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AS · Practice questions · Translocation: loading the phloem and mass flow

Load, osmosis, pressure, flow.

Six original Cambridge-style questions on translocation, from defining sources and sinks to explaining the effect of a respiratory inhibitor and interpreting a ringing experiment.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Energy at the source, flow down the gradient.

01
Recall
[3 marks]

Name two assimilates transported in the phloem and explain what is meant by a source and a sink.

  • Sucrose and amino acids (dissolved in water). ✓
  • Source: a part of the plant where assimilates are made or released into the phloem, for example a photosynthesising leaf. ✓
  • Sink: a part of the plant where assimilates are used or stored, for example a root, growing tip or developing fruit. ✓
02
Explain
[5 marks]

Explain how companion cells load sucrose into phloem sieve tubes.

  • Companion cells respire aerobically in their many mitochondria to make ATP. ✓
  • Proton pumps in the companion cell membrane use ATP to pump hydrogen ions out of the cell. ✓
  • This creates a high concentration of hydrogen ions outside the cell (a proton gradient). ✓
  • Hydrogen ions diffuse back in through cotransporter proteins, carrying sucrose with them against its concentration gradient. ✓
  • Sucrose then moves from the companion cell into the sieve tube element through plasmodesmata. ✓
03
Explain
[4 marks]

Explain how sucrose moves from a source to a sink by mass flow.

  • Sucrose loaded at the source lowers the water potential of the sieve tube sap. ✓
  • Water enters from the xylem by osmosis, so the hydrostatic pressure at the source is high. ✓
  • At the sink sucrose is removed (used in respiration or stored as starch), water leaves by osmosis and the pressure is lower. ✓
  • Sap moves down the hydrostatic pressure gradient from source to sink, carrying the dissolved sucrose with it. ✓
04
Analysis
[4 marks]

Compare the transport of water in the xylem with the transport of sucrose in the phloem.

  • Both involve movement of a solution by mass flow down a pressure gradient. ✓
  • Xylem transport is one way, from roots to leaves; phloem transport is from source to sink, up or down the plant. ✓
  • Xylem contents are under tension (pressure below atmospheric); phloem sap is under a high positive hydrostatic pressure. ✓
  • Xylem transport needs no energy from the plant's cells; phloem transport needs ATP for loading at the source (xylem cells are dead, sieve tube elements are living). ✓
05
Data
[3 marks]

A ring of bark, including the phloem but not the xylem, was removed from the stem of a tree. After several weeks the bark just above the ring was swollen and the roots had stopped growing. The leaves above the ring did not wilt. Explain these observations.

  • Removing the phloem stopped translocation of sucrose from the leaves (source) down past the ring. ✓
  • Sucrose built up in the tissues just above the ring, so they swelled; the roots (sink) received no sucrose for respiration and growth. ✓
  • The xylem was intact, so water still reached the leaves and they did not wilt. ✓
06
Data
[3 marks]

The sucrose concentration of sieve tube sap was 0.80 mol dm-3 near a leaf and 0.30 mol dm-3 near a root. Calculate the difference as a percentage of the concentration near the leaf, and explain how this difference relates to mass flow.

  • Difference = 0.80 - 0.30 = 0.50 mol dm-3; 0.50 ÷ 0.80 × 100 = 62.5%. ✓
  • The high sucrose concentration at the leaf (source) gives a low water potential, so water enters and the hydrostatic pressure is high. ✓
  • At the root (sink) sucrose is removed, so the pressure is lower; the sap flows down this pressure gradient from leaf to root. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.