Biology HL

1. 1.

   Compare a pluripotent embryonic stem cell with a multipotent bone-marrow stem cell. Explain how stem-cell niches support tissue renewal and why a differentiated cell can differ in function from another cell with essentially the same genome.

   [4 marks] · no calculator

   Worked explanation

   1. Distinguish developmental potential from the number of times a cell divides.
   2. A niche is a regulating microenvironment, not merely an empty physical location.
   3. Specialization normally alters expression rather than requiring each tissue to lose all unused genes.

   Common mistake: Pluripotent is not synonymous with totipotent, and differentiated cells do not generally discard every gene they are not expressing.

   Marking points

   • Pluripotent cells can form cell types across the body's major tissue lineages, but not alone an entire organism including extraembryonic tissues.
   • Multipotent marrow stem cells have a more restricted range, such as blood-cell lineages.
   • A niche supplies signals that help maintain stem cells or regulate their proliferation and differentiation.
   • Different patterns of gene expression produce different proteins and functions despite essentially shared DNA sequences.

   Examiner tip: Pluripotent is not synonymous with totipotent, and differentiated cells do not generally discard every gene they are not expressing.

2. 2.

   Type I pneumocytes are extremely thin; type II pneumocytes contain many secretory vesicles that release surfactant. Explain the contribution of each type to alveolar function and why having two specialized cell types can be advantageous.

   [4 marks] · no calculator

   Worked explanation

   1. Relate small diffusion distance specifically to the exchange role of type I cells.
   2. Relate secretory machinery to surfactant supply rather than claiming type II cells generate oxygen.
   3. The tissue combines distinct adaptations; not every cell must maximize both thinness and secretion.

   Common mistake: Surfactant reduces surface tension; it does not raise oxygen partial pressure or actively pump gases across the alveolar wall.

   Marking points

   • Thin type I cells shorten the diffusion path for gases across the alveolar wall.
   • Type II vesicles store and secrete surfactant into the alveolar lumen.
   • Surfactant reduces surface tension, reducing the tendency of alveoli to collapse.
   • Division of functions allows a thin exchange surface to coexist with cells equipped for substantial secretion.

   Examiner tip: Surfactant reduces surface tension; it does not raise oxygen partial pressure or actively pump gases across the alveolar wall.

3. 3.

   A skeletal muscle fibre is long and contains many nuclei; a cardiac muscle cell is shorter and branched. Both contain contractile myofibrils. Explain how these observations relate to specialization and why the skeletal fibre challenges an overly simple claim that every animal cell is small with one nucleus.

   [4 marks] · no calculator

   Worked explanation

   1. Identify the shared contractile machinery before comparing architecture.
   2. Link geometry and nuclear number to the functional demands of the large fibre rather than treating it as many separate mature cells in a row.
   3. Use the fibre as a qualification of an oversimplified cell description, not as a refutation that muscle is cellular.

   Common mistake: Several nuclei do not automatically mean a mature fibre is several separate membrane-bounded cells.

   Marking points

   • Contractile myofibrils enable force production in both muscle types.
   • Long skeletal fibres support contraction along a substantial length; multiple nuclei support a large cytoplasmic volume.
   • Branching of cardiac cells supports an interconnected tissue that contracts coordinately.
   • A skeletal fibre is a membrane-bounded multinucleate cellular unit, so a universal one-nucleus/small-cell description has exceptions.

   Examiner tip: Several nuclei do not automatically mean a mature fibre is several separate membrane-bounded cells.

4. 4.

   A hypothetical tissue stem cell divides, producing a daughter that remains a stem cell and a daughter that differentiates. Explain how this can support long-term tissue renewal and why multipotency does not mean the cell can form every body-cell type.

   [3 marks] · no calculator

   Worked explanation

   1. Distinguish maintaining the renewing population from producing a specialised output.
   2. The described division illustrates one possible balance; it does not mean every stem-cell division must produce these exact two outcomes.
   3. Potency describes the range of possible differentiated descendants, not the rate of division.

   Common mistake: Multipotent is not synonymous with pluripotent, and differentiation alone does not maintain a stem-cell pool.

   Marking points

   • Retention of a stem-cell daughter supports self-renewal of the stem-cell pool.
   • The differentiating daughter can replace specialised cells lost from the tissue.
   • Multipotent cells can form a restricted range of related cell types, not all body-cell types.

   Examiner tip: Multipotent is not synonymous with pluripotent, and differentiation alone does not maintain a stem-cell pool.

5. 5.

   Explain how type I and type II pneumocytes contribute differently to gas exchange in mammalian alveoli. Predict a consequence of reduced surfactant secretion without claiming that type II cells normally provide the thinnest gas-diffusion surface.

   [3 marks] · no calculator

   Worked explanation

   1. Identify the gas-diffusion role separately from the surface-tension role.
   2. A thin barrier helps diffusion, whereas surfactant helps keep a usable exchange surface open.
   3. The prediction follows from altered surface tension, not from assuming surfactant itself transports oxygen.

   Common mistake: Surfactant reduces surface tension; it does not carry oxygen across the alveolar wall as a respiratory pigment.

   Marking points

   • Thin type I pneumocytes form much of the alveolar surface, providing a short gas-diffusion path.
   • Type II pneumocytes secrete surfactant, reducing surface tension at the air-liquid interface.
   • Reduced surfactant increases the tendency of alveoli to collapse and the work needed to inflate them, impairing effective gas exchange.

   Examiner tip: Surfactant reduces surface tension; it does not carry oxygen across the alveolar wall as a respiratory pigment.

Original, unofficial practice questions from SubjectScout — not official past-paper questions or complete syllabus coverage. Added English teaching explanations are AI-authored; independent teacher review is pending.