igcse model questions vol1 2021 biology | Practical

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Question 1 View Details
A student examines a prepared slide of an onion epidermal cell using a compound microscope. The microscope has an ocular lens of 10× magnification and the 40× objective lens is used. The scale in the eyepiece reticle indicates that 1 mm on the reticle corresponds to 0.025 mm on the specimen at this magnification.
Question Parts
(a)
Calculate the total magnification of the microscope.
(b)
If a cell measures 12 mm on the reticle, determine its actual length in millimetres.
(c)
Draw a labelled diagram of a typical plant cell, indicating at least five structures and state one function for each structure.
(d)
Suggest two possible sources of error that could affect the measurement in part (b) and propose one way to minimise each error.
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Question 2 View Details
A student prepares a fresh root‑tip squash of an onion and observes it under a microscope using a 40× objective and a 10× ocular lens. From a random field containing 120 cells the student records the following numbers of cells in each stage of mitosis:
Question Parts
(a)
Calculate the percentage of cells observed in each stage of mitosis (prophase, metaphase, anaphase, telophase) and the percentage of cells in interphase.
(b)
The observation is repeated on three different fields giving the following percentages of cells in mitosis (i.e. the sum of the four mitotic stages): 45 %, 48 % and 42 %. Calculate the mean percentage of cells in mitosis and the standard deviation (to one decimal place). Use the sample standard deviation formula.
(c)
Based on the results, discuss what the proportion of cells in mitosis indicates about the growth activity of the onion root tip.
(d)
Identify one procedural error that could lead to an over‑estimation of the mitotic index and suggest how to avoid it.
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Question 3 View Details
A student is investigating how temperature influences the activity of the enzyme amylase in breaking down starch. The procedure is as follows: 1. Prepare 50 mL of 1 % starch solution. 2. Add 5 mL of amylase solution to the starch and place the mixture in a water‑bath set at the required temperature. 3. Start a stop‑watch as soon as the enzyme is added and, at 10‑second intervals, add a few drops of iodine solution until the mixture no longer turns blue (indicating starch has been digested). 4. Record the total time taken for the colour to disappear. The experiment is repeated at the temperatures shown in the table below, and the time taken for the starch to be completely digested is recorded. | Temperature (°C) | Time for starch to disappear (s) | |------------------|----------------------------------| | 0 | 120 | | 20 | 80 | | 30 | 45 | | 40 | 30 | | 50 | 35 | | 60 | 55 | | 70 | 110 | Using this data, answer the questions that follow.
Question Parts
(a)
Calculate the rate of the reaction at each temperature, expressing the rate as the reciprocal of time (s⁻¹). Record your answers in a table.
(b)
From the rates obtained, state the optimum temperature for amylase activity.
(c)
Explain why the rate of starch digestion decreases at temperatures above the optimum.
(d)
Identify two possible sources of error in this experiment and suggest one way to minimise each.
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Question 4 View Details
A student investigates how the concentration of glucose affects the rate of respiration in germinating beans. The set‑up is a sealed 250 mL flask containing 5 germinating beans and a known amount of glucose solution. The flask is connected to a gas syringe that measures the volume of CO₂ produced. For each glucose concentration, the volume of CO₂ collected after exactly 30 minutes is recorded. | Glucose concentration (% w/v) | CO₂ collected (mL) after 30 min | |-------------------------------|-----------------------------------| | 0 | 5 | | 2 | 12 | | 4 | 20 | | 6 | 27 | | 8 | 33 | Using this data, answer the following questions.
Question Parts
(a)
Calculate the average rate of CO₂ production (mL min⁻¹) for each glucose concentration.
(b)
Describe the relationship between glucose concentration and the rate of respiration based on the calculated rates.
(c)
Calculate the percentage increase in the respiration rate when the glucose concentration is increased from 2 % to 6 %.
(d)
Give one physiological reason why the respiration rate does not continue to increase proportionally at glucose concentrations above 6 %.
(e)
Design a controlled experiment to investigate the effect of temperature on the same respiration system. State the independent variable, the dependent variable and at least two variables that must be kept constant.
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