| Date | November 2016 | Marks available | 2 | Reference code | 16N.3.HL.TZ0.3 |
| Level | Higher level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Describe | Question number | 3 | Adapted from | N/A |
Question
The diagram shows the development of potato plants (Solanum tuberosum) over 14 weeks. New tubers start growing from week 9. These are modified underground stems serving as a starch reserve and bearing buds from which new plants arise.
Scientists planted several potato plants in a greenhouse. The sucrose and amino acids in potato plant phloem exudates were measured during several weeks.
Describe briefly how scientists obtained leaf phloem sap from the potato plants.
Suggest reasons for different amounts of sucrose in the leaf phloem sap of the potato plants.
Markscheme
a. aphids insert stylet in «potato» plants/feed from «potato» plants
“Aphids” is essential for the mark.
b. phloem exudates/sap obtained from severed stylets
“Stylets” is essential for the mark.
a. sucrose produced by leaves during photosynthesis
b. sucrose moves/translocates from source/leaves to sink/roots/tubers
OR
sucrose carried by phloem to tuber
c. «wk 5» high sucrose with increased leaf growth/photosynthesis / OWTTE
d. «wk 5-7» more sucrose used for general plant growth / OWTTE
e. «wk 7-11» concentration sucrose increases due to greater production/photosynthesis «than usage/storage» / OWTTE
f. sucrose transformed into starch in tuber «from week 9»
g. contribution of amino acids unknown so difficult to know about different amounts of sucrose / OWTTE
Award the mark for realizing that amino acids play a role in the ratio
h. «abiotic» conditions in greenhouse may vary over time / OWTTE
Accept abiotic factors only if variation through time is explicit.
