Form 4 · Worked answers

Introduction to Biology and Laboratory Rules, worked answers

Fully worked answers for Introduction to Biology and Laboratory Rules, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Identifying the three types of variable in a given experiment.
Writing a valid hypothesis for a described investigation.
Stating safety precautions or correcting an unsafe practice.
Describing the trend shown by a table or graph of results in words.
Suggesting one specific improvement to a described experimental procedure.
Matching a piece of apparatus to the quantity it is used to measure.
Explaining why a particular variable must be kept fixed in a given experiment.
Converting a set of raw readings into an appropriately titled data table.
Explaining why fewer variables can be controlled in a field investigation than in a laboratory one.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

A student places five identical potato chips, each cut to the same size, into test tubes containing sucrose solution at concentrations of 0.0, 0.2, 0.4, 0.6 and 0.8 mol dm⁻³ for 30 minutes, then reweighs each chip. (a) State the manipulated variable. (b) State the responding variable. (c) State one variable that should be kept fixed, other than the size of the potato chip.

[3]

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(a) The manipulated variable is the concentration of the sucrose solution. (b) The responding variable is the change in mass of the potato chip (or percentage change in mass). (c) A suitable fixed variable is the immersion time (30 minutes for every tube) or the temperature of the solutions, since either factor could otherwise affect how much water moves in or out of the chip independently of concentration.

manipulated variableresponding variablefixed variableconcentration

Using the same investigation, write a suitable hypothesis relating the concentration of the sucrose solution to the change in mass of the potato chip.

[2]

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As the concentration of the sucrose solution increases, the potato chip loses more mass (or gains less mass), because a more concentrated external solution draws more water out of the chip's cells by osmosis. A hypothesis must state this direction of change, not simply that concentration 'affects' mass change.

hypothesisdirectionosmosis

State the apparatus a student should use to measure exactly 20 cm³ of the sucrose solution for each test tube, and explain why it is more suitable than pouring the solution directly from a beaker marked in 50 cm³ divisions.

[2]

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A measuring cylinder (or a burette, for a smaller volume with finer control) should be used. Its finer graduation marks allow the meniscus to be read to a smaller interval than a beaker's wide divisions, so the volume measured is closer to the true 20 cm³ each time, reducing error between the five test tubes.

measuring cylindergraduationaccuracy

A student's drawing of an onion epidermal cell, made using a light microscope, shows the cell shaded in pencil and its label lines crossing one another. State two corrections needed for this drawing to meet biological-drawing convention, and explain each.

[4]

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First, the shading should be removed and replaced with a clean, unbroken outline, since biological drawings represent structure using outline alone, not tone or texture. Second, the label lines should be redrawn straight and non-crossing, each ending exactly on the structure it names, because crossing label lines make it unclear which line belongs to which structure and cost marks even if the correct structures are identified.

biological drawingshadinglabel linesoutline

A student records the time taken for a starch–iodine colour change to disappear at five different temperatures. Write only the two column headings needed for an appropriately constructed results table (do not include any data values).

[3]

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Temperature / °C (left-hand column, since temperature is the manipulated variable) and Time taken for the colour to disappear / s (right-hand column, the responding variable). The unit is placed once in each heading using a forward slash, so that every value entered beneath it is recorded as a plain number.

column headingunitmanipulated variable

Student A measures the volume of a solution three times using a burette and obtains 24.9, 25.0 and 25.1 cm³. Student B, using a burette with an uncorrected zero error, obtains 23.0, 23.1 and 23.0 cm³ for the same solution. State which student's readings are more precise and which are more accurate, explaining your answer.

[3]

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Student A's readings are both precise and accurate: they are close to one another (within 0.2 cm³) and close to the likely true volume. Student B's readings are precise, since they cluster closely together (within 0.1 cm³), but they are not accurate, because the uncorrected zero error shifts every reading away from the true value by roughly the same amount.

precisionaccuracyzero error

A group of students compares the percentage cover of a weed species at five points along a line transect running from a school pond towards a car park. State one factor that would be difficult to keep fixed in this field investigation, and suggest one way the reliability of the results could still be improved.

[3]

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Soil moisture (or light intensity, or soil pH) would be difficult to keep fixed, since it naturally changes with distance from the pond and cannot be controlled outdoors the way it could be in a laboratory. Reliability can still be improved by placing more than one transect across the same site and averaging the percentage cover recorded at each equivalent distance, rather than relying on a single transect line.

transectfield investigationreliabilityuncontrolled variable

Phrasing that earns marks

Fields and careers in biology: Biology branches into areas such as medicine, genetics, microbiology and ecology, each leading to specific careers.
Laboratory safety: Correct handling of apparatus, chemicals and specimens, plus fire and glassware rules, prevents accidents.
Manipulated, responding and fixed variables: A fair test changes one manipulated variable, measures the responding variable, and keeps all other (fixed) variables constant.
Hypothesis: A testable statement predicting how the responding variable depends on the manipulated variable.
Scientific investigation: The ordered steps of aim, hypothesis, variables, apparatus, procedure, results and conclusion.
Biological drawing: Clear line drawings with a title, magnification and labels using straight, non-crossing label lines.
Communicating in biology: Findings are recorded as labelled diagrams, data tables with units placed in the column heading, and graphs with a sensible scale; each format has its own conventions, and an answer loses marks if a label, a unit or an axis title is missing.
SI units and precision: Standard units such as cm, g, s and °C are used throughout biology so that measurements can be compared and repeated by anyone. Readings within one table are recorded to the same number of decimal places so precision is not overstated.

Frequently asked questions

What is the difference between a manipulated and a fixed variable?

The manipulated variable is the one you deliberately change in an experiment. Fixed variables are all the others you keep constant so the test is fair, so that any change in the responding variable is due only to the manipulated variable.

How do I write a good hypothesis for SPM?

State a clear, testable relationship with direction between the manipulated and responding variables, for example 'as light intensity increases, the rate of photosynthesis increases'. Avoid vague wording like 'affects'.

Do I need to memorise careers in biology?

You should know the main fields and one or two careers each, as short-answer questions sometimes ask you to match a field to a career or a career to what it studies.