A-LevelBiologyTransport in plants9700/23 · May/June 2022 · Q311 Marks

Hakea spp. are xerophytic plants native to Australia. The leaves of Hakea have adaptations for a xerophytic mode of life.

(a)Fig. 3.1 is a photomicrograph of a transverse section of part of a leaf of H. laurina. [Figure 3.1] Describe the xerophytic features of the waxy cuticle and the stoma shown in Fig. 3.1 and explain how these features adapt the plant to a xerophytic mode of life.[3]
(b)(i)Meristem cells have a similar role to stem cells in animals. Suggest how meristematic tissue activity leads to the formation of root clusters.[3]
(b)(ii)Organic anions (negatively charged organic compounds) are released into the soil by rootlets. The concentration of these organic anions can become higher in the soil solution than in the rootlet cells. Suggest and explain how the concentration of organic anions in the soil solution can become higher than in the rootlet cells.[3]
(b)(iii)Explain how the formation of root clusters can lead to an increase in the uptake of phosphate ions and absorption of water from the soil solution.[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 (May/June 2022). The mark or mark type is shown in brackets.

[3] any three from: decreases / reduces, transpiration; A lower transpiration rate accept in the correct context once only counts towards max two I evaporation max two for cuticle thick (waxy) cuticle; increased waterproof layer; increased (diffusion) distance for water vapour / less water vapour lost ; in context of cuticular transpiration max two for stoma sunken stoma; A stoma in depression / AW moist/humid, air collects in area near to external environment; AW decreases water potential gradient; in context of between sub-stomatal air space or area above stoma and external environment alternative acceptable statement for the transpiration mark reduces diffusion of water vapour (out) via the stoma ;
[3] any three from: (many) mitotic cell cycles / mitoses ; AW ref. to division of (meri)stem cell also produces more (meri)stem cells ; formation of rootlets by mitosis is, growth / increase in number of cells ; idea of producing large number of cells R cell growth differentiation / specialisation ; in context of the new cells formed by stem cell division
[3] any three from: active transport occurring ; (pumped/moved, out) against the(ir) concentration gradient ; (move out through), carrier / pump, protein; R channel protein using, ATP / energy; A have ATPase activity AVP; e.g. membrane impermeable to entry of anions suggestion that move out by exocytosis allow ATP mp if also stated;
[2] any two from: increased surface area; (because, many / more) root hairs / root hair cells ; increase in, carrier proteins / channel proteins (for ion uptake); A transport proteins / aquaporins (for water) R channel proteins AVP; increase in water uptake osmotically to follow ion uptake / AW;