Translocation in phloemAQA A-Level Biology: Subtopic test

1. 1
   A potato plant makes sugars in its leaves during the summer. Over the same period its underground tubers swell with stored starch, while the shoot tips continue to grow.

   (a)

   Which tissue carries sucrose from the leaves to the developing tubers?

   [1 mark]

   • AXylem vessels
   • BPalisade mesophyll cells
   • CPhloem sieve tube elements
   • DRoot hair cells

   (b)

   Which part of the potato plant is a sink for sucrose during the summer?

   [1 mark]

   • AThe developing tubers
   • BThe mature leaves in full sunlight
   • CThe xylem vessels in the stem
   • DThe palisade mesophyll cells

   (c)

   Explain why the concentration of sucrose in the phloem is higher in the leaves than in the tubers.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a sugar beet plant, sucrose made in the leaves is moved to the storage root through sieve tube elements. Each sieve tube element has a companion cell beside it.

   (a)

   Which statement describes how sucrose is loaded into the phloem at the leaf?

   [1 mark]

   • ACompanion cells use ATP to pump sucrose directly into the xylem
   • BWater moves out of the companion cells by osmosis, drawing sucrose in with it
   • CSucrose moves through the pores of the sieve plates into the companion cells
   • DCompanion cells use ATP to move hydrogen ions out, and the hydrogen ions re-enter with sucrose by co-transport

   (b)

   How does water enter the sieve tube elements at the leaf?

   [1 mark]

   • ABy active transport using ATP from the companion cell
   • BBy osmosis from the xylem, because sucrose lowers the water potential of the sieve tube
   • CBy co-transport with hydrogen ions
   • DBy transpiration pull from the leaf

   (c)

   Explain why the hydrostatic pressure in the sieve tube elements is higher in the leaf than in the storage root.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Two students investigated transport in a woody shrub in summer. Student A removed a complete ring of bark, including the phloem but leaving the xylem intact, from one stem and observed it for two weeks. Student B supplied one leaf on a similar shrub with air containing radioactively labelled carbon dioxide (¹⁴CO₂) for 30 minutes. After 24 hours she cut thin sections of the stem and placed them against X-ray film.

   (a)

   Explain the changes Student A would observe in the stem, above and below the ring, over two weeks.

   [3 marks]

   (b)

   Explain how Student B's results would show that organic substances are transported in the phloem, and state one limitation of this evidence for the mass flow hypothesis.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Scientists studying translocation in a willow tree collected the following evidence. (1) When an aphid feeding on a stem was cut away, leaving its stylet in a sieve tube, sap continued to flow out of the stylet, and the flow was faster close to the leaves than lower down the stem. (2) The concentration of sucrose in the sieve tubes was higher in the leaves than in the roots. (3) When a metabolic inhibitor was applied to the leaves, the rate of translocation fell sharply. (4) Different solutes in the phloem were found to move at different rates, and some moved in opposite directions in different sieve tubes. (5) Sieve plates lie across the sieve tubes at intervals.

   (a)

   Explain how the mass flow hypothesis accounts for the movement of sucrose from the leaves to the roots of the tree.

   [6 marks]

   (b)

   Evaluate the evidence for and against the mass flow hypothesis.

   [6 marks]

   Total for question 4: 12 marks

End of questions