 hours. The counts obtained are shown in Table 2.\n\nTable 2 – Colchicine‑treated root tip (total cells examined = 400)\nProphase: 100 cells\nMetaphase: 200 cells\nAnaphase: 50 cells\nTelophase: 30 cells\nInterphase: 20 cells
Question Parts
(a)
Calculate the mitotic index (percentage of cells in mitosis) for the untreated sample.
(b)
Determine the percentage of cells in each phase (prophase, metaphase, anaphase, telophase, interphase) for the untreated sample.
(c)
Calculate the mitotic index for the colchicine‑treated sample and discuss the effect of colchicine on cell division based on the indices obtained.
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Question 3 View Details
A group of students carried out an experiment to determine the carbohydrate content of a sample of mango pulp using Benedict's test. They dried 2.00 g of mango pulp, ground it to a fine powder and added 10 mL of distilled water. The mixture was filtered and 5.0 mL of the clear filtrate was mixed with 5.0 mL of Benedict's solution in a test tube. The tube was heated in a boiling water bath for 5 minutes and the colour of the solution was compared with a standard colour chart. The colour chart was prepared as follows: standard glucose solutions containing 0.20 g, 0.40 g, 0.60 g, 0.80 g and 1.00 g of glucose in 10 mL of water gave colour intensities 1, 2, 3, 4 and 5 respectively. The unknown mango filtrate produced a colour intensity of 3.5 on the chart.
Question Parts
(a)
Calculate the percentage (by mass) of carbohydrate present in the original mango pulp sample.
(b)
Identify two possible sources of error in this experiment and suggest a method to minimise each error.
(c)
Briefly explain why Benedict's test is specific for reducing sugars.
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Question 4 View Details
Students investigated the effect of temperature on the rate of respiration in germinating beans using a closed respirometer. The respirometer consisted of a sealed bottle fitted with a cotton plug and a graduated glass tube (inner diameter 1.0 cm) inverted in a water trough. Ten germinating beans were placed in the bottle, the bottle was filled with water, sealed and the fall in water level was recorded after 30 minutes. At 25°C the water fell 2.0 cm; at 35°C the water fell 3.2 cm. Atmospheric pressure was 760 mmHg. The water vapour pressure at 25°C is 23.8 mmHg and at 35°C is 42.2 mmHg. Assume the gas collected behaves ideally and that the volume of water displaced equals the volume of gas consumed.
Question Parts
(a)
Calculate the volume of O₂ consumed at each temperature (in mL), correcting for water vapour pressure.
(b)
Determine the rate of respiration (mL O₂ per minute) at each temperature.
(c)
Explain the observed effect of temperature on the rate of respiration, referring to enzyme activity.
(d)
Identify one limitation of this experimental method and propose an improvement.
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