Chemical control in plantsEdexcel A-Level Biology B: Subtopic test

1. 1
   A maltster prepares barley for brewing by soaking the grains and letting them germinate. During germination the embryo releases gibberellin, which diffuses to the aleurone layer around the starch-rich endosperm. The aleurone cells then make and release amylase. The maltster stops germination by heating the grains once enough starch has been broken down.

   (a)

   Which substance in the barley endosperm is hydrolysed by amylase?

   [1 mark]

   • AStarch
   • BCellulose
   • CProtein
   • DGibberellin

   (b)

   Which statement describes the role of gibberellin in the germinating grain?

   [1 mark]

   • AIt is the substrate that amylase hydrolyses
   • BIt stimulates the aleurone cells to make amylase
   • CIt hydrolyses starch directly to maltose
   • DIt prevents the embryo respiring until the grain is dry

   (c)

   Explain how the production of amylase allows the barley embryo to grow.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A gardener has three identical single-stemmed saplings of a shrub. On plant X she removes the apical bud. Plant Y is left intact. On plant Z she removes the apical bud and immediately applies a paste containing auxin to the cut surface. After two weeks plant X has produced many lateral shoots, whereas plants Y and Z have produced almost none.

   (a)

   Which term describes the suppression of lateral buds by the apical bud?

   [1 mark]

   • APhototropism
   • BPhotomorphogenesis
   • CApical dominance
   • DEtiolation

   (b)

   What do the results for plant Z show?

   [1 mark]

   • AAuxin is made in lateral buds
   • BCytokinin is made in the apical bud
   • CAuxin has no effect on lateral buds
   • DAuxin applied to the cut surface can replace the apical bud in suppressing lateral buds

   (c)

   Explain why plant X produced many lateral shoots.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates the effect of gibberellic acid on amylase production. She cuts barley grains in half and keeps only the halves without an embryo. She places five halves, cut face down, on each of four starch agar plates containing gibberellic acid at 0, 1, 10 and 100 mg dm⁻³. After 48 hours she removes the grain halves and floods each plate with iodine solution. The mean diameter of the clear zone around the grain halves is 0 mm, 6 mm, 12 mm and 14 mm respectively.

   (a)

   Explain the appearance of the plates after they were flooded with iodine solution, including the plate that contained no gibberellic acid.

   [3 marks]

   (b)

   Assume each clear zone is circular. Calculate the percentage increase in the area of the clear zone when the concentration of gibberellic acid rises from 1 to 10 mg dm⁻³. Suggest why the increase from 10 to 100 mg dm⁻³ is much smaller.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A nursery grows chrysanthemums, a short-day plant, as cut flowers. Young plants are raised from stem cuttings, and the cut bases are dipped in a rooting powder containing synthetic auxin. To make the plants flower out of season, the grower covers them with blackout cloth each evening so that they receive about 14 hours of continuous darkness.

   (a)

   Explain how auxin brings about elongation of cells in stems, and why the rooting powder promotes root growth in the cuttings.

   [6 marks]

   (b)

   The chrysanthemums are covered each evening to give 14 hours of continuous darkness. A brief flash of red light in the middle of the dark period stops flowering, but a flash of far-red light straight after the red flash allows flowering. Explain these observations in terms of phytochrome, and why the grower uses blackout cloth.

   [6 marks]

   Total for question 4: 12 marks

End of questions