1. 1
   A plant physiologist injected a radioactive tracer into the sucrose made by photosynthesis in a mature leaf of a bean plant and later found the tracer concentrated in the roots, while very little was found in a nearby young, actively growing leaf.

   (a)

   Through which tissue had the labelled sucrose been transported?

   [1 mark]

   • APhloem
   • BXylem
   • CEpidermis
   • DCortex only, without any specialised tissue

   (b)

   What term describes the mature leaf in this investigation, which released sucrose into the transport tissue?

   [1 mark]

   • AA sink
   • BA source
   • CA stoma
   • DA lacteal

   (c)

   Explain why the roots accumulated much more of the radioactive tracer than the nearby young, actively growing leaf.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Later in the growing season, the same bean plant began producing developing seed pods that needed large amounts of sucrose and amino acids for growth. The physiologist found that translocation was now directed mainly towards the pods rather than the roots.

   (a)

   What term best describes the developing seed pods at this stage?

   [1 mark]

   • AA stimulus
   • BA source
   • CA sink
   • DA stoma

   (b)

   Which substances, in addition to sucrose, are transported in the phloem towards the developing seed pods?

   [1 mark]

   • ACellulose
   • BOxygen
   • CUrea
   • DAmino acids

   (c)

   Explain why the roots of the bean plant can act as both a source and a sink at different times during the plant's life.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A farmer noticed that potato tubers, which are underground storage organs, grew much larger when the plant's leaves were left undamaged, but grew poorly when most of the leaves were eaten by pests early in the growing season.

   (a)

   A farmer noticed that potato tubers (underground storage organs) grew much larger when the plant's leaves were left undamaged, but grew poorly when most of the leaves were removed by pests early in the season. Explain, using the terms source and sink, why removing the leaves reduced tuber growth.

   [3 marks]

   (b)

   Suggest and explain four adaptations of phloem tissue that make it suited to translocating sucrose and amino acids efficiently from source to sink.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research team studied translocation in a sugar beet plant by measuring sucrose concentration and hydrostatic pressure at different points in the phloem, from a mature source leaf to a storage root acting as a sink.

   (a)

   Describe how sucrose is loaded into the phloem at a source, such as a photosynthesising leaf, and explain how this leads to the translocation of sucrose towards sinks elsewhere in the plant.

   [6 marks]

   (b)

   The research team found that when they applied a chemical that specifically blocked the active loading of sucrose into companion cells at the source leaf, translocation to the storage root almost completely stopped, even though the phloem tubes themselves remained undamaged. Explain this result, and discuss what it shows about the mechanism driving translocation.

   [6 marks]

   Total for question 4: 12 marks

End of questions