IB Biology Practice Questions and Exams
Build confidence for IB Biology with focused practice that follows the current syllabus. Work through questions by topic, compare your response with a clear worked answer, or generate a complete SL or HL practice exam when you are ready for a timed attempt. IBBase questions use familiar command terms and mark allocations without claiming to reproduce official papers. The aim is straightforward: help you identify what you know, see where marks are being lost, and choose the next area to revise. Start with one of the syllabus themes below, then move from short practice to a full paper at your own pace.
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Sample IB Biology questions
Question 1
Explain why phospholipids form a bilayer when placed in water and how this structure contributes to selective permeability.
Worked answer
The hydrophilic phosphate heads face the water while the hydrophobic fatty-acid tails turn inward, producing a stable bilayer. Its hydrophobic centre allows small non-polar molecules to diffuse through but restricts ions and most polar molecules, which require specific membrane proteins.
Question 2
A mutation changes one amino acid near an enzyme's active site. Explain how this may reduce the rate of an enzyme-controlled reaction.
Worked answer
The substitution can change interactions within the protein and alter the active site's three-dimensional shape or chemistry. The substrate then binds less effectively, so fewer enzyme–substrate complexes form per unit time and the reaction rate falls.
Question 3
Outline why DNA replication is described as semiconservative and state the role of complementary base pairing.
Worked answer
Each original strand acts as a template, so every daughter DNA molecule contains one parental strand and one newly synthesized strand. Complementary pairing—A with T and C with G—determines the nucleotide sequence of each new strand and supports accurate copying.
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