IB DP Biology Theme B: Form and Function — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

Related: Theme B: Form and Function study guide and revision notes

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Section A

1. Explain why cellulose’s molecular structure suits it to a structural role in plant cell walls, while starch’s structure suits it to energy storage. [2]

2. State what determines a protein’s specific function, according to this theme’s central idea. [2]

3. Name one passive and one active method by which substances cross a cell membrane, and state the key difference between them. [2]

Section B

4. A cell biologist observes that a liver cell and a skin cell in the same organism contain identical DNA but produce very different sets of proteins.

(a) Explain how this is possible in terms of gene expression. [2] (b) State what this shows about the relationship between differentiation and genetic material. [1]

5. A student is asked to explain why alveoli are efficient gas exchange surfaces.

(a) Name two properties shared by efficient gas-exchange surfaces. [2] (b) For each property named, explain why it increases the rate of diffusion. [2]

6. Explain why dividing a eukaryotic cell into membrane-bound compartments (organelles) is functionally useful, giving two distinct reasons. [2]

7. (HL only) Outline how the structure of contractile proteins and the movement of ions across membranes together enable muscle contraction, referring to the sliding-filament mechanism. [3]

8. A desert plant has unusually small, thick leaves with a waxy surface. Explain how this structural adaptation suits the plant to its environment. [3]

9. A phospholipid bilayer allows small non-polar molecules to cross freely but requires specific transport proteins for larger or charged molecules. Explain this difference in terms of the bilayer’s structure. [3]

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Answers

1. Cellulose’s specific bonding arrangement produces long, strong, non-branching chains, which suits it to providing structural strength in plant cell walls [1]. Starch’s structure is more compact and can be readily broken down when energy is needed, which suits it to storage rather than a structural role [1].

2. A protein’s specific function is determined by its three-dimensional folded shape [1], which is itself determined by its underlying sequence of amino acids [1].

3. Passive: diffusion, facilitated diffusion, or osmosis, all of which move substances down a concentration gradient without requiring energy [1]. Active: active transport, which requires ATP and can move substances against a concentration gradient [1]. The key difference is whether energy input is required.

4. (a) Both cells contain the same genes, but differential gene expression means each cell type expresses only the subset of genes relevant to its own specialised role [1], so a liver cell and a skin cell produce different proteins despite an identical genome [1]. (b) Differentiation occurs through differences in which genes are expressed, not through cells gaining or losing genetic material [1].

5. (a) Any two of: permeability, thin tissue layer, moisture, large surface area [2]. (Blood supply and ventilation are not properties of the surface; they maintain the concentration gradient across it, so they do not answer this part.) (b) For example: a large surface area provides more area across which diffusion can occur simultaneously [1]; a thin tissue layer shortens the diffusion distance the gas must travel [1]. (Also creditable, one mark each: moisture lets gases dissolve before diffusing across; permeability lets oxygen and carbon dioxide pass through the surface.)

6. Compartmentalization allows chemically incompatible processes to occur simultaneously in separate parts of the cell [1], and allows each organelle to maintain the specific internal conditions (such as pH or enzyme concentration) that its function requires, rather than every process sharing one set of conditions [1].

7. The structure of contractile proteins (such as actin and myosin) allows filaments to slide past one another when triggered [1]. This triggering occurs through the movement of ions (such as calcium) across membranes, which initiates the interaction between the contractile proteins [1]. Together, ion movement across membranes and the sliding of the protein filaments past each other produce the shortening of the muscle fibre described by the sliding-filament mechanism [1].

8. Small, thick leaves reduce the leaf’s surface area relative to its volume, which reduces the area available for water loss through transpiration [1]. A waxy surface (cuticle) provides an additional barrier that further reduces water loss by limiting evaporation directly through the leaf surface [1]. Together, these structural features suit the plant to surviving in an environment where water conservation is a major selective pressure, directly illustrating this theme’s structure-function organising question [1].

9. The phospholipid bilayer has hydrophilic phosphate heads facing outward on both surfaces and hydrophobic fatty-acid tails facing inward [1]. Small non-polar molecules can pass relatively freely through the hydrophobic interior of the bilayer [1], whereas larger or charged (polar) molecules cannot cross this hydrophobic interior directly and instead require specific transport proteins embedded in the membrane to cross [1].

Why these question styles recur

Section B questions in this set deliberately mirror the applied style DP Biology exams use across Theme B: rather than asking for a bare definition, each question gives a specific scenario (a named cell type, a named organism, a described structure) and asks the structure-function link to be explained in relation to that scenario specifically. Practising this applied form – rather than only memorising the underlying facts in isolated form – is what best prepares a Theme B response for the scenario-based style Paper 1 and Paper 2 consistently use.