DP Biology (first assessment 2025)

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Question 22N.2.HL.TZ0.7c

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Date November 2022 Marks available [Maximum mark: 7] Reference code 22N.2.HL.TZ0.7c
Level HL Paper 2 Time zone TZ0
Command term Explain Question number c Adapted from N/A
c.
[Maximum mark: 7]
22N.2.HL.TZ0.7c
(c)

Explain how carbohydrates are transported from plant leaves.

[7]

Markscheme
  1. translocation/movement by mass flow;
  2. in phloem sieve tubes;
  3. sieve plates/pores in end walls/lack of organelles allows flow (of sap);
  4. carbohydrates (principally) transported as sucrose;
  5. (sucrose/glucose/sugar/carbohydrate) loaded (into phloem) by active transport;
  6. loading/pumping in (of sugars) by companion cells;
  7. high solute concentration generated (at the source);
  8. water enters by osmosis (due to the high solute concentration);
  9. hydrostatic pressure increased/high hydrostatic pressure generated;
  10. pressure gradient causes flow (from source to sink);
  11. leaves are a source because carbohydrates are made there;
  12. transport to the sink where carbohydrates are used/stored;
Examiners report

This was answered well by many candidates, with correct use of terminology. Some answers gave considerable detail about methods used to load assimilate into phloem. The specific structures used for transport (sieve tubes) were not always named and their adaptations were rarely included. Many candidates stated that phloem transport is bidirectional and that xylem transport is not — a hypothesis that has been falsified! Transport in a single phloem sieve tube can only be in one direction at one time, though the direction can be reversed if the hydrostatic pressure gradient switches, for example when a growing leaf changes from being a sink to a source. Xylem transport can also be bidirectional over time, as xylem sap sinks down to the roots of deciduous trees when leaves are lost in the fall and some plants allow xylem sap to sink to the roots every night when transpiration stops.

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