SPM Biology Paper 3 practical technique
Paper 3 marks come from five skills done well on paper: a clear hypothesis, correctly identified variables, a proper table, a well-drawn graph, and a conclusion that uses the actual data.
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The five skills that earn Paper 3 marks
| Skill | What earns the mark |
|---|---|
| Hypothesis | A directional statement linking the manipulated and responding variables |
| Variables | The manipulated, responding and fixed variables correctly named |
| Table | Headings with units, and consistent, sensible values |
| Graph | Labelled axes, a sensible scale, and an accurate plot |
| Conclusion | A statement that uses the data and says whether the hypothesis is supported |
Identifying the three variables
- Manipulated variable: the one factor you deliberately change between trials.
- Responding variable: the one factor you measure, which changes because of the manipulated variable.
- Fixed (controlled) variables: everything else that could affect the result and must be kept the same.
- Name each variable specifically, "temperature of the water bath" scores where "temperature" alone may not.
Writing a hypothesis and building the table
A strong hypothesis names both variables and states the direction of the relationship, for example "as the concentration of the enzyme increases, the rate of reaction increases" rather than a vague "concentration affects the rate". It should read as something the experiment can actually test and either support or reject.
A results table needs a heading with the variable and its unit at the top of each column, not repeated in every cell, with values recorded to a consistent number of decimal places or significant figures across a trial.
Plotting the graph
- Put the manipulated variable on the x-axis and the responding variable on the y-axis, each labelled with its unit.
- Choose a scale that uses most of the grid, with even, easy-to-read intervals.
- Plot each point accurately and give the graph a clear title stating what it shows.
- Draw a single best-fit line or curve through the points where the relationship is continuous, rather than joining the points dot to dot.
Conclusion and evaluation
Using the planning and answering time
The practical gives you 5 minutes planning to plan before 40 minutes answering of writing. Use the planning time to read the question twice, underline the manipulated and responding variables, and decide the range of values you will use, so that when writing begins you are filling in a plan rather than inventing one.
During the answering time, work in the order the marks fall: variables and hypothesis first because they are quick and certain, then the table, then the graph, then the conclusion and evaluation. Leaving the graph until the variables and table are correct means the axes and scale follow directly from work you have already checked.
Describing results and reading the graph
When a question asks you to describe what a graph or table shows, state the overall trend first, then quote figures from the data to support it. A description such as 'as the temperature rises from 20 to 40 °C the rate increases, reaching a peak at 40 °C, then falls' scores where a bare 'the rate changes' does not.
If the relationship is directly proportional, say so; if it rises to a peak and falls, name the peak as the optimum. To read a value between two plotted points, follow the line up from the x-axis to the curve and across to the y-axis, and state the value you read with its unit.
Worked Paper 3-style question
Scenario. A student investigated how temperature affects the rate at which yeast releases carbon dioxide during respiration. Equal volumes of yeast suspension and glucose solution were mixed in tubes kept at 20, 30, 40, 50 and 60 °C, and the number of gas bubbles released in one minute was counted at each temperature.
(a) Hypothesis. As the temperature increases from 20 °C towards about 40 °C, the rate of carbon dioxide release increases; above the optimum temperature, the rate decreases. (b) Variables. Manipulated: temperature of the water bath; responding: number of gas bubbles released per minute; fixed: volume and concentration of glucose, volume and concentration of yeast suspension, and the counting time.
(c) Tabulation. Draw a table with a first column headed 'Temperature / °C' and a second column headed 'Number of gas bubbles per minute'; record the temperatures 20 to 60 in the first column and the matching counts in the second, keeping each count to a whole number. (d) Inference. The rate is highest at the optimum temperature and lower on either side; above the optimum the enzymes in the yeast denature, their active sites change shape, so fewer substrate molecules are broken down and less carbon dioxide is released.
Technique errors that quietly lose marks
Source:SRC-FORMAT
Frequently asked questions
What is the difference between the manipulated and responding variable?
How do I write a good hypothesis for Paper 3?
What makes a conclusion score well?
How should I use the planning time in Paper 3?
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