igcse model questions vol1 2017 biology | Practical

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Question 1 View Details
You are given a prepared slide of a thin transverse section of an onion (Allium cepa) epidermis. Using a light microscope equipped with an ocular micrometer (scale = 0.01 mm per division) and a calibrated stage micrometer (scale = 0.1 mm per division), you are to determine the average size of the cells and evaluate the surface‑area‑to‑volume ratio. The lengths and widths of five representative cells measured on the slide are given below (all measurements in millimetres).
Question Parts
(a)
Calculate the mean length and the mean width of the five cells.
(b)
Assuming a uniform thickness of 0.02 mm for all cells, calculate:
(c)
Determine the average surface‑area‑to‑volume ratio and comment on how this ratio influences the suitability of the cell for diffusion of nutrients.
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Question 2 View Details
You are investigating the rate of cell division in the root tip of a fast‑growing bean (Phaseolus vulgaris). A 1 cm segment of the root tip is harvested, fixed, stained with aceto‑orcein and a slide is prepared. In a randomly selected field of view you count 200 cells and record the number of cells observed in each stage of mitosis as shown below. Prophase: 45 cells Metaphase: 30 cells Anaphase: 25 cells Telophase: 20 cells Interphase (non‑dividing): 80 cells
Question Parts
(a)
Calculate the mitotic index (percentage of cells in mitosis) for the observed field.
(b)
Determine the percentage of cells in each phase of mitosis.
(c)
Based on the percentages obtained, identify which phase is likely to be the rate‑limiting step of mitosis and give a brief explanation.
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Question 3 View Details
A student is investigating the effect of temperature on the activity of the enzyme amylase in breaking down starch. Five test tubes each containing 5 mL of 1 % (w/v) starch solution were prepared. To each tube 1 mL of amylase solution (concentration constant) was added and the mixture was incubated for 5 minutes at the temperatures shown in the table below. After incubation 1 mL of iodine solution was added and the colour intensity was recorded using the scale 0 = no colour (starch completely hydrolysed) to 5 = deep blue (starch present). The experiment was repeated three times for each temperature. | Temperature (°C) | Replicate 1 | Replicate 2 | Replicate 3 | |------------------|------------|------------|------------| | 10 | 4 | 3 | 4 | | 25 | 2 | 3 | 2 | | 37 | 0 | 0 | 0 | | 50 | 3 | 4 | 3 | | 70 | 5 | 5 | 4 | Using the data above, answer the following questions:
Question Parts
(a)
Calculate the mean colour intensity for each temperature and present the results in a table.
(b)
Draw a line graph of mean colour intensity (y‑axis) against temperature (x‑axis). From the graph state the optimum temperature for amylase activity.
(c)
Explain why the colour intensity is lowest at the optimum temperature and why it increases at the lower (10 °C) and higher (70 °C) temperatures.
(d)
Identify two possible sources of error in this experiment and suggest a method to minimise each error.
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Question 4 View Details
A group of students measured the rate of respiration of germinating broad‑bean (Vigna unguiculata) seeds using a closed‑system respirometer. The set‑up consisted of a sealed container containing 10 seeds, a water‑filled inverted graduated tube (capacity 100 mL) and a temperature‑controlled water‑bath. The change in water level in the graduated tube was recorded for 5 minutes at two different bath temperatures. The volume displaced (ml) for each trial is shown below. | Temperature (°C) | Trial 1 | Trial 2 | Trial 3 | |------------------|---------|---------|---------| | 20 | 12 | 13 | 11 | | 30 | 22 | 24 | 23 | Assume that the volume change is solely due to O₂ consumption (1 mL of water displaced ≈ 1 mL of O₂). Answer the following questions:
Question Parts
(a)
Calculate the mean rate of respiration (in mL O₂ min⁻¹) at 20 °C and at 30 °C.
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