Biology Extended Essay Exemplar: What is the effect of different…

What is the effect of different concentrations of zinc sulphate in varying concentrations (25, 50, 100, 150, 200 millimoles per litre), 35% concentration tea tree oil, and a mixture of zinc sulphate and tea tree oil at each concentration (25, 50, 100, 150, 200 millimoles per litre), on the zones of inhibition of gram-positive (Micrococcus luteus) and gram-negative (Escherichia coli) bacteria after 48 hour and 24 hour incubation in 25ºC respectively, on soy agar, using the disc diffusion method?SL

Official IB Result

29/34

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

Good

Criteria Feedback

The student communicates a clear, biologically grounded topic with a relevant experimental aim.
The student's method is detailed and generally well matched to a disc-diffusion investigation.
The student shows awareness of how the selected method and preliminary trials relate to the question.

The research question is overloaded and would be sharper if narrowed to one main comparison.
Some treatment descriptions are inconsistent, especially around the tea tree oil concentration and the exact conditions being compared.
The method needs stronger control design and clearer handling of replication to support fair comparison.

1.1·Weakness

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The research question is workable, but it is overloaded: it combines concentration, treatment type, two species, incubation time, medium, and method in one very long sentence. Narrowing the wording would make the focus sharper and easier to test. There is also a mismatch here between “35% concentration tea tree oil” and the later description of pure TTO.

1.2·Strength

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The title communicates a clear biological comparison and identifies the two organisms and treatments immediately. This helps the reader understand the investigation’s scope from the outset.

1.3·Weakness

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The aim introduces a comparison, but the design is not yet fully focused because the treatment logic is inconsistent across the question and method. The student should state exactly which conditions are being compared and keep the concentration description identical in the aim, question, and procedure.

1.4·Question

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What is the single central comparison the student most wants to answer: concentration effect, treatment type, or bacterial type? Identifying one primary focus would make the research question more precise and the later analysis easier to follow.

1.5·Suggestion

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Consider narrowing the rationale to what this investigation can actually support: in-vitro inhibition on two bacterial species. Claims about medical implants are interesting, but they need a more cautious phrasing unless supported by implant-specific evidence.

1.6·Weakness

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The dependent variable is clear, but the sentence is awkwardly phrased and could be more concise. A cleaner definition would improve clarity and reduce the risk of ambiguity about the measurement being taken.

1.7·Strength

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The student shows selection rationale here by explaining how preliminary trials narrowed the concentration range. That is exactly the kind of evidence-based decision-making that strengthens method justification.

1.8·Strength

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This is a strong methodological justification because it links the chosen method to the existing literature. The student is not just describing the method, but showing why it is appropriate for the question.

1.9·Weakness

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This is a major methodological limitation because the zinc-only discs and the combined-treatment discs do not carry the same zinc volume. That makes direct comparison between conditions less clean, because the treatment changes in more than one way at once.

1.10·Suggestion

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Consider making the control strategy explicit here. If deionised water was the carrier for some discs, stating a water-only control would help show that any inhibition came from the antibacterial agents rather than the disc moisture itself.

1.11·Strength

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This is a precise and practical measurement choice. Using a digital calliper increases measurement precision and supports more reliable quantitative data.

1.12·Suggestion

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A stronger method section would also explain how a control disc was handled consistently across all plates. A zero-treatment or deionised-water control would make the inhibition zones easier to interpret against baseline diffusion.

1.13·Weakness

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This creates a likely replication problem: the five discs on one plate may not be independent experimental replicates. The student should explain whether each disc counted as a technical replicate within a plate or whether separate plates were used for true replication.

1.14·Weakness

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The student should clarify what counts as a “trial” here, because the earlier description suggests multiple discs on one plate. Without that clarification, it is difficult to judge whether the replication is truly independent.

Criteria B: Knowledge and Understanding

5/6

Criteria StrandsPro

B.1Application of source material

Excellent

B.2Use of terminology and concepts

Excellent

Criteria Feedback

The student uses scientific terminology accurately and consistently in most places.
The student applies source material to explain biological mechanisms rather than only describing facts.
The student's explanation shows clear understanding of relevant concepts such as bacterial cell structure and inhibition.

Some phrasing is imprecise or occasionally oversimplified, which weakens scientific clarity.
A few claims are stronger than the evidence directly supports, especially when discussing mechanisms and real-world implications.
Some statistical and biological wording would benefit from more careful precision.

2.1·Strength

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The student uses source material to establish the real-world significance of the topic. This helps the investigation feel purposeful rather than purely descriptive.

2.2·Weakness

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This explanation is mostly accurate but oversimplified. The student should be more careful with wording such as “the gram of a bacteria,” and should distinguish Gram staining from the structural features that actually cause the staining outcome.

2.3·Weakness

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The examples are relevant, but the sentence would be stronger with more precise scientific phrasing. “Athletes foot” should be “athlete’s foot,” and the explanation should distinguish established antibacterial evidence from common use claims.

2.4·Strength

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The student explains mechanism rather than just naming zinc as antibacterial, which shows stronger biological understanding. This makes later discussion of the results more meaningful.

2.5·Strength

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The hypothesis is testable and clearly linked to the investigation variables. It predicts a directional relationship, which gives the later analysis a clear benchmark.

2.6·Suggestion

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It would strengthen the knowledge base to explain one key limitation of disc diffusion as well as its suitability. Showing both sides would demonstrate deeper understanding of what the method can and cannot reveal.

2.7·Weakness

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A very high

R^{2}

alone does not prove reliability, especially when the line is a polynomial fitted to only five concentrations. The student should interpret the statistic more cautiously and avoid overstating what the trendline demonstrates.

2.8·Strength

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This is a strong example of applying source material directly to explain the observed pattern. It connects mechanism to result instead of leaving the biology at the descriptive level.

2.9·Suggestion

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Consider tying this claim more explicitly to the earlier data table or graph values. Linking the mechanism to specific numbers would make the application of source material more convincing.

2.10·Weakness

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This explanation is plausible, but it is presented as established fact even though the experiment did not directly test membrane entry or synergy. The student should phrase it more cautiously as an interpretation supported by the pattern, not as a proven mechanism.

Criteria C: Critical Thinking

11/12

Criteria StrandsPro

C.1Research analysis

Moderate

C.2Discussion and evaluation

Moderate

C.3Argument development

Moderate

Criteria Feedback

The student presents a coherent line of argument from rationale to conclusion.
The student analyses the results with quantitative evidence, including means, spread, and statistical testing.
The student identifies several relevant limitations and attempts to explain patterns using biological reasoning.

Some conclusions go beyond what the data and tests can securely show.
The statistical treatment does not always match the central claims, especially where interaction or synergy is implied.
Certain aspects of the design limit how confidently the patterns can be interpreted.

3.1·Strength

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The graph is relevant to the research question and presents the comparison clearly. It gives the reader a concise visual summary of the main treatment pattern before the written analysis.

3.2·Weakness

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The explanation is interesting, but it is speculative because the investigation did not measure stress response directly. The student should distinguish between observed data and proposed explanation more clearly.

3.3·Weakness

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This conclusion overstates what the trendline can prove. A high

R^{2}

suggests a good fit to the plotted points, but it does not by itself establish reliability or validity of the underlying experiment.

3.4·Suggestion

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This is a strong observation. To deepen the analysis, the student could link the overlap more explicitly to the statistical test results and explain whether the apparent pattern is still meaningful despite the variability.

3.5·Weakness

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The inference is too strong. ANOVA shows that at least one group mean differs, but it does not mean every concentration has a significant difference or that one variable influenced the other at all concentrations.

3.6·Weakness

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The notation here appears inconsistent with the earlier treatment label, and the second-degree and fourth-degree polynomial fits should be justified more carefully. A very high

R^{2}

still does not show that the model is biologically meaningful.

3.7·Strength

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The argument is coherent because it ties the pattern in the results to a plausible biological mechanism. This gives the discussion a logical thread rather than listing results in isolation.

3.8·Weakness

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This evaluation is too general to be persuasive. The student should explain which sources were most useful, how they shaped the argument, and whether any sources had limitations or bias.

3.9·Weakness

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The conclusion is stronger than the evidence shown here, because synergy requires a specific interaction analysis or better-controlled comparison. The student should describe this as a possible synergistic pattern unless that claim has been statistically demonstrated.

3.10·Question

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What evidence in this experiment directly demonstrates synergy, rather than simply a larger zone when both substances are present? Answering that would help separate a plausible mechanism from a fully supported conclusion.

3.11·Suggestion

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Consider qualifying the conclusion by noting the methodological limitations that affect how confidently the hypothesis can be accepted. That would show a more balanced evaluation of the evidence.

Criteria D: Presentation

3/4

Criteria StrandsPro

D.1Structure appropriateness

Moderate

D.2Layout and formatting

Moderate

Criteria Feedback

The report includes the major expected sections and is generally easy to navigate.
Headings, tables, graphs, and captions help guide the reader through the investigation.
Most scientific formatting conventions are applied in a recognisable way.

The structure is disrupted by repeated introductory material and inaccurate page references.
Some pagination and layout choices make the document less polished and less consistent.
A few tables, figures, and captions are harder to read or could be formatted more cleanly.

4.1·Weakness

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The student should check that the contents page matches the actual section order and page numbers throughout the report. Inaccurate cross-referencing weakens presentation even when the underlying science is strong.

4.2·Strength

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The report includes a contents page, which helps the reader navigate the investigation’s main sections. That supports clarity and makes the structure easier to follow.

4.3·Weakness

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The structure becomes confusing when section numbering starts over with a second “Introduction” later in the contents. The student should keep the section hierarchy consistent so the report reads as one coherent investigation.

4.4·Weakness

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The contents page appears to give an incorrect page reference for the methodology section. Accurate pagination matters because it helps the reader locate sections efficiently and signals careful organization.

4.5·Weakness

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The section order is understandable, but the duplicated introductory material before this point makes the overall structure less efficient. Streamlining the front matter would improve readability.

4.6·Suggestion

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The student has included sample calculations, which is useful. It would be even clearer if the calculation steps were formatted so the reader can follow the formula, substituted values, and final answer without interruption.

4.7·Weakness

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The graph title is informative, but it is very long and difficult to scan. A shorter title with the essential variables would improve readability while still preserving the scientific detail.

4.8·Strength

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The table title clearly identifies the organism and what the table contains, which helps the reader interpret the data quickly. Clear labeling like this supports good presentation.

4.9·Weakness

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The graph title repeats the same long structure as Graph 1, which makes the presentation repetitive. The student could improve readability by keeping the title concise and using the caption or surrounding text for extra detail.

4.10·Weakness

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The figure is relevant, but the repeated figure label and nearby captioning suggest the layout needs checking for consistency. The student should ensure figure numbering and captions are unique and clearly aligned with the text.

4.11·Suggestion

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The bibliography is clearly separated from the main body, which helps readability. To strengthen presentation further, the student should ensure every in-text citation has a matching, consistently formatted bibliography entry.

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IB Biology SL Extended Essay Example

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