A-LevelBiologyControl and coordination9700/41 · Oct/Nov 2021 · Q512 Marks

Gibberellin is a plant growth regulator involved in barley seed germination. Production of gibberellin is stimulated by the uptake of water. State the location of gibberellin synthesis in a barley seed during germination.

(a)Gibberellin is a plant growth regulator involved in barley seed germination. Production of gibberellin is stimulated by the uptake of water. State the location of gibberellin synthesis in a barley seed during germination.[1]
(b)(i)Barley seeds germinate when placed on blotting (absorbent) paper soaked in water. The germination of barley seeds placed on blotting paper soaked in solutions of different water potential was investigated. The success of germination was measured as a germination index for: • barley seeds placed on blotting paper soaked in water • barley seeds placed on blotting paper soaked in 5 solutions of different water potential. The results are shown in Fig. 5.1. The higher the germination index value, the more successful the germination of the barley seeds. [Figure 5.1] With reference to Fig. 5.1, describe the relationship between the germination index of barley seeds and water potential.[2]
(b)(ii)Suggest explanations for the relationship shown in Fig. 5.1.[3]
(c)During barley seed germination, gibberellin stimulates the synthesis of enzymes. State the name of one of these enzymes and the precise location of its synthesis. enzyme location[2]
(d)(i)Some plants are grown commercially for their flowers. Many of these plants are varieties that have short stems. Two factors that affect the height of stems are: • gibberellin • the Le/le gene. The Le/le gene has two alleles, Le and le. Suggest an advantage of growing a short-stemmed variety of a flowering plant.[1]
(d)(ii)Explain how the Le/le gene and gibberellin are involved in affecting the height of plant stems.[3]

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 (Oct/Nov 2021). The mark or mark type is shown in brackets.

[1] embryo;
[1] as water potential increases the germination index increases; ora
[1] water potential in MPa and index figure given for two points ;
[1] low / negative, water potential decreases water, uptake / in, seed; ora
[1] water, needed / used, to activate embryo / produce gibberellin ;
[1] water, needed / used, for hydrolysis (reactions);
[1] starch to maltose / maltose to glucose ;
[1] water, needed / used, as a medium for reactions;
[1] solutes (lowering water potential) may, be toxic / inhibit enzymes ;
[1] AVP;
[1] amylase / maltase / protease ;
[1] aleurone layer;
[1] less, energy / resources, to make stem so, more to / larger, flowers or less likely to be damaged by, wind / storm / heavy rain or takes less, water / fertiliser / time to flower ;
[1] short / dwarf, plants: are, homozygous recessive / lele ;
[1] le / recessive allele, codes for, non-functional / no, enzyme ;
[1] le / recessive allele / non-functional enzyme, gives inactive gibberellin ;
[1] DELLA proteins, not broken down / stay bound to PIF ;
[1] PIF cannot, bind to promoter / start transcription ;
[1] of gene(s) that promote, growth / cell division / cell elongation ;