CCEA GCE Biology (1010) Β· Unit A2 3: Practical Skills in Biology
Mini-Lesson Β· A-level
Practical Skills in Biology
CCEA Unit A2 3 assesses your practical skills across the A2 course. This mini-lesson works through the A2 techniques β the respirometer and RQ, redox indicators (methylene blue and DCPIP), pigment chromatography, and ecological sampling β and the statistics and evaluation that the written paper actually rewards.
Work through each screen, answer the questions as you go β several are A-level calculations β and collect β stars. Press Start when you are ready.
Respirometer
Using a respirometer
A respirometer measures the gas exchange of a small organism (germinating seeds, maggots, woodlice) sealed in a tube connected to a manometer.
With soda lime: the COβ produced is absorbed, so the only change in gas volume is caused by oxygen consumption. The manometer fluid moves towards the organism, and the distance moved gives the volume of Oβ taken up.
Without soda lime: the change reflects the difference between Oβ consumed and COβ produced β and combining the two runs lets you calculate the RQ.
Control tube: an identical tube containing glass beads of the same volume. It has no respiring organism, so any movement in it must be caused by changes in temperature or atmospheric pressure β and your reading is corrected accordingly.
Keep the whole apparatus in a water bath at constant temperature: the gas laws mean even a small temperature change moves the manometer fluid far more than respiration does.
1A respirometer shows that an organism consumes 5.0 cmΒ³ of oxygen and produces 3.5 cmΒ³ of carbon dioxide in the same period. Calculate its RQ.
RQ
Hint: RQ = COβ Γ· Oβ = 3.5 Γ· 5.0.
Quick check
Identify the substrate
?An organism has an RQ of 0.7. Which respiratory substrate is it mainly using?
Redox indicators
Methylene blue and DCPIP
Both are artificial hydrogen (electron) acceptors β they take the place of the natural coenzymes, and they change colour when reduced. That makes an invisible reaction visible and timeable.
Methylene blue (respiration) β blue when oxidised, colourless when reduced. Add it to a yeast suspension: dehydrogenase enzymes strip hydrogen from the respiratory substrate and pass it to the dye, so the faster respiration is going, the faster the blue colour disappears. Layer oil on top or use a stoppered tube, because oxygen re-oxidises the dye back to blue.
DCPIP (photosynthesis β the Hill reaction) β blue when oxidised, colourless when reduced. Add it to a suspension of isolated chloroplasts and illuminate them: DCPIP substitutes for NADP, accepting the electrons from the light-dependent reaction. The time taken to decolourise is a measure of the rate of the light-dependent stage. Keep the chloroplasts cold and in an isotonic buffer so they are not damaged, and include a dark control, which should stay blue.
The dark control is the single most important control here: if the tube kept in darkness also decolourises, then something other than the light-dependent reaction is reducing the dye, and your result is worthless.
Calculate
Your turn β pigment Rf
2In a chromatogram of chloroplast pigments, one pigment travels 4.5 cm and the solvent front travels 9.0 cm. Calculate the Rf value.
Rf
Hint: Rf = 4.5 Γ· 9.0.
Statistics
The chi-squared test
Use chi-squared when you have categorical (counted) data and want to know whether your observed results differ significantly from the results you expected β the classic case being a genetic cross.
ΟΒ² = Ξ£ (O β E)Β² Γ· Edegrees of freedom = (number of categories β 1)
State the null hypothesis: there is no significant difference between the observed and expected results; any difference is due to chance.
Calculate the expected numbers from the predicted ratio.
Calculate ΟΒ².
Find the critical value in the table at p = 0.05 and the correct degrees of freedom.
ΟΒ² less than the critical value β accept the null hypothesis: the difference is due to chance. ΟΒ² greater than or equal to the critical value β reject it: the difference is significant, and something else (linkage, epistasis, selection) is going on.
p = 0.05 means there is a 5 % probability that a difference this large could have arisen by chance alone. It is a convention, not a law of nature β and ΟΒ² needs reasonably large numbers to be valid.
Calculate
Your turn β chi-squared
3A dihybrid cross predicts a 9:3:3:1 ratio. From 160 offspring the observed numbers are 100, 26, 28 and 6. Calculate ΟΒ² = Ξ£ (O β E)Β² Γ· E. Give your answer to 2 decimal places.
?Your ΟΒ² is 3.38. With 3 degrees of freedom, the critical value at p = 0.05 is 7.82. What do you conclude?
Match it
Which statistical tool?
Tap the question on the left, then the right tool.
The question you are asking
Tool
Uncertainty and error
Uncertainty, accuracy and precision
Accuracy β how close a reading is to the true value.
Precision β how closely repeated readings agree with each other. A miscalibrated balance gives precise but inaccurate results.
Random error β varies unpredictably; reduced by taking many repeats and a mean.
Systematic error β the same size and direction every time (a zero error, a balance reading 0.2 g high). Repeats will not help; you must calibrate.
% error = (uncertainty Γ· measured value) Γ 100uncertainty is usually half the smallest scale division
Always take the largest practical measurement, because the percentage error falls as the measured value rises. Identify anomalies, exclude them from the mean, and say why β an anomaly you cannot explain is a finding, not an embarrassment.
Calculate
Your turn β percentage error
4A balance has an uncertainty of Β±0.005 g. A student weighs a sample as 2.50 g. Calculate the percentage error.
%
Hint: (0.005 Γ· 2.50) Γ 100.
Experimental design
Designing an A2 investigation
Independent variable β the one you change. Choose a sensible range and at least five values, so a trend can be seen.
Dependent variable β the one you measure, as objectively and precisely as the apparatus allows.
Controlled variables β everything else that could affect the result: temperature (water bath), pH (buffer), volume and concentration of solutions, and the age, mass and species of the organism. Controlling them is what makes the experiment valid.
Control experiment β an identical set-up with the key factor removed (boiled enzyme, glass beads, a tube kept in the dark). It proves that your result is really caused by what you think it is.
Replicates β repeats make the mean more reliable and let you spot anomalies.
Ethics and safety are marked too: risk-assess the reagents, handle organisms humanely and return them to their habitat, and follow aseptic technique with microorganisms.
Sort it
Which kind of variable?
Tap an item, then tap the type of variable it is in an investigation into the effect of temperature on enzyme activity.
ποΈ Independent
π Dependent
π Controlled
Calculate
Your turn β rate from a colorimeter
5In a starchβamylase reaction, the absorbance falls from 0.80 to 0.20 over 4.0 minutes. Calculate the mean rate of change of absorbance per minute.
?A student finds a strong, statistically significant correlation between soil nitrate concentration and plant height in a field. What is the strongest valid conclusion?
Recap
The big ideas to know
Respirometer: soda lime absorbs COβ, so manometer movement measures oxygen uptake. Include a control tube with glass beads to correct for changes in temperature and atmospheric pressure.
Redox indicators:methylene blue and DCPIP are artificial hydrogen/electron acceptors β they become colourless when reduced, so the time taken to decolourise measures dehydrogenase activity.
Rf = distance moved by the pigment Γ· distance moved by the solvent front.
ΟΒ² = Ξ£ (O β E)Β² Γ· E, with degrees of freedom = (number of classes β 1). If ΟΒ² is less than the critical value at p = 0.05, accept the null hypothesis: the difference is due to chance.
% error = (uncertainty Γ· measured value) Γ 100. Random errors are reduced by repeating; systematic errors are not β the instrument must be calibrated.
Anomalies should be identified and excluded from the mean, and the reason given β never quietly deleted.
Correlation is not causation. A statistically significant correlation still needs a plausible mechanism before you can claim a cause.
You have covered the A2 practical skills. Press Finish to see your score.
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