WJEC Biology for AS: Study and Rev Guide

Isla’s answer Ceri’s answer a) Ψ S = Ψ − Ψ P / i.e. -100 -200 ✓ = -300 ✗ q b) (i) Nitrate ions use a carrier protein ✓ because above point B, further increase in ion concentration has no effect on the rate of uptake ✓ w (iii) I would repeat the experiment in the absence of oxygen to see if uptake stopped. ✓ This would confirm that it is active transport as oxygen is needed to produce ATP rather than facilitated diffusion ✓ e a) Ψ S = Ψ − Ψ P / i.e. -100 -200 ✓ = -300 kPa; ✓ b) (i) Nitrate ions are absorbed by diffusion? ✗ because it is directly affected by concentration of nitrate ions between A and B? ✗ q (ii) I would repeat the experiment ✗ w Examiner commentary q Isla did not include units for her part a) answer so lost one mark. w Isla could have included in i) that this is because all the channel proteins are in use. e Part ii) is a perfect answer: It names both methods and a sensible idea of how to distinguish between them based upon the knowledge that active transport requires ATP. Isla achieves 5/6 marks Examiner commentary q Whilst it is true that rate of diffusion is proportional to concentration, this explanation fails to explain what happens at B – C. w Repeating the experiment in part ii) would just provide more evidence and so improve reliability of the conclusion. Ceri achieves 2/6 marks Exam tip Repeating an experiment only improves reliability: it does not give more evidence. Q & A 4 AS Biology: Study and Revision Guide 132 (a) Water enters root hair cells by osmosis. The water potential ( Ψ ) of the soil water is Ψ –100kPa, and the pressure potential ( Ψ P ) inside the root hair cell is +200kPa. Calculate the solute potential ( Ψ S ) of the root hair cell if there is no net movement of water using the formula Ψ = Ψ S + Ψ P . Show your working. [2] b) An experiment was carried out to determine how nitrate ions enter the roots of plants. The results are shown opposite. (i) What conclusion can you draw from the evidence provided as to how nitrate ions are absorbed? [2] (ii) What further evidence would be needed to confirm exactly which method of uptake is involved? [2] Nitrate ion concentration / mg per litre Rate of uptake A B C