9.1 Essential ideas

9.1.2 Transport in the phloem of plants

Phloem is the tissue through which the products of photosynthesis are translocated from the source of production – the leaves – to areas of growth and storage – the sinks, including fruits, seeds, stems and roots. Translocation in the phloem, unlike the transpiration stream in the xylem, is multidirectional, and requires active transport.

Structure of the phloem

Phloem tissue is composed of:

  • sieve tube elements – individual enucleated cells lined up from end to end and connected by sieve plates
  • companion cells – nucleated cells positioned along the length of the sieve tube elements, attached by many plasmodesmata.

phloem elements Figure 9.1.2a – Phloem elements

Source-to-sink transport

  • Sugars are produced through photosynthesis in the mesophyll layers of leaves (source).
  • Following production, sugars diffuse from the mesophyll through the companion cells of the phloem and are loaded into the sieve tube member.
  • Loading in the sieve tube is an active process that requires ATP and a chemiosmotic mechanism using proton pumps and a symport protein.
  • Loading results in a much higher concentration of solute inside the sieve tube member than in the mesophyll.
  • High solute concentration in the sieve tube at the loading points causes water to enter the phloem by osmosis from surrounding tissues.
  • This creates an area of high hydrostatic pressure at the source end.
  • Hydrostatic pressure is relieved at the sink – as sucrose is removed, water exits the phloem by osmosis to surrounding tissues.
  • Companion cells at the sink do not accumulate sugars – they are transported to the growing or storage organs.
  • In this way, plant food always moves from source to sink.
  • Normally, the source and sink are located relatively close together on a plant. For example, lower leaves will supply sugars to growing roots, and higher leaves to flowers and seeds.

overview of translocationFigure 9.1.2b – Overview of translocation

Key questions

  • Define source, sink, translocation.
  • Explain how structure and function are related in phloem.
  • Identify the types of membrane transport involved in translocation.

xylem vs phloem vesselFigure 9.1.2c – Comparison of xylem and phloem vessels

Extended essay

  • Translocation is a process that requires active transport. Think about modified storage organs such as tubers (e.g. potatoes) or bulbs (e.g. onions), which act as either sources or sinks, depending on the season.
  • Is it possible that a potato simultaneously acts as a source and a sink? Which factors influence when a potato acts as a source or a sink? How could you test it?

In the lab

Analyse data collected by aphid stylectomy. See: Page 9.2.4