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

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IB Biology SL Extended Essay Example

To what extent do increasing concentrations (0.0%- 2.0%(± 0.1%)) of common culinary spices Allium sativum (garlic), and Curcuma longa (turmeric) extracts, affect the post-Penicillin (100 µg/cm³) regrowth (±5%) of beneficial gut microbiota Lactobacillus acidophilus, as measured by OD₆₀₀ [±0.002 SD] over 48-hour period?SL

Official IB Result

31/34

General feedback

26/28

No overall summary is available for this report.

6.1·Weakness

Page 33• Click to view

The graph title is informative, but the wording is cumbersome and the phrasing "with 5% error bar" is unclear. The student should shorten the title and state exactly what the error bars represent.

Criteria A: Focus and Method

6/6

Criteria StrandsPro

A.1Topic communication and explanation

Good

A.2Research question formulation and focus

Moderate

A.3Methodology appropriateness and completeness

Poor

Criteria Feedback

The student chooses a relevant and researchable biological topic with a clear link between the organism, treatments, and measured outcome.
The student's research question is specific enough to guide an investigation and includes the key variables needed for a practical study.
The student shows awareness of an appropriate basic method by using a measurable growth proxy and a standardized culture approach.
The student connects the topic to a wider biological context in a way that shows clear planning intent.

The focus becomes less precise because the question combines two extracts, a dose variable, and a recovery claim in one line.
Some of the explanation stretches beyond what the method can actually test, especially when moving from single-culture results to gut-level claims.
The rationale for several methodological choices is not consistently justified or clearly reconciled with the procedure.
The proposed method contains enough ambiguity that the independent and dependent variables are not always fully controlled or transparent.

1.1·Strength

Page 1• Click to view

The research question is clearly framed around a specific organism, treatments, concentration range, and measurable outcome. That level of precision gives the investigation a workable direction and shows strong methodological intent.

1.2·Weakness

Page 1• Click to view

The question is still too broad because it combines two extract types, a dose variable, and a recovery claim in one sentence. A narrower version would improve focus, for example by separating garlic and turmeric into distinct comparisons or making the extract type and concentration the two explicit independent variables.

1.3·Weakness

Page 9• Click to view

The wording implies a very specific recovery percentage and antibiotic concentration, but those values are not yet fully justified here. The student should ensure the question matches the actual procedure and controls, otherwise the investigation’s focus and claims become less internally consistent.

1.4·Strength

Page 16• Click to view

The choice of MRS broth is appropriate and clearly linked to the organism under study. This shows that the student selected a medium that supports the target bacterium rather than a generic culture medium.

1.5·Weakness

Page 16• Click to view

The method description overstates what

O D_{600}

can measure. It estimates turbidity and cell density, but not metabolic state directly. The student should describe it as an indirect proxy and acknowledge that color or particulate interference can distort the reading.

1.6·Weakness

Page 17• Click to view

The rationale for

2 5^{\circ}

C is unclear and appears biologically inconsistent with optimal growth conditions for

L . a c i d o p hi l u s

. The student should justify temperature using organism-specific evidence, because an unsupported choice can affect growth and weaken the method’s validity.

1.7·Suggestion

Page 19• Click to view

Consider revising the temperature choice or justifying it with a source specific to

L . a c i d o p hi l u s

. If the goal is to model optimal culture recovery, the incubation temperature should clearly align with the organism’s known growth conditions.

1.8·Weakness

Page 21• Click to view

The justification for the plant source is not convincing. Fresh garlic and turmeric from a farm do not guarantee identical composition, so the student should not present source similarity as controlled unless the extracts are chemically standardized.

1.9·Weakness

Page 23• Click to view

This procedure is clearly described, but the later concentration steps must be consistent with this stock preparation. The student should show how the crude extract dilution produces the final treatment concentrations used in the tubes, since the current logic is difficult to follow.

1.10·Weakness

Page 24• Click to view

The concentration sequence is sensible, but the preparation calculation is not transparent enough to verify the final treatment strength. The student should show step-by-step dilution math so the stated percentages can be traced from stock extract to culture tube.

1.11·Weakness

Page 27• Click to view

Using tablets as the antibiotic source introduces uncertainty because excipients may be present alongside the active compound. A clearer method would justify tablet use or, better, use a characterized pure penicillin preparation so the independent variable is unambiguous.

1.12·Weakness

Page 28• Click to view

The stated stock concentration is plausible, but the final concentration in each culture is still unclear after later dilution steps. The student should calculate and report the actual penicillin concentration in the test tubes, not only the stock concentration.

1.13·Suggestion

Page 30• Click to view

The student could strengthen the method by adding a brief limitation statement here:

O D_{600}

is useful for comparing turbidity, but it cannot distinguish live from dead cells. That would make the interpretation more scientifically careful.

1.14·Weakness

Page 30• Click to view

This blank choice is incomplete for crude plant extracts. Because garlic and turmeric can add their own turbidity or absorbance, the student should use extract-matched blanks so that the OD readings reflect bacterial growth rather than extract colour or particles.

Criteria B: Knowledge and Understanding

4/6

Criteria StrandsPro

B.1Application of source material

Moderate

B.2Use of terminology and concepts

Moderate

Criteria Feedback

The student uses a strong range of scientific terminology and most of it is applied appropriately.
The student shows accurate understanding of key microbiology ideas such as beta-lactam action, growth measurement, and culture conditions.
The student draws on source material to support the biological background for the extracts and the bacterium.
The student's discussion generally shows that they understand the main concepts behind the investigation rather than relying on description alone.

Some terminology is used too broadly, especially when describing turbidity, viability, and metabolic state as if they were the same thing.
The discussion sometimes treats single-species results as if they directly represent gut microbiome behaviour.
Several source-based claims are relevant in a general sense but are not always matched closely enough to the exact in-vitro system used here.
A few biological statements are imprecise or overextended, which weakens consistency.

2.1·Strength

Page 10• Click to view

The student uses source material to connect the two plants to plausible bioactive compounds and a microbiota-related rationale. That creates a clear factual foundation for the investigation rather than leaving the choice of extracts unexplained.

2.2·Weakness

Page 10• Click to view

This sentence blends two different ideas without clearly distinguishing them: antimicrobial action and potential prebiotic support. The student should separate these mechanisms and explain which one is actually relevant to the single-species assay used in the experiment.

2.3·Weakness

Page 12• Click to view

The explanation is mostly correct, but it treats

O D_{600}

as if it were directly proportional to bacterial growth in every circumstance. The student should note that turbidity is only an indirect proxy and can be affected by extract particles or cell aggregation.

2.4·Weakness

Page 13• Click to view

The discussion of gut bacteria is broadly relevant, but the figure-based explanation risks oversimplifying the organism’s role. The student should be careful not to treat a single cultured strain as equivalent to the full gut microbiome.

2.5·Weakness

Page 15• Click to view

The causal link is too direct here. A decrease in absorbance may reflect reduced cell density, but it does not by itself prove gut-population decline or a specific biological mechanism. The student should keep the claim tied to the in-vitro culture result.

2.6·Weakness

Page 16• Click to view

The student should revise the terminology here.

O D_{600}

estimates turbidity/cell density; it does not measure metabolic state directly. More precise terminology would improve the scientific accuracy of the explanation.

2.7·Weakness

Page 19• Click to view

This is a useful attempt to explain control variables, but the wording is vague and some effects are not biologically precise. The student should explain exactly how temperature affects enzyme activity and growth rate, and why the chosen temperature is appropriate for the bacterium.

2.8·Strength

Page 20• Click to view

This is a strong use of microbiological terminology because it shows awareness that growth depends on environmental pH. The student’s method is more convincing when control variables are tied to a concrete range rather than described vaguely.

2.9·Weakness

Page 27• Click to view

The claim goes a little too far. Matching a McFarland standard helps standardize inoculum density, but it does not guarantee identical culture behavior across treatments. The student should describe it as improved consistency rather than complete uniformity.

2.10·Question

Page 43• Click to view

What evidence supports calling

L a c t o ba c i l l u s

aerobic here? If the student rechecks this classification, the explanation of why SCFAs might influence the growth environment will become much more accurate.

2.11·Weakness

Page 56• Click to view

This glossary entry is accurate and concise, which is helpful. The student should apply the same precision earlier in the report, where

O D_{600}

is sometimes described as if it measured viability or metabolic state directly.

Criteria C: Critical Thinking

12/12

Criteria StrandsPro

C.1Research analysis

Poor

C.2Discussion and evaluation

Poor

C.3Argument development

Poor

Criteria Feedback

The student builds a recognizable argument from context, method, results, and evaluation, so the report has a clear line of development.
The student identifies a coherent pattern in the data and relates it back to their question rather than simply describing the numbers.
The student attempts quantitative analysis, including means, variation, and formal statistical testing, which shows initiative and analytical ambition.
The student includes an evaluation that acknowledges several real limitations and suggests sensible improvements.
The student's conclusion does answer the investigation in a direct way and stays linked to the observed pattern.

The argument is not always tightly controlled, because several mechanisms are discussed without clearly separating what the experiment can and cannot show.
Some statistical treatment is not fully convincing, especially where model fit and visual patterns are interpreted too strongly.
The analysis depends on OD values without sufficient controls for extract colour or turbidity, which limits confidence in the findings.
The conclusion is sometimes more confident than the evidence allows, especially when it shifts toward therapeutic or microbiome-wide claims.

3.1·Strength

Page 32• Click to view

The student shows the calculation behind the mean, which makes the processing transparent and easier to check. Showing at least one worked example is good practice because it demonstrates how the raw values were transformed.

3.2·Weakness

Page 33• Click to view

The calculation is difficult to follow and mixes several ideas at once. The student should rewrite it in a clean step-by-step format, clearly define the reference control, and show how uncertainty was propagated so the percentage growth value can be verified.

3.3·Weakness

Page 33• Click to view

The regression is not presented clearly enough to interpret confidently. The student should specify whether the model is quadratic, show the equation in standard form, and explain why a polynomial fit is biologically justified with only five concentration levels.

3.4·Weakness

Page 35• Click to view

This interpretation is incorrect because

R^{2}

does not by itself establish a linear relationship, and the data are explicitly non-linear. The student should describe

R^{2}

as model fit, not as proof of linearity or causation.

3.5·Weakness

Page 35• Click to view

Overlapping error bars do not automatically show statistical non-significance. The student should rely on the reported statistical test results instead, or state this only as a visual hint rather than a conclusion.

3.6·Weakness

Page 38• Click to view

This uncertainty discussion misses a major source of error: extract color and turbidity affecting

O D_{600}

. The student should also quantify or at least acknowledge systematic uncertainty from the blank, since it directly affects the validity of the growth analysis.

3.7·Strength

Page 39• Click to view

The student attempts a higher-level statistical test and reports both the

F

statistic and

p

value. That shows initiative and an understanding that the data need formal comparison rather than visual inspection alone.

3.8·Weakness

Page 39• Click to view

The statistical reporting is incomplete because the assumptions and full ANOVA table are not shown. The student should report degrees of freedom, factor structure, and assumption checks so the reader can judge whether the test was applied appropriately.

3.9·Weakness

Page 41• Click to view

The reasoning moves too quickly from turbidity to therapeutic efficacy. The student should limit the claim to measured culture density, because efficacy cannot be established without viability controls, extract blanks, and a broader biological endpoint.

3.10·Weakness

Page 44• Click to view

The conclusion is stronger than the evidence because it assumes the OD increase represents microbial regrowth rather than a combined effect of biomass, extract interference, or residual antibiotic conditions. The student should qualify the conclusion to match the limits of the assay.

3.11·Strength

Page 45• Click to view

This is a strong evaluative statement because it correctly identifies a key weakness of the method. The student shows genuine critical awareness here by distinguishing what the assay measures from what it cannot prove.

3.12·Weakness

Page 46• Click to view

This is a good improvement idea, but it would be stronger if tied directly to the current conclusion. The student should explain that such methods would test whether the observed OD changes reflect viable recovery rather than simple turbidity shifts.

Criteria D: Presentation

4/4

Criteria StrandsPro

D.1Structure appropriateness

Good

D.2Layout and formatting

Moderate

Criteria Feedback

The student's report has a clear overall structure and is easy to follow from introduction through conclusion.
Tables, graphs, and section headings help the reader move through the investigation logically.
The student includes the main components expected in a formal research report, including supporting lists and reference material.
Page numbering and other basic navigation features are present.

The section order becomes inconsistent in places, which makes the document feel less polished.
Some figures and references are not formatted with enough consistency or precision.
The graph and some labels are harder to read or interpret than they should be.
The bibliography section is present but needs much tighter formatting and consistency.

4.1·Strength

Page 2• Click to view

The report has a clear signposted structure, and the contents page makes the investigation easy to navigate. That improves readability and helps the reader follow the progression from background to evaluation.

4.2·Weakness

Page 2• Click to view

This heading appears out of sequence relative to the surrounding section numbers. Consistent numbering matters because it helps the reader track the argument and signals careful organization.

4.3·Weakness

Page 2• Click to view

The section numbering is inconsistent, with repeated and out-of-order numbering later on the contents page. The student should renumber sections so the contents page matches the body text precisely.

4.4·Weakness

Page 3• Click to view

The glossary is useful, but its placement after the bibliography and supporting lists disrupts the report flow. The student should group appendices or supplementary material more deliberately so the main argument remains central.

4.5·Weakness

Page 3• Click to view

The section comes after the bibliography, which makes the overall order less intuitive. Supporting lists such as apparatus and chemicals are usually placed before the bibliography so the end of the report feels conclusive.

4.6·Strength

Page 18• Click to view

Major sections such as the hypothesis are clearly labelled, which helps the reader understand the logic of the investigation. This consistent signposting is a strength in presentation.

4.7·Strength

Page 31• Click to view

The move from methodology to data collection is clearly marked, which supports a logical progression through the report. The student maintains a recognisable investigation structure.

4.8·Strength

Page 32• Click to view

The tables organise raw and processed values clearly, which helps the reader see the pattern across concentrations. Presenting the raw trials alongside the mean and SD is a strong presentation choice.

4.9·Weakness

Page 33• Click to view

The graph is useful because it compares both extracts in one place, but the title and displayed trendlines make interpretation harder than necessary. The student should keep the visual simpler and make sure the plotted model matches the data actually collected.

4.10·Weakness

Page 47• Click to view

The bibliography is clearly separated as its own section, which is helpful. However, the entries beneath it are not consistently formatted, so the section works structurally but still needs tighter presentation discipline.

4.11·Weakness

Page 47• Click to view

This reference list is populated with relevant sources, but repeated entries and inconsistent citation details reduce precision. The student should standardise every citation and remove duplicates to make the bibliography easier to trust and verify.

Loading scores...

Official IB Result

31/34

General feedback

26/28

No overall summary is available for this report.

6.1·Weakness

Page 33• Click to view

The graph title is informative, but the wording is cumbersome and the phrasing "with 5% error bar" is unclear. The student should shorten the title and state exactly what the error bars represent.

Criteria A: Focus and Method

6/6

Criteria StrandsPro

A.1Topic communication and explanation

Good

A.2Research question formulation and focus

Moderate

A.3Methodology appropriateness and completeness

Poor

Criteria Feedback

The student chooses a relevant and researchable biological topic with a clear link between the organism, treatments, and measured outcome.
The student's research question is specific enough to guide an investigation and includes the key variables needed for a practical study.
The student shows awareness of an appropriate basic method by using a measurable growth proxy and a standardized culture approach.
The student connects the topic to a wider biological context in a way that shows clear planning intent.

The focus becomes less precise because the question combines two extracts, a dose variable, and a recovery claim in one line.
Some of the explanation stretches beyond what the method can actually test, especially when moving from single-culture results to gut-level claims.
The rationale for several methodological choices is not consistently justified or clearly reconciled with the procedure.
The proposed method contains enough ambiguity that the independent and dependent variables are not always fully controlled or transparent.

1.1·Strength

Page 1• Click to view

1.2·Weakness

Page 1• Click to view

1.3·Weakness

Page 9• Click to view

1.4·Strength

Page 16• Click to view

1.5·Weakness

Page 16• Click to view

The method description overstates what

O D_{600}

1.6·Weakness

Page 17• Click to view

The rationale for

2 5^{\circ}

C is unclear and appears biologically inconsistent with optimal growth conditions for

L . a c i d o p hi l u s

. The student should justify temperature using organism-specific evidence, because an unsupported choice can affect growth and weaken the method’s validity.

1.7·Suggestion

Page 19• Click to view

Consider revising the temperature choice or justifying it with a source specific to

L . a c i d o p hi l u s

. If the goal is to model optimal culture recovery, the incubation temperature should clearly align with the organism’s known growth conditions.

1.8·Weakness

Page 21• Click to view

1.9·Weakness

Page 23• Click to view

1.10·Weakness

Page 24• Click to view

1.11·Weakness

Page 27• Click to view

1.12·Weakness

Page 28• Click to view

1.13·Suggestion

Page 30• Click to view

The student could strengthen the method by adding a brief limitation statement here:

O D_{600}

is useful for comparing turbidity, but it cannot distinguish live from dead cells. That would make the interpretation more scientifically careful.

1.14·Weakness

Page 30• Click to view

Criteria B: Knowledge and Understanding

4/6

Criteria StrandsPro

B.1Application of source material

Moderate

B.2Use of terminology and concepts

Moderate

Criteria Feedback

The student uses a strong range of scientific terminology and most of it is applied appropriately.
The student shows accurate understanding of key microbiology ideas such as beta-lactam action, growth measurement, and culture conditions.
The student draws on source material to support the biological background for the extracts and the bacterium.
The student's discussion generally shows that they understand the main concepts behind the investigation rather than relying on description alone.

Some terminology is used too broadly, especially when describing turbidity, viability, and metabolic state as if they were the same thing.
The discussion sometimes treats single-species results as if they directly represent gut microbiome behaviour.
Several source-based claims are relevant in a general sense but are not always matched closely enough to the exact in-vitro system used here.
A few biological statements are imprecise or overextended, which weakens consistency.

2.1·Strength

Page 10• Click to view

2.2·Weakness

Page 10• Click to view

2.3·Weakness

Page 12• Click to view

The explanation is mostly correct, but it treats

O D_{600}

2.4·Weakness

Page 13• Click to view

2.5·Weakness

Page 15• Click to view

2.6·Weakness

Page 16• Click to view

The student should revise the terminology here.

O D_{600}

estimates turbidity/cell density; it does not measure metabolic state directly. More precise terminology would improve the scientific accuracy of the explanation.

2.7·Weakness

Page 19• Click to view

2.8·Strength

Page 20• Click to view

2.9·Weakness

Page 27• Click to view

2.10·Question

Page 43• Click to view

What evidence supports calling

L a c t o ba c i l l u s

aerobic here? If the student rechecks this classification, the explanation of why SCFAs might influence the growth environment will become much more accurate.

2.11·Weakness

Page 56• Click to view

This glossary entry is accurate and concise, which is helpful. The student should apply the same precision earlier in the report, where

O D_{600}

is sometimes described as if it measured viability or metabolic state directly.

Criteria C: Critical Thinking

12/12

Criteria StrandsPro

C.1Research analysis

Poor

C.2Discussion and evaluation

Poor

C.3Argument development

Poor

Criteria Feedback

The student builds a recognizable argument from context, method, results, and evaluation, so the report has a clear line of development.
The student identifies a coherent pattern in the data and relates it back to their question rather than simply describing the numbers.
The student attempts quantitative analysis, including means, variation, and formal statistical testing, which shows initiative and analytical ambition.
The student includes an evaluation that acknowledges several real limitations and suggests sensible improvements.
The student's conclusion does answer the investigation in a direct way and stays linked to the observed pattern.

The argument is not always tightly controlled, because several mechanisms are discussed without clearly separating what the experiment can and cannot show.
Some statistical treatment is not fully convincing, especially where model fit and visual patterns are interpreted too strongly.
The analysis depends on OD values without sufficient controls for extract colour or turbidity, which limits confidence in the findings.
The conclusion is sometimes more confident than the evidence allows, especially when it shifts toward therapeutic or microbiome-wide claims.

3.1·Strength

Page 32• Click to view

3.2·Weakness

Page 33• Click to view

3.3·Weakness

Page 33• Click to view

3.4·Weakness

Page 35• Click to view

This interpretation is incorrect because

R^{2}

does not by itself establish a linear relationship, and the data are explicitly non-linear. The student should describe

R^{2}

as model fit, not as proof of linearity or causation.

3.5·Weakness

Page 35• Click to view

3.6·Weakness

Page 38• Click to view

This uncertainty discussion misses a major source of error: extract color and turbidity affecting

O D_{600}

. The student should also quantify or at least acknowledge systematic uncertainty from the blank, since it directly affects the validity of the growth analysis.

3.7·Strength

Page 39• Click to view

The student attempts a higher-level statistical test and reports both the

F

statistic and

p

value. That shows initiative and an understanding that the data need formal comparison rather than visual inspection alone.

3.8·Weakness

Page 39• Click to view

3.9·Weakness

Page 41• Click to view

3.10·Weakness

Page 44• Click to view

3.11·Strength

Page 45• Click to view

3.12·Weakness

Page 46• Click to view

Criteria D: Presentation

4/4

Criteria StrandsPro

D.1Structure appropriateness

Good

D.2Layout and formatting

Moderate

Criteria Feedback

The student's report has a clear overall structure and is easy to follow from introduction through conclusion.
Tables, graphs, and section headings help the reader move through the investigation logically.
The student includes the main components expected in a formal research report, including supporting lists and reference material.
Page numbering and other basic navigation features are present.

The section order becomes inconsistent in places, which makes the document feel less polished.
Some figures and references are not formatted with enough consistency or precision.
The graph and some labels are harder to read or interpret than they should be.
The bibliography section is present but needs much tighter formatting and consistency.

4.1·Strength

Page 2• Click to view

4.2·Weakness

Page 2• Click to view

This heading appears out of sequence relative to the surrounding section numbers. Consistent numbering matters because it helps the reader track the argument and signals careful organization.

4.3·Weakness

Page 2• Click to view

The section numbering is inconsistent, with repeated and out-of-order numbering later on the contents page. The student should renumber sections so the contents page matches the body text precisely.

4.4·Weakness

Page 3• Click to view

4.5·Weakness

Page 3• Click to view

4.6·Strength

Page 18• Click to view

Major sections such as the hypothesis are clearly labelled, which helps the reader understand the logic of the investigation. This consistent signposting is a strength in presentation.

4.7·Strength

Page 31• Click to view

The move from methodology to data collection is clearly marked, which supports a logical progression through the report. The student maintains a recognisable investigation structure.

4.8·Strength

Page 32• Click to view

4.9·Weakness

Page 33• Click to view

4.10·Weakness

Page 47• Click to view

4.11·Weakness

Page 47• Click to view