Biology IA Exemplar: How does varying carbon dioxide concentration… | RevisionDojo

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IB Biology HL Internal Assessment Example

How does varying carbon dioxide concentration through using varying sodium bicarbonate masses(2g, 4g, 6g, 8g and 10g)(±0.1g) affect the rate of photosynthesis in Elodea Canadensis, as measured by the rate of oxygen evolution through a measure of the duration for the float time Filter Paper Disk Assay in seconds(±0.1s)?HL

Official IB Result

21/24

Criteria A: Research Design

6/6

Criteria StrandsPro

A.1Research question context

Good

A.2Methodological considerations

Moderate

A.3Methodology description

Moderate

Criteria Feedback

The student framed the investigation in a specific biological context and identified a clear organism, variable, and measurement approach.
The student included several relevant controls and methodological steps that make the investigation broadly reproducible.
The student showed awareness that the experiment needed repeated trials and a defined procedure.
The student used scientific language that generally keeps the investigation focused on photosynthesis rather than a vague topic.

The wording of the investigation does not fully match the variable actually changed, since the student varied sodium hydrogen carbonate mass rather than directly measuring carbon dioxide concentration.
Some background claims go beyond what the experiment can support and weaken the precision of the setup.
Important biological controls were not fully standardised, such as the size, age, health, or amount of plant material.
A few methodological details are still missing, which leaves some uncertainty about exact reproduction and validity.

1.1·Weakness

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This background overreaches beyond the investigation and introduces claims that are not well controlled or directly relevant to the research question. The student should narrow the context to aquatic photosynthesis and bicarbonate availability, because that would build a more disciplined justification for the experiment.

1.2·Strength

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The research question is specific and clearly operationalised: the student identifies the organism, the independent variable, and the measurement method. That clarity helps the investigation remain focused and makes the intended relationship measurable rather than vague.

1.3·Suggestion

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Consider rewriting the aim in more precise scientific language. Referring to “food-synthesis” is informal, so using “photosynthetic rate” or “rate of oxygen evolution” would better match IB expectations and make the purpose easier to evaluate against the data.

1.4·Question

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What evidence supports the claim that 10 g represents an “optimum-level” of CO2? The student would strengthen the design by avoiding assumptions of optimality unless they are justified by preliminary data or literature.

1.5·Weakness

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The research question repeatedly claims to vary CO2 concentration, but the method actually varies sodium bicarbonate mass. The student should align the wording with the true manipulated variable or explain clearly why mass is a valid proxy for dissolved inorganic carbon concentration.

1.6·Strength

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This control is well chosen because it identifies a major confounder in photosynthesis experiments and gives a concrete way to hold it constant. That specificity makes the investigation more reproducible and improves validity.

1.7·Weakness

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The dependent variable is described clearly, but the method of measuring it is still vulnerable to timing error because the endpoint relies on human judgment of when the disk first floats. The student should acknowledge this limitation more explicitly in the design and consider a more direct measurement approach.

1.8·Strength

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The student gives a clear and reproducible preparation step that reduces variation in disk size and surface area. This is strong methodology because it helps ensure differences in float time are more likely due to the independent variable than to disk-to-disk variation.

1.9·Suggestion

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The student could improve reproducibility by stating exactly how the buffer was prepared and whether the final pH was checked after NaHCO3 addition. That would make it clearer that pH was actually controlled rather than only intended to be controlled.

1.10·Weakness

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This step is not sufficiently reproducible because it does not standardise the Elodea material. The student should specify plant length, mass, leaf area, age, and whether the same plant or a fresh sample is used for each trial, since biological variation could meaningfully affect the results.

1.11·Weakness

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The repetition plan is useful, but the wording does not clarify whether new plant material and fresh disks are used each time. Without that detail, another student could not fully reproduce the investigation and may inadvertently introduce dependence between trials.

Criteria B: Data Analysis

4/6

Criteria StrandsPro

B.1Communication of data recording and processing

Good

B.2Consideration of uncertainties

Moderate

B.3Data processing quality

Poor

Criteria Feedback

The student presents the raw data in a clear table and follows it with processed values that are easy to compare across treatments.
The student calculates means, standard deviations, reciprocal rates, and includes a graph, so the reader can follow the trend in more than one way.
The student shows some awareness of uncertainty by discussing timing precision and variability across trials.
The student's qualitative observations help connect the numerical results to what was actually seen during the experiment.

The uncertainty treatment is not consistently clear, and some values are not communicated in a mathematically reliable way.
The outlier handling is not fully justified, and the treatment of that value is not handled consistently across the analysis.
Some processing steps appear inaccurate or inconsistent, which weakens confidence in the statistical conclusions.
The statistical test and graph interpretation are not fully secure enough to support the strength of the claims being made.

2.1·Strength

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The qualitative observations are useful because they describe both timing and visible bubble formation. This helps the reader connect the numerical trends to what was actually seen during the experiment, which strengthens the communication of the data.

2.2·Weakness

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Recording to two decimal places does not in itself guarantee precision, especially when the endpoint depends on reaction time. The student should distinguish between display precision and actual measurement uncertainty, because the former can look more exact than the data really are.

2.3·Weakness

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The student explains why an outlier was removed, which is good, but the criterion for exclusion is still not rigorous enough. A stronger analysis would define an objective rule in advance and then apply it consistently, otherwise the omission can bias the mean.

2.4·Question

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If the standard deviation decreases with increasing mass, what does that imply about the consistency of the measurements, and does the 10 g treatment fit that pattern or break it? This is worth considering before drawing reliability claims from the full set of treatments.

2.5·Weakness

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The standard-deviation formula is not clearly communicated, and it appears to use

5

even where the 10 g outlier was excluded. The student should state whether the sample or population formula is being used and keep the denominator consistent with the number of included values.

2.6·Weakness

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This uncertainty is not mathematically plausible for a reciprocal rate expressed in

s^{- 1}

. The student should revisit the propagation method, because the uncertainty is larger than the value itself and therefore cannot be interpreted meaningfully.

2.7·Weakness

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The statistical claim is weakened because the ANOVA table does not match the structure of the data shown. The student should verify the degrees of freedom and the grouping used, because the reported test does not convincingly support the conclusion about differences across six treatments.

2.8·Weakness

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The direction of the uncertainty trend is misread here: the values decrease rather than increase. The student should check whether the uncertainty is being reported as absolute or relative uncertainty and interpret the trend consistently with the calculations.

2.9·Suggestion

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The student could improve the analysis by interpreting the regression gradient in its actual units rather than as a percentage change. Doing so would make the trend easier to evaluate quantitatively and avoid overstating what the slope represents.

2.10·Weakness

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This interpretation goes beyond what the graph alone can justify unless the error bars are defined and calculated clearly. The student should state what the bars represent and how they were obtained before using them to claim low random error or high precision.

Criteria C: Conclusion

5/6

Criteria StrandsPro

C.1Conclusion relevance and consistency

Moderate

C.2Scientific context comparison

Moderate

Criteria Feedback

The student answers the research question directly by describing the overall trend in float time and rate across the treatments.
The student supports the conclusion with numerical evidence from the data rather than making only a general statement.
The student makes a relevant comparison to scientific ideas about carbon availability and aquatic photosynthesis.
The student's final claim stays connected to the observed pattern in the investigation.

Some parts of the conclusion rely on analysis that is not fully reliable, which weakens confidence in the final interpretation.
A few statements are too definitive for the evidence available, especially where the student moves from the data to broader scientific or environmental claims.
The comparison to scientific context is relevant but not always tightly evaluated against the actual conditions of the student's experiment.
The discussion sometimes overstates what the processed results can prove.

3.1·Strength

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The conclusion answers the research question directly and is anchored in the observed trend across the treatments. That makes it relevant and clearly connected to the investigation rather than becoming a generic summary.

3.2·Weakness

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The conclusion overstates what

R^{2}

shows. A strong

R^{2}

indicates a good linear fit, but it does not by itself prove minimal error or causation, so the student should use more cautious wording here.

3.3·Weakness

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This statement is too definitive for the system being studied. The student should qualify it as a source of dissolved inorganic carbon rather than claiming it simply decomposes to CO2, because that distinction matters for scientific accuracy.

3.4·Weakness

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This explanation weakens the conclusion because it repeats an uncertainty trend incorrectly. The student should make sure the uncertainty values are interpreted correctly before using them to justify confidence in the final claim.

3.5·Strength

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The student makes a relevant comparison to scientific context by linking the result to another aquatic-plant study. That strengthens the conclusion because it shows the pattern is not isolated to this dataset alone.

3.6·Weakness

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This broader environmental claim is not well supported by the investigation and distracts from the actual conclusion. The student should keep the comparison focused on photosynthesis and dissolved oxygen, unless additional evidence is provided for the ecological claims.

Criteria D: Evaluation

6/6

Criteria StrandsPro

D.1Methodological weaknesses

Excellent

D.2Improvements

Excellent

Criteria Feedback

The student identifies several specific limitations rather than staying at a general level, which makes their evaluation insightful.
The student explains the likely effect of several weaknesses on reliability, validity, or precision.
The student proposes realistic improvements that are clearly linked to the problems they identified.
The student shows a strong understanding that some limitations affect the quality of the investigation more seriously than others.

Some explanations could separate random error, systematic error, and validity issues more clearly.
A few improvements would be even stronger if the student explained more precisely how they would change the quality of the data.
Some statements about the effect of changes are a little overstated and need tighter scientific wording.
The evaluation would benefit from more careful discussion of how each weakness affected the final pattern in the results.

4.1·Strength

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The student identifies a specific weakness rather than using a generic statement, and links it to the highest-treatment data. That specificity is strong evaluation practice because it shows the student can locate where the investigation was least stable.

4.2·Weakness

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This limitation is well chosen, but the explanation should separate timing delay from variability more carefully. A slower or faster human response does not automatically create the observed spread unless the student explains how it varied across trials.

4.3·Weakness

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This is a valid limitation, but the discussion would be stronger if it explained the relative impact on the results. The student should clarify whether gas exchange would introduce random error, systematic bias, or both, because that affects how seriously the issue changes the interpretation.

4.4·Suggestion

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This is an appropriate and realistic improvement. The student could make it even stronger by explaining that frame-by-frame analysis gives a more objective endpoint than manual stopwatch timing, which would better address the specific weakness.

4.5·Strength

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This is a strong improvement because it is directly linked to the identified limitation and explains how the change would improve validity. The student is not just naming a fix, but showing why it would make the independent variable more meaningful.

Loading scores...

Official IB Result

21/24

Criteria A: Research Design

6/6

Criteria StrandsPro

A.1Research question context

Good

A.2Methodological considerations

Moderate

A.3Methodology description

Moderate

Criteria Feedback

The student framed the investigation in a specific biological context and identified a clear organism, variable, and measurement approach.
The student included several relevant controls and methodological steps that make the investigation broadly reproducible.
The student showed awareness that the experiment needed repeated trials and a defined procedure.
The student used scientific language that generally keeps the investigation focused on photosynthesis rather than a vague topic.

The wording of the investigation does not fully match the variable actually changed, since the student varied sodium hydrogen carbonate mass rather than directly measuring carbon dioxide concentration.
Some background claims go beyond what the experiment can support and weaken the precision of the setup.
Important biological controls were not fully standardised, such as the size, age, health, or amount of plant material.
A few methodological details are still missing, which leaves some uncertainty about exact reproduction and validity.

1.1·Weakness

Page 2• Click to view

1.2·Strength

Page 2• Click to view

1.3·Suggestion

Page 3• Click to view

1.4·Question

Page 3• Click to view

1.5·Weakness

Page 3• Click to view

1.6·Strength

Page 4• Click to view

1.7·Weakness

Page 4• Click to view

1.8·Strength

Page 5• Click to view

1.9·Suggestion

Page 5• Click to view

1.10·Weakness

Page 6• Click to view

1.11·Weakness

Page 6• Click to view

Criteria B: Data Analysis

4/6

Criteria StrandsPro

B.1Communication of data recording and processing

Good

B.2Consideration of uncertainties

Moderate

B.3Data processing quality

Poor

Criteria Feedback

The student presents the raw data in a clear table and follows it with processed values that are easy to compare across treatments.
The student calculates means, standard deviations, reciprocal rates, and includes a graph, so the reader can follow the trend in more than one way.
The student shows some awareness of uncertainty by discussing timing precision and variability across trials.
The student's qualitative observations help connect the numerical results to what was actually seen during the experiment.

The uncertainty treatment is not consistently clear, and some values are not communicated in a mathematically reliable way.
The outlier handling is not fully justified, and the treatment of that value is not handled consistently across the analysis.
Some processing steps appear inaccurate or inconsistent, which weakens confidence in the statistical conclusions.
The statistical test and graph interpretation are not fully secure enough to support the strength of the claims being made.

2.1·Strength

Page 7• Click to view

2.2·Weakness

Page 7• Click to view

2.3·Weakness

Page 7• Click to view

2.4·Question

Page 8• Click to view

2.5·Weakness

Page 8• Click to view

The standard-deviation formula is not clearly communicated, and it appears to use

5

even where the 10 g outlier was excluded. The student should state whether the sample or population formula is being used and keep the denominator consistent with the number of included values.

2.6·Weakness

Page 9• Click to view

This uncertainty is not mathematically plausible for a reciprocal rate expressed in

s^{- 1}

. The student should revisit the propagation method, because the uncertainty is larger than the value itself and therefore cannot be interpreted meaningfully.

2.7·Weakness

Page 10• Click to view

2.8·Weakness

Page 10• Click to view

2.9·Suggestion

Page 11• Click to view

2.10·Weakness

Page 11• Click to view

Criteria C: Conclusion

5/6

Criteria StrandsPro

C.1Conclusion relevance and consistency

Moderate

C.2Scientific context comparison

Moderate

Criteria Feedback

The student answers the research question directly by describing the overall trend in float time and rate across the treatments.
The student supports the conclusion with numerical evidence from the data rather than making only a general statement.
The student makes a relevant comparison to scientific ideas about carbon availability and aquatic photosynthesis.
The student's final claim stays connected to the observed pattern in the investigation.

Some parts of the conclusion rely on analysis that is not fully reliable, which weakens confidence in the final interpretation.
A few statements are too definitive for the evidence available, especially where the student moves from the data to broader scientific or environmental claims.
The comparison to scientific context is relevant but not always tightly evaluated against the actual conditions of the student's experiment.
The discussion sometimes overstates what the processed results can prove.

3.1·Strength

Page 12• Click to view

3.2·Weakness

Page 12• Click to view

The conclusion overstates what

R^{2}

shows. A strong

R^{2}

indicates a good linear fit, but it does not by itself prove minimal error or causation, so the student should use more cautious wording here.

3.3·Weakness

Page 12• Click to view

3.4·Weakness

Page 12• Click to view

3.5·Strength

Page 13• Click to view

3.6·Weakness

Page 13• Click to view

Criteria D: Evaluation

6/6

Criteria StrandsPro

D.1Methodological weaknesses

Excellent

D.2Improvements

Excellent

Criteria Feedback

The student identifies several specific limitations rather than staying at a general level, which makes their evaluation insightful.
The student explains the likely effect of several weaknesses on reliability, validity, or precision.
The student proposes realistic improvements that are clearly linked to the problems they identified.
The student shows a strong understanding that some limitations affect the quality of the investigation more seriously than others.

Some explanations could separate random error, systematic error, and validity issues more clearly.
A few improvements would be even stronger if the student explained more precisely how they would change the quality of the data.
Some statements about the effect of changes are a little overstated and need tighter scientific wording.
The evaluation would benefit from more careful discussion of how each weakness affected the final pattern in the results.

4.1·Strength

Page 13• Click to view

4.2·Weakness

Page 14• Click to view

4.3·Weakness

Page 14• Click to view

4.4·Suggestion

Page 14• Click to view

4.5·Strength

Page 14• Click to view