The same five mistakes appear in IB Biology exam scripts year after year. They are not caused by gaps in knowledge. They are caused by habits that feel correct during revision but fail under exam conditions — because revision is low-stakes and exams are not.
Each of the five mistakes below is entirely preventable once you know to look for it.
This is the most expensive mistake in IB Biology. A student knows a topic well, sees a familiar keyword in the question, and writes everything they know about that keyword — ignoring the command term and the specific scope of the question.
Example: the question says "Outline the process of transcription." The student writes a detailed account of both transcription and translation. Result: zero extra marks for the translation content, and less time available for questions they could have scored on.
The fix: Before writing anything, underline the command term and circle the specific process or structure being asked about. Write only what those two constraints require.
"Describe" asks you to state what happens, in sequence if necessary. "Explain" asks you to state what happens and give the mechanistic reason why. A description of competitive inhibition names what occurs. An explanation of competitive inhibition states what occurs and explains why — at the molecular level.
The fix: When you see "Explain," train yourself to append the word because after every statement. If you cannot complete the sentence with a mechanistic reason, you have written a description, not an explanation.
IB examiners require that comparison questions address the same feature for both subjects in the same sentence or adjacent sentences. Listing features of Subject A and then features of Subject B in separate paragraphs is not a comparison and will not earn comparison marks.
Wrong: "Mitosis produces two cells. Meiosis produces four cells with genetic variation."
Right: "Mitosis produces two genetically identical daughter cells, whereas meiosis produces four genetically distinct haploid daughter cells."
When a question asks you to "draw" or "sketch," the diagram must satisfy specific criteria: it must be large enough to label clearly, straight lines must be drawn with a ruler, and labels must use a ruled horizontal line leading to the structure — not a freehand arrow.
The most commonly penalised errors: labels with no clear leader line, structures that are not recognisably distinct from each other, and diagrams so small that labelling becomes impossible.
The fix: Reserve at least one-third of the available space for your diagram. Practise drawing the ten most frequently examined structures — mitochondrion, chloroplast, cell surface membrane cross-section, DNA double helix — until they are automatic.
Paper 2 data-based questions are designed to be answerable using only the provided data — even for an organism or experiment you have never encountered. Students who rely entirely on memorised knowledge, rather than the data in front of them, frequently miss what the question is asking.
The specific numbers, trends, and anomalies in the graph or table are the answer. A question asking you to "suggest a reason for the result at 40°C" requires you to describe what the data shows at 40°C, then anchor your biological explanation to that specific data point.
The fix: Before answering any data-based question, spend 20 seconds annotating the graph or table. Mark the peak, the plateau, the anomalies, and the direction of the trend. These annotations become the scaffolding for your answer and prevent you from drifting into generic biology.