A-LevelBiologyControl and coordination9700/42 · Oct/Nov 2018 · Q813 Marks

Gibberellin is a plant growth hormone that has a role in germination and in stem elongation.

(a)Outline how gibberellin is involved in activating genes for stem elongation.[2]
(b)Outline the role played by gibberellin in the germination of wheat seeds.[4]
(c)The length of stem in pea plants is controlled by a single gene. Pea plants can be either tall or short. A study was carried out to investigate the effect of applying gibberellin to short pea plants. Two groups of short pea seedlings were used, group P and group Q. • Group P consisted of 20 seedlings to which a paste containing gibberellin had been applied two days after germination. • Group Q consisted of 20 seedlings to which a paste without gibberellin had been applied two days after germination. • The length of stem of the pea plants was recorded at intervals over 20 days. The results are shown in [Figure 8.1]. With reference to Fig. 8.1, describe the results of the investigation and compare the growth rate of plants in group P and group Q.[4]
(d)Explain the role of the gene controlling stem length in pea plants.[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 2018). The mark or mark type is shown in brackets.

[2] any two from 1 idea that DELLA proteins prevent the activation of genes (for stem elongation); 2 gibberellin binds to receptors (on cell surface membrane); 3 causes breakdown of DELLA proteins; 4 (so) transcription / gene expression / gene activation / mRNA synthesis, can occur; 5 AVP; e.g. ref. to transcription factors / PIF
[4] any four from 1 seed absorbs water ; 2 embryo produces gibberellin; 3 gibberellin, moves to / acts on / stimulates, aleurone layer; 4 (where) production of amylase occurs ; 5 amylase, hydrolyses / breaks down, starch in endosperm ; 6 to, maltose / glucose ; 7 embryo uses sugars for, respiration / growth ; 8 AVP; e.g. gibberellins affect, gene / synthesis of mRNA, coding for amylase
[4] any four from 1 both P and Q same until, day 2/ paste applied ; 2 P, greater stem length / greater height / taller, than Q (after, day 2/ paste applied); 3 P 35 cm and Q 15cm / P 20cm longer than Q, at, end (of investigation) / day 20 ; 4 P greater rate than Q/AW; 5 comparative calculated growth rates for P and Q ; e.g. 1.75 (cm day⁻¹) and 0.75 (cm day⁻¹) or 1.89 (cm day⁻¹) and 0.78 (cm day⁻¹)
[3] any three from 1 tall pea plants have, dominant allele / Le; 2 (which codes for) enzyme that produces active gibberellin ; 3 dwarf pea plants (only) have, recessive alleles / le ; 4 (so) no (active) gibberellin formed; 5 GA₁ is the active form of gibberellin ;

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.