Biology

1. 1.

   Explain why oxygen can cross a phospholipid bilayer directly whereas sodium ions usually require a membrane protein.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Permeability depends on charge and lipid solubility, not size alone. A channel provides a hydrophilic route through a barrier that an ion cannot readily enter.

   Marking points

   • The bilayer interior is hydrophobic.
   • Small nonpolar oxygen can dissolve in and diffuse across it.
   • Charged sodium ions are excluded by that interior and need a protein pathway.

   Examiner tip: An ion channel can mediate passive transport without ATP.

2. 2.

   A plant cell is placed in a solution of lower water potential than its cytoplasm. Explain the direction of net water movement and the possible effect on the cell membrane.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The wall does not prevent water loss. As the protoplast shrinks, it can detach from the relatively rigid wall, so the cell loses turgor rather than the wall contracting equally.

   Marking points

   • Water moves out by osmosis down the water-potential gradient.
   • The vacuole and cytoplasm lose volume.
   • The membrane may pull away from the wall during plasmolysis.

   Examiner tip: State water potential, not just 'high concentration', to make direction unambiguous.

3. 3.

   A potato cylinder increases from 2.40 g to 2.70 g in a solution; its surface is blotted consistently before both weighings. Assume mass change is due only to net water movement. Calculate percentage mass change and infer the net water movement.

   [3 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Use the initial mass as denominator: (2.70 - 2.40)/2.40 x 100. Under the stated assumption, positive mass change indicates water entry rather than solute synthesis.

   Marking points

   • Mass gain = 0.30 g.
   • Percentage change = +12.5%.
   • Net water entered the tissue, assuming mass change is due to osmosis.

   Examiner tip: Blot surfaces consistently before weighing so external droplets do not mimic uptake.

4. 4.

   Explain sodium-glucose cotransport in an intestinal epithelial cell, including the indirect role of ATP.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The cotransporter couples a favourable sodium movement to an unfavourable glucose movement. ATP powers the separate pump that restores the sodium gradient, rather than directly binding each glucose transport event.

   Marking points

   • The sodium-potassium pump uses ATP to maintain low intracellular sodium.
   • Sodium enters down its electrochemical gradient through the cotransporter.
   • This drives glucose uptake against its gradient.

   Examiner tip: Distinguish the cotransporter from the ATP-powered pump.

5. 5.

   Beetroot discs heated at different temperatures release pigment. Design controls that allow absorbance to be interpreted as membrane permeability rather than unequal tissue amounts or cutting damage.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Absorbance depends on pigment concentration. Standardising tissue and liquid amounts prevents a concentration difference unrelated to permeability; pre-rinsing removes initial injury leakage before the temperature treatment begins.

   Marking points

   • Use discs of equal dimensions and number from comparable tissue.
   • Rinse discs before exposure to remove pigment released by cutting.
   • Keep solution volume, pH and exposure time constant.
   • Use replicated temperatures and a solvent blank for absorbance.

   Examiner tip: Measure pigment in the bathing solution, not colour remaining in the discs.

6. 6.

   Uptake of a solute plateaus as external concentration rises and falls when respiration is inhibited. Evaluate whether this proves active transport.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate evidence for carrier mediation from evidence for energy coupling. Neither saturation nor an inhibitor alone uniquely identifies the transport mode; gradient direction and cell health constrain alternative explanations.

   Marking points

   • A plateau supports a limited number of saturable transport proteins.
   • Facilitated diffusion can also saturate.
   • Respiration inhibition suggests energy dependence but may have nonspecific cellular effects.
   • Demonstrating uptake against a measured electrochemical gradient with viability controls provides stronger evidence.

   Examiner tip: An inhibitor response is not automatically a specific ATP-dependence test.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.