Biology Extended Essay Exemplar: To what extent do serial dilutions… | RevisionDojo

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IB Biology SL Extended Essay Example

To what extent do serial dilutions of an aqueous Mangifera indica L. extract (v/v %, 0%, 25%, 50%, 75%, 100%) affect the growth rate of Saccharomyces cerevisiae cells under ethanol-induced oxidative stress as measured using optical density (OD) at 635nm, over a 6h period?SL

Official IB Result

25/34

Criteria A: Focus and Method

5/6

Criteria StrandsPro

A.1Topic communication and explanation

Excellent

A.2Research question formulation and focus

Good

A.3Methodology appropriateness and completeness

Moderate

Criteria Feedback

The student communicates a clear, biologically relevant investigation with a strong real-world rationale.
The student's research question is specific enough to show a genuine experimental focus.
The student includes a relevant range of methods and controls, with a thoughtful rationale for several design choices.

The investigation shifts between antioxidant and antifungal framing, which reduces precision.
Some aspects of the method are not fully aligned with the stated question, especially around how stress and growth are measured.
There are a few scientific explanation errors, including inconsistent interpretation of optical density.

1.1·Strength

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The title is clear, specific, and biologically relevant. It immediately tells the reader the organism, treatment, and general outcome being investigated, which strengthens topic communication from the first page.

1.2·Strength

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This opening effectively frames the investigation in a broader sustainability context. Linking mango peel waste to antioxidant potential gives the topic clear significance and helps justify why the experiment matters beyond the classroom.

1.3·Weakness

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This sentence broadens the topic too far and blurs the distinction between antioxidant and antifungal claims. The student should keep the rationale tightly aligned to the single phenomenon the method can actually test, otherwise the investigation’s purpose becomes less precise.

1.4·Question

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What direct evidence in the method shows that ethanol actually induced oxidative stress rather than only changing growth? Reflecting on this would help the student separate what is assumed from what is demonstrated.

1.5·Weakness

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This appears inconsistent with the surrounding method, which describes a much smaller cuvette volume. The student should check whether the intended unit is

uL

rather than

m L

, because an incorrect unit here would seriously affect reproducibility and validity.

1.6·Weakness

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This explanation reverses the relationship between absorbance and scattering, which weakens the scientific accuracy of the method description. The student should state the OD relationship consistently and check that every later interpretation uses the same direction.

1.7·Strength

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This is a good methodological awareness point. The student identifies a realistic source of measurement interference and shows an understanding that colorimeter readings can be affected by sample pigmentation, which is important for evaluating the suitability of OD.

1.8·Weakness

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This contradicts the earlier explanation and creates confusion about how growth is being interpreted. The student should choose one correct definition of OD and apply it consistently throughout the report, especially in the data discussion.

1.9·Suggestion

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It would strengthen focus to explicitly state the comparison being made in one concise sentence, for example: extract concentration with and without ethanol stress. That would make the experimental design easier to follow before the analysis begins.

1.10·Weakness

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This is too confident for the evidence provided. A pre-grown culture does not, by itself, prove that all later changes are caused only by the treatments; the student should acknowledge other possible growth-cycle effects and explain how the design reduces them rather than eliminating them.

1.11·Weakness

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This assumption needs stronger justification or a direct limitation statement. OD can be influenced by turbidity, pigment, and cell debris, so the student should make clear that it is only a proxy for growth rather than a direct measure of population size.

1.12·Strength

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The hypotheses are organized clearly and separately for ethanol, extract concentration, and interaction effects. This is strong methodological planning because it matches the factorial structure of the investigation and prepares the reader for the later statistical analysis.

Criteria B: Knowledge and Understanding

4/6

Criteria StrandsPro

B.1Application of source material

Good

B.2Use of terminology and concepts

Moderate

Criteria Feedback

The student uses a broad range of relevant biology and statistics vocabulary appropriately.
The student connects background knowledge about ROS, oxidative stress, and mango peel compounds to their investigation.
The student generally integrates source material into the rationale rather than listing facts in isolation.

Some source-based claims go beyond what the student's method can actually show.
There are a few important conceptual inaccuracies, especially around statistics and optical density.
At times the student treats related ideas as interchangeable when they need more careful distinction.

2.1·Strength

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This is a relevant and accurate explanation of oxidative stress. The student uses it well to build conceptual background for why ethanol might affect yeast growth and why antioxidants could matter.

2.2·Strength

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The student demonstrates solid subject knowledge by defining ROS and connecting them to cell damage. This kind of precise background explanation helps establish the biological basis for the investigation.

2.3·Strength

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The terminology here is mostly accurate and appropriate for IB Biology. Defining antioxidants before using them in the argument helps the reader follow the later interpretation of mango peel compounds.

2.4·Weakness

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This source is being used to support a strong antifungal claim, but the student’s own method only measures OD changes, not fungal inhibition directly. The student should avoid importing source conclusions that are broader than the evidence gathered in this investigation.

2.5·Weakness

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The student treats ethanol stress and oxidative stress as essentially identical here, but that relationship needs more careful wording. It would be stronger to explain that ethanol may contribute to oxidative stress rather than assuming the two are interchangeable in every context.

2.6·Strength

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This qualitative evidence is useful because it helps explain a real limitation of the OD measurements. Showing sedimentation and visible color differences demonstrates that the student is not relying on raw numbers alone.

2.7·Weakness

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The graph caption omits the 75% category even though the investigation uses five concentrations elsewhere. The student should make sure every graph and table includes the full set of relevant treatment levels to avoid confusion about the dataset.

2.8·Weakness

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This is conceptually incorrect.

R^{2}

cannot be negative, and it is not the same as a correlation coefficient

r

. The student should correct the statistic and report it with the proper interpretation.

Criteria C: Critical Thinking

9/12

Criteria StrandsPro

C.1Research analysis

Moderate

C.2Discussion and evaluation

Moderate

C.3Argument development

Moderate

Criteria Feedback

The student develops a clear scientific argument that moves from rationale to method, analysis, discussion, and conclusion.
The student includes substantial quantitative analysis and uses statistical testing to support parts of their interpretation.
The student responds to unexpected results thoughtfully and shows genuine awareness of the limitations of their evidence.

The central argument becomes less coherent when the focus shifts away from the original question.
Some claims are stronger than the data can support, especially where mechanism is inferred from OD changes alone.
A few of the post-hoc and interpretive steps are not fully robust or sufficiently justified.

3.1·Strength

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This is a strong example of analysis linked directly to statistical evidence. The student identifies the key result clearly and uses the p-value appropriately to support the claim, which shows good control of the data.

3.2·Weakness

Page 18• Click to view

This is a major shift in focus that weakens the coherence of the argument. The student should explain why collapsing the stressed and unstressed data is methodologically justified, and also acknowledge that this no longer answers the original research question as written.

3.3·Strength

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The student shows flexibility in response to unexpected data by reconsidering the direction of the investigation. This is a valuable analytical move, because good scientific writing should adapt to results rather than forcing the original expectation.

3.4·Weakness

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This conclusion goes beyond what OD data alone can establish. The student should distinguish correlation from mechanism: lower OD may indicate reduced turbidity or growth, but it does not prove antifungal action or rule out other explanations.

3.5·Strength

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This is a good use of post-hoc interpretation because it names the specific groups that differ rather than making a vague statement about the whole dataset. The student is appropriately narrowing the claim to what the statistical comparison actually shows.

3.6·Weakness

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This is not a robust or standard post-hoc strategy, because sequential lumping can change the comparison structure and inflate interpretation problems. The student should justify this method much more carefully or replace it with a standard post-hoc approach aligned to the ANOVA.

3.7·Question

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What evidence shows a true biological threshold rather than just a statistical difference between groups? Thinking about this distinction would help the student avoid over-interpreting pairwise significance as a mechanism.

3.8·Weakness

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This conclusion is too absolute because the evidence only supports some concentration effects in the first two hours, not the full claim about the investigation as a whole. The student should make the conclusion more cautious and tie it directly to the scope of the tested data.

3.9·Weakness

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This is an overreach from OD data. The student can say the extract was associated with lower OD, but not that it definitively caused cellular damage without a viability assay or direct mechanistic evidence.

3.10·Weakness

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This wording is too strong. The result shows that the treatment did not produce a significant effect in this dataset; it does not prove that 7.5% ethanol is universally ineffective. The student should use more careful statistical language.

3.11·Strength

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This is a strong evaluation point because it identifies a genuine interpretive limitation of the method. The student shows awareness that a proxy measure can be useful but still incomplete, which is exactly the kind of critical reflection expected.

3.12·Strength

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The student is thinking critically about method suitability by comparing the chosen design with more direct antioxidant assays. That comparison strengthens the evaluation because it shows the limitations are being judged against the actual research aim.

Criteria D: Presentation

3/4

Criteria StrandsPro

D.1Structure appropriateness

Excellent

D.2Layout and formatting

Excellent

Criteria Feedback

The student's report is organized into clear sections that are easy to follow.
Tables, figures, and appendices are generally placed and labelled in a reader-friendly way.
The overall layout is polished and shows care with formatting and presentation.

Some figure details are inconsistent or incomplete, which interrupts clarity.
There are a few formatting issues in captions, labels, and referencing style.
The report exceeds the word limit, which suggests the presentation could be more concise.

4.1·Suggestion

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Consider checking that all figure and table numbering is sequential and consistent across the body text, captions, and appendices. A fully consistent numbering system would make navigation easier and reduce confusion when the reader cross-references results.

4.2·Strength

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The numbered sectioning is consistent and helps the reader see the progression from raw data to interpretation. Clear signposting like this is effective presentation because it mirrors the logic of an investigation.

4.3·Strength

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The report has a clear scientific structure with distinct sections for methodology, data collection, analysis, discussion, evaluation, conclusion, and bibliography. This organization makes it easy for the reader to navigate the investigation and follow the argument logically.

4.4·Strength

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This table title is informative because it tells the reader exactly what is being summarized. Clear table naming improves presentation by allowing the reader to interpret the data without having to search for the explanation elsewhere.

4.5·Weakness

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The graph caption is missing the 75% concentration, which makes the presentation of results incomplete. The student should ensure every figure caption matches the full dataset shown in the investigation.

4.6·Weakness

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The axis label is inconsistent with the stated measurement at 635 nm. The student should correct the label so that the figure and method use the same wavelength throughout the report.

4.7·Weakness

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This axis label is cramped and unclear as written. Even though the overall layout is acceptable, the student should keep figure labels clean and unambiguous so the reader can interpret the graph quickly.

4.8·Strength

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The bibliography is included and placed in the correct overall position at the end of the report. That supports a logical presentation flow and helps separate the argument from its sources.

4.9·Weakness

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The reference list shows inconsistent citation formatting across entries, and this one is a good example of missing uniform punctuation and style. The student should standardize every citation entry so the bibliography looks professionally edited.

Loading scores...

Official IB Result

25/34

Criteria A: Focus and Method

5/6

Criteria StrandsPro

A.1Topic communication and explanation

Excellent

A.2Research question formulation and focus

Good

A.3Methodology appropriateness and completeness

Moderate

Criteria Feedback

The student communicates a clear, biologically relevant investigation with a strong real-world rationale.
The student's research question is specific enough to show a genuine experimental focus.
The student includes a relevant range of methods and controls, with a thoughtful rationale for several design choices.

The investigation shifts between antioxidant and antifungal framing, which reduces precision.
Some aspects of the method are not fully aligned with the stated question, especially around how stress and growth are measured.
There are a few scientific explanation errors, including inconsistent interpretation of optical density.

1.1·Strength

Page 1• Click to view

1.2·Strength

Page 3• Click to view

1.3·Weakness

Page 4• Click to view

1.4·Question

Page 6• Click to view

1.5·Weakness

Page 8• Click to view

This appears inconsistent with the surrounding method, which describes a much smaller cuvette volume. The student should check whether the intended unit is

uL

rather than

m L

, because an incorrect unit here would seriously affect reproducibility and validity.

1.6·Weakness

Page 9• Click to view

1.7·Strength

Page 9• Click to view

1.8·Weakness

Page 10• Click to view

1.9·Suggestion

Page 10• Click to view

1.10·Weakness

Page 10• Click to view

1.11·Weakness

Page 10• Click to view

1.12·Strength

Page 11• Click to view

Criteria B: Knowledge and Understanding

4/6

Criteria StrandsPro

B.1Application of source material

Good

B.2Use of terminology and concepts

Moderate

Criteria Feedback

The student uses a broad range of relevant biology and statistics vocabulary appropriately.
The student connects background knowledge about ROS, oxidative stress, and mango peel compounds to their investigation.
The student generally integrates source material into the rationale rather than listing facts in isolation.

Some source-based claims go beyond what the student's method can actually show.
There are a few important conceptual inaccuracies, especially around statistics and optical density.
At times the student treats related ideas as interchangeable when they need more careful distinction.

2.1·Strength

Page 4• Click to view

This is a relevant and accurate explanation of oxidative stress. The student uses it well to build conceptual background for why ethanol might affect yeast growth and why antioxidants could matter.

2.2·Strength

Page 4• Click to view

2.3·Strength

Page 5• Click to view

2.4·Weakness

Page 5• Click to view

2.5·Weakness

Page 6• Click to view

2.6·Strength

Page 12• Click to view

2.7·Weakness

Page 15• Click to view

2.8·Weakness

Page 19• Click to view

This is conceptually incorrect.

R^{2}

cannot be negative, and it is not the same as a correlation coefficient

r

. The student should correct the statistic and report it with the proper interpretation.

Criteria C: Critical Thinking

9/12

Criteria StrandsPro

C.1Research analysis

Moderate

C.2Discussion and evaluation

Moderate

C.3Argument development

Moderate

Criteria Feedback

The student develops a clear scientific argument that moves from rationale to method, analysis, discussion, and conclusion.
The student includes substantial quantitative analysis and uses statistical testing to support parts of their interpretation.
The student responds to unexpected results thoughtfully and shows genuine awareness of the limitations of their evidence.

The central argument becomes less coherent when the focus shifts away from the original question.
Some claims are stronger than the data can support, especially where mechanism is inferred from OD changes alone.
A few of the post-hoc and interpretive steps are not fully robust or sufficiently justified.

3.1·Strength

Page 17• Click to view

3.2·Weakness

Page 18• Click to view

3.3·Strength

Page 18• Click to view

3.4·Weakness

Page 19• Click to view

3.5·Strength

Page 21• Click to view

3.6·Weakness

Page 21• Click to view

3.7·Question

Page 22• Click to view

3.8·Weakness

Page 23• Click to view

3.9·Weakness

Page 24• Click to view

3.10·Weakness

Page 25• Click to view

3.11·Strength

Page 25• Click to view

3.12·Strength

Page 25• Click to view

Criteria D: Presentation

3/4

Criteria StrandsPro

D.1Structure appropriateness

Excellent

D.2Layout and formatting

Excellent

Criteria Feedback

The student's report is organized into clear sections that are easy to follow.
Tables, figures, and appendices are generally placed and labelled in a reader-friendly way.
The overall layout is polished and shows care with formatting and presentation.

Some figure details are inconsistent or incomplete, which interrupts clarity.
There are a few formatting issues in captions, labels, and referencing style.
The report exceeds the word limit, which suggests the presentation could be more concise.

4.1·Suggestion

Page 2• Click to view

4.2·Strength

Page 2• Click to view

4.3·Strength

Page 2• Click to view

4.4·Strength

Page 14• Click to view

4.5·Weakness

Page 15• Click to view

4.6·Weakness

Page 15• Click to view

The axis label is inconsistent with the stated measurement at 635 nm. The student should correct the label so that the figure and method use the same wavelength throughout the report.

4.7·Weakness

Page 19• Click to view

4.8·Strength

Page 29• Click to view

The bibliography is included and placed in the correct overall position at the end of the report. That supports a logical presentation flow and helps separate the argument from its sources.

4.9·Weakness

Page 31• Click to view