A-LevelBiologyTransport in plants9700/22 · Feb/Mar 2023 · Q313 Marks

Fig. 3.1 is a diagram of an area of phloem tissue from a transverse section through the stem of a squash plant, Cucurbita pepo. X Y companion cell Fig. 3.1

(a)(i)Cell X and cell Y in Fig. 3.1 are sieve tube elements. Explain why cell X and cell Y have very different appearances in this transverse section.[2]
(a)(ii)Sucrose is formed from the glucose synthesised by mesophyll cells in the leaves of C. pepo. Explain how companion cells are involved in the transfer of sucrose into phloem sieve tubes.[4]
(b)(i)Hydrogen bonding is important in the movement of water in xylem. Explain how hydrogen bonding occurs between two water molecules.[2]
(b)(ii)Outline how hydrogen bonding is involved in water transport in the xylem of a plant stem.[3]
(b)(iii)Hydrogen bonding between water molecules gives water a relatively high latent heat of vaporisation. Suggest why it is important to plants that water has a high latent heat of vaporisation.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2023). The mark or mark type is shown in brackets.

[2] Question 3(a)(i) Answer cell Y shows, a sieve plate / sieve pores; ora idea that the sieve plates are at different heights in the stem / the section misses the sieve plate in cell X/AW;
[4] Question 3(a)(ii) Answer any four from: 1 (protein in) companion cell surface membrane, moves / pumps, protons into, cell wall / apoplast; 2 using ATP / by active transport ; 3 establishes a proton gradient ; 4 protons move into companion cell, by facilitated diffusion ; A described e.g. down the, electrochemical / concentration, gradient 5 protons cotransport sucrose / protons and sucrose pass through a cotransporter (protein), into companion cell ; 6 sucrose moves against its concentration gradient ; 7 diffusion of sucrose into phloem sieve tube, through plasmodesmata ; A all marking points from an annotated diagram
[2] Question 3(b)(i) Answer this could be answered diagrammatically or in text, e.g. [Image showing H-O-H with δ+ on H and δ- on O, and a dashed line from δ- O to δ+ H of another molecule] any two from: idea that oxygen has a small negative charge and hydrogen has a small positive charge; idea that a hydrogen bond forms between the oxygen of one water molecule and the hydrogen of a neighbouring water molecule; AVP; e.g. oxygen is more electronegative than hydrogen
[3] Question 3(b)(ii) Answer any three from: 1 ref. to cohesion between water molecules ; 2 ref. to adhesion / hydrogen bonding, between water molecules and, cellulose / hydrophilic parts of lignin (in cell wall) ; A hydrophilic parts of cell wall 3 formation of continuous column / AW; 4 (column) pulled up by, transpiration / evaporation / transpiration pull;
[2] Question 3(b)(iii) Answer any two from: ref. to plant leaves heat up, when light absorbed for photosynthesis / at high (ambient) temperatures; (high latent heat of vaporisation leads to large cooling effect / removal of (large quantities of) heat energy, by evaporation ; reduces, protein / enzyme, denaturation ; reduces rate of water loss by, transpiration / evaporation, at high temperatures ; AVP;