Question 22N.2.HL.TZ0.7
| Date | November 2022 | Marks available | [Maximum mark: 15] | Reference code | 22N.2.HL.TZ0.7 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Describe, Draw, Explain | Question number | 7 | Adapted from | N/A |
A wide variety of organic compounds are used by living organisms.
Draw a diagram to show the ring structure of D-ribose.
[3]
- ring with four carbons and one oxygen atom;
- CH2OH attached to C4;
- OH and H attached by single bonds to C1, C2 and C3 with OH facing downwards on C2 and C3;
Numbering of carbons not required for mpa.
Carbons in the ring can be shown as a junction of single bonds without the letter C, but oxygen must be shown as O.
For mpc OH can face up or down.
Well-prepared candidates were able to draw the structure of ribose with every atom correct. Most others got at least part of the molecule. Common faults were putting six atoms in the ring or drawing deoxyribose instead of ribose.
Describe how ATP is produced by Photosystem II in the light-dependent stage of photosynthesis.
[5]
- light (energy) absorbed by pigments/chlorophyll/photosystems;
- excited electrons passed to electron carriers/electron transport chain;
- protons/hydrogen ions pumped into thylakoid (space);
- proton gradient/high proton concentration generated;
- protons pass via ATP synthase to the stroma;
- ATP synthase phosphorylates ADP/ATP synthase converts ADP to ATP;
- photophosphorylation/chemiosmosis;
- ATP synthase/electron carriers/proton pumps/photosystems/pigment are in the thylakoid membrane;
There was evidence that many candidates had prepared carefully for this topic and were fully familiar with the sequence of events in the light-dependent reactions that result in ATP production. In some of the weaker answers the protons were moving in the wrong direction and some candidates were confused about the differences in structure between chloroplasts and mitochondria and therefore the nature of the proton gradient.
Explain how carbohydrates are transported from plant leaves.
[7]
- translocation/movement by mass flow;
- in phloem sieve tubes;
- sieve plates/pores in end walls/lack of organelles allows flow (of sap);
- carbohydrates (principally) transported as sucrose;
- (sucrose/glucose/sugar/carbohydrate) loaded (into phloem) by active transport;
- loading/pumping in (of sugars) by companion cells;
- high solute concentration generated (at the source);
- water enters by osmosis (due to the high solute concentration);
- hydrostatic pressure increased/high hydrostatic pressure generated;
- pressure gradient causes flow (from source to sink);
- leaves are a source because carbohydrates are made there;
- transport to the sink where carbohydrates are used/stored;
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.
