A-LevelBiologyTransport in plants9700/22 · May/June 2021 · Q27 Marks

Sugars are transported within phloem sieve tubes from a source, such as a mature leaf, to a young leaf, which acts as a sink. The young leaf also needs water and dissolved mineral ions, which arrive at the leaf within xylem vessels.

(a)As the young leaf matures, the quantity of sugar taken up by the leaf decreases to zero, but the need for water increases. Suggest and explain why the quantity of sugar taken up by the developing leaf decreases to zero over time, but the need for water increases.[3]
(b)The features listed in Table 2.1 are present in one or more of the three cell types: • companion cell • phloem sieve tube element • xylem vessel element. Complete Table 2.1 using a tick (✔) if the feature is present and a cross (X) if the feature is absent. Table 2.1 feature companion cell phloem sieve tube element xylem vessel element cytoplasm cell surface membrane lignified cell wall nucleus[4]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[3] any three from decrease in sugar 1 idea that changing / transition, from sink to source e.g. leaf becomes the source leaf no longer the sink sugars are (now) being moved away from leaf was a growing area, now translocating (sugars) 2 photosynthesis provides enough sugar / (rate of) photosynthesis increases ; A leaf (now) makes enough sugar I makes own food increase in water to max 2 3 increase in size / more cells / increase in (leaf) surface area, (so proportionate increase in water); 4 (more cells that need) water to, maintain turgidity / prevent flaccidity / prevent wilting, (because of transpiration); 5 increased transpiration / greater number of stomata ; A increased evaporation (greater internal leaf area) 6 water is, a reactant/ needed, for photosynthesis ; (in context of increased photosynthesis, more water needed) 7 AVP; e.g. maturing leaf growth rate slows / mature leaf no growth (more cells, so more) water needed for, cellular reactions / AW e.g. for hydrolysis of starch (more) water needed for, cell elongation / enlarging vacuoles
[4] one mark each correct row ;;; column mark can be used instead of row mark (if higher mark achieved) [table with rows and columns] cytoplasm: companion cell (✔), phloem sieve tube element (✔), xylem vessel element (X); cell surface membrane: companion cell (✔), phloem sieve tube element (✔), xylem vessel element (X); lignified cell wall: companion cell (X), phloem sieve tube element (X), xylem vessel element (✔); nucleus: companion cell (✔), phloem sieve tube element (X), xylem vessel element (X);