Biology IA Example: Substrate Type and Yeast Respiration Rate | RevisionDojo

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IB Biology HL Internal Assessment Example

What is the substrate type's (Glucose, Fructose, Galactose, Sucrose, Maltose, Starch) effect on the rate (ppm/min) of respiration in yeast (Saccharomyces cerevisiae), as measured by the change in CO2 concentration (ppm) over 600 seconds?HL

Official IB Result

14/24

Criteria A: Research Design

4/6

Criteria StrandsPro

A.1Research question context

Good

A.2Methodological considerations

Moderate

A.3Methodology description

Moderate

Criteria Feedback

The student asked a focused, measurable question that clearly identified the organism, substrates, dependent variable, units, and time frame.
The student showed awareness of relevant biochemical and practical factors that could affect fermentation results.
The student's procedure includes enough detail for much of the investigation to be followed and repeated.

The student's substrate comparison is not fully fair because equal masses do not represent equal molar amounts or equal fermentable availability.
Some important methodological conditions were not fully controlled or justified, especially around starch preparation and anaerobic conditions.
There are internal inconsistencies that reduce reproducibility, including different replicate numbers across treatments.

1.1·Strength

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The research question is tightly focused: it names the organism, the substrate categories, the dependent variable, units, and the 600-second time frame. That level of specificity makes the investigation measurable and gives the analysis a clear target.

1.2·Weakness

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This justification is incomplete and ambiguous, so the methodological rationale is weakened. The student should restate the sentence clearly and explain why 5 minutes is sufficient for thermal equilibration and why this duration is appropriate for all treatments.

1.3·Weakness

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This control is not actually equivalent across substrates because 10 g of each carbohydrate does not represent the same molar amount or fermentable-glucose-equivalent amount. The student should justify how this allows a fair comparison of substrate type, or redesign the concentrations so the treatments are comparable in biochemical availability.

1.4·Suggestion

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The equipment list is detailed, but the student would strengthen reproducibility by stating which bottle size was actually used in the trial setup and how much headspace remained. Those details matter because gas volume and pressure conditions can affect CO2 readings.

1.5·Weakness

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The starch procedure is not methodologically comparable with the other substrates because it uses a different preparation route and then bypasses the standard dilution step. The student should either standardise all substrate preparations or explicitly explain how this exception preserves equivalence.

1.6·Suggestion

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The procedure would be more reproducible if the student specified how long the bottle was left open during transfer and how the seal was checked. Small timing differences here can change oxygen exposure and CO2 loss, especially in a fermentation investigation.

1.7·Question

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What happens to the interpretation if the meter only records the start and end points rather than the full time course? The student should consider whether transient changes or lag phases in fermentation might be missed by this design.

1.8·Weakness

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This instruction conflicts with the later data tables, where some substrates have 3 trials and starch has 5. Internal consistency is essential for reproducibility; the student should make the planned number of trials match the reported dataset and explain any deviations.

Criteria B: Data Analysis

4/6

Criteria StrandsPro

B.1Communication of data recording and processing

Excellent

B.2Consideration of uncertainties

Moderate

B.3Data processing quality

Moderate

Criteria Feedback

The student recorded and organised the raw and processed data clearly, with units, worked calculations, and a graph that directly addresses the question.
The student showed awareness of variability by including error bars and standard deviations.
The student processed the results in a way that allows comparisons among substrates to be made.

The student's uncertainty treatment is not fully correct, especially in how it is calculated and interpreted.
The replication is inconsistent across treatments, which weakens confidence in the comparisons.
Some calculated values, including the glucose spread, should be checked carefully for accuracy.

2.1·Strength

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Including a no-substrate control is a strong design choice because it provides a baseline for CO2 change unrelated to added carbohydrate. That makes the treatment comparisons more meaningful and helps separate fermentation effects from background activity.

2.2·Suggestion

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The processing would be clearer if each substrate had a worked example using the same format, then a summary table of all calculated rates. That would make the calculation chain easier to follow and help readers verify each result quickly.

2.3·Suggestion

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The graph is appropriate for categorical substrates, but the presentation would be stronger if the caption stated exactly what the error bars represent. That would make the variability easier to interpret and prevent confusion between standard deviation, standard error, and uncertainty.

2.4·Weakness

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The reported glucose standard deviation should be checked against the raw values, because the spread looks unusually large and may have been calculated incorrectly. If the standard deviation is wrong, the comparison of variability across substrates becomes unreliable.

2.5·Weakness

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The mean increase for the control is clearly processed, but the processing quality is weakened by inconsistent replication elsewhere in the dataset. To improve precision, the student should keep replicate numbers identical across all substrates before comparing mean rates.

2.6·Weakness

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The uncertainty treatment is mathematically incorrect: percentage uncertainty should not be calculated separately from the initial and final CO2 readings and then added as a proxy for uncertainty in the change. The student should propagate the absolute uncertainties of both measurements into the difference, then convert that result into the uncertainty of the rate.

2.7·Weakness

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This single percentage is presented as though it represents the trial’s total measurement uncertainty, but it is not clearly derived from the measured change in CO2 concentration. The student should show a valid uncertainty propagation method and distinguish instrument uncertainty from variability in the biological response.

2.8·Question

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How would the interpretation change if the glucose spread is driven by one unusually high or low trial rather than genuine biological variability? The student should consider whether an outlier test or repeat trial would be more appropriate than drawing a ranking from the mean alone.

Criteria C: Conclusion

3/6

Criteria StrandsPro

C.1Conclusion relevance and consistency

Moderate

C.2Scientific context comparison

Moderate

Criteria Feedback

The student's conclusion stays connected to the research question and uses the processed results to make a direct claim.
The student compares their findings with expected scientific ideas and recognises when the results do not match expectations.
The student acknowledges that methodological limitations may have affected the pattern they observed.

The student generalises too far from the specific treatment tested to broader biological claims.
Some claims in the conclusion are stronger than the analysis can support, especially where variability and missing controls remain.
The student treats the observed rate ranking as a biological preference ranking, which the evidence does not prove.

3.1·Weakness

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This conclusion overgeneralises from one polysaccharide, starch, to polysaccharides as a whole. The student should limit the claim to the specific treatment tested unless the analysis includes additional evidence showing that other polysaccharides behave similarly.

3.2·Strength

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The conclusion is clearly tied to the research question and uses the processed results to make a direct claim. That keeps the final judgment anchored to the investigation rather than drifting into unsupported summary.

3.3·Weakness

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The student labels the rank order as a 'preference' ranking, but the data only show observed CO2 production under this procedure. The conclusion should distinguish measured fermentation rate from actual biological preference, which cannot be proved from this experiment alone.

3.4·Weakness

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This explanation is plausible, but it is asserted too confidently given the method. The student should evaluate whether the yeast and conditions used could actually hydrolyse starch sufficiently, rather than assuming starch automatically became accessible glucose.

3.5·Question

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Does the control really prove that sugar is needed, or only that some CO2 production occurs even without added substrate? The student should think about alternative explanations such as residual nutrients in the yeast or contamination in the water.

3.6·Suggestion

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This is a sensible start, but the conclusion would be stronger if it named the most likely limiting factors and linked each one to a specific observed anomaly, such as the incomplete dissolution of galactose and maltose or the unusually variable glucose results.

Criteria D: Evaluation

3/6

Criteria StrandsPro

D.1Methodological weaknesses

Good

D.2Improvements

Moderate

Criteria Feedback

The student identifies several specific weaknesses that are clearly connected to the procedure and results.
The student explains many of the limitations in terms of how they could affect substrate availability, reliability, or carbon dioxide production.
The student suggests realistic improvements that are generally linked to the problems they identified.

Some limitations are not clearly prioritised by seriousness, so the most important issues are not distinguished from the less important ones.
A few improvements are too general and need more precise scientific justification.
The investigation would be strengthened by a clearer fix for the substrate-comparison problem, especially standardising concentrations on a comparable chemical basis.

4.1·Strength

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This is a clear causal explanation of how the limitation affected the outcome. It strengthens the evaluation because the student connects the procedural weakness to a likely directional bias in the data.

4.2·Suggestion

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A stronger improvement would state the exact temperature or preparation method needed to make the solutions comparable. The student should also explain why this would reduce the limitation, rather than only suggesting a general pre-heating step.

4.3·Strength

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This is a strong limitation because it is specific, linked directly to the starch treatment, and explained in terms of its effect on substrate contact and CO2 production. The student is doing more than listing flaws; they are showing how the issue could distort the results.

4.4·Weakness

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The limitation is valid, but the impact is stated too generally. The student should explain how losing one replicate changes confidence in the mean and whether it affected specific substrates more than others, because that would show the relative seriousness of the problem.

4.5·Weakness

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The limitation is identified, but the explanation does not yet prioritise it against the other issues. The student should compare this uncertainty with the non-comparable substrate concentrations and dissolution problems, because some limitations are more serious than others for validity.

4.6·Suggestion

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These are thoughtful improvement ideas, but the student should explain which one best addresses the uncertainty about aerobic versus anaerobic respiration. Adding that justification would show stronger prioritisation of the most important methodological fix.

Loading scores...

Official IB Result

14/24

Criteria A: Research Design

4/6

Criteria StrandsPro

A.1Research question context

Good

A.2Methodological considerations

Moderate

A.3Methodology description

Moderate

Criteria Feedback

The student asked a focused, measurable question that clearly identified the organism, substrates, dependent variable, units, and time frame.
The student showed awareness of relevant biochemical and practical factors that could affect fermentation results.
The student's procedure includes enough detail for much of the investigation to be followed and repeated.

The student's substrate comparison is not fully fair because equal masses do not represent equal molar amounts or equal fermentable availability.
Some important methodological conditions were not fully controlled or justified, especially around starch preparation and anaerobic conditions.
There are internal inconsistencies that reduce reproducibility, including different replicate numbers across treatments.

1.1·Strength

Page 1• Click to view

1.2·Weakness

Page 3• Click to view

1.3·Weakness

Page 3• Click to view

1.4·Suggestion

Page 4• Click to view

1.5·Weakness

Page 5• Click to view

1.6·Suggestion

Page 5• Click to view

1.7·Question

Page 6• Click to view

1.8·Weakness

Page 6• Click to view

Criteria B: Data Analysis

4/6

Criteria StrandsPro

B.1Communication of data recording and processing

Excellent

B.2Consideration of uncertainties

Moderate

B.3Data processing quality

Moderate

Criteria Feedback

The student recorded and organised the raw and processed data clearly, with units, worked calculations, and a graph that directly addresses the question.
The student showed awareness of variability by including error bars and standard deviations.
The student processed the results in a way that allows comparisons among substrates to be made.

The student's uncertainty treatment is not fully correct, especially in how it is calculated and interpreted.
The replication is inconsistent across treatments, which weakens confidence in the comparisons.
Some calculated values, including the glucose spread, should be checked carefully for accuracy.

2.1·Strength

Page 7• Click to view

2.2·Suggestion

Page 9• Click to view

2.3·Suggestion

Page 10• Click to view

2.4·Weakness

Page 10• Click to view

2.5·Weakness

Page 10• Click to view

2.6·Weakness

Page 11• Click to view

2.7·Weakness

Page 11• Click to view

2.8·Question

Page 12• Click to view

Criteria C: Conclusion

3/6

Criteria StrandsPro

C.1Conclusion relevance and consistency

Moderate

C.2Scientific context comparison

Moderate

Criteria Feedback

The student's conclusion stays connected to the research question and uses the processed results to make a direct claim.
The student compares their findings with expected scientific ideas and recognises when the results do not match expectations.
The student acknowledges that methodological limitations may have affected the pattern they observed.

The student generalises too far from the specific treatment tested to broader biological claims.
Some claims in the conclusion are stronger than the analysis can support, especially where variability and missing controls remain.
The student treats the observed rate ranking as a biological preference ranking, which the evidence does not prove.

3.1·Weakness

Page 11• Click to view

3.2·Strength

Page 11• Click to view

3.3·Weakness

Page 12• Click to view

3.4·Weakness

Page 12• Click to view

3.5·Question

Page 12• Click to view

3.6·Suggestion

Page 12• Click to view

Criteria D: Evaluation

3/6

Criteria StrandsPro

D.1Methodological weaknesses

Good

D.2Improvements

Moderate

Criteria Feedback

The student identifies several specific weaknesses that are clearly connected to the procedure and results.
The student explains many of the limitations in terms of how they could affect substrate availability, reliability, or carbon dioxide production.
The student suggests realistic improvements that are generally linked to the problems they identified.

Some limitations are not clearly prioritised by seriousness, so the most important issues are not distinguished from the less important ones.
A few improvements are too general and need more precise scientific justification.
The investigation would be strengthened by a clearer fix for the substrate-comparison problem, especially standardising concentrations on a comparable chemical basis.

4.1·Strength

Page 13• Click to view

4.2·Suggestion

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4.3·Strength

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4.4·Weakness

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4.5·Weakness

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4.6·Suggestion

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