Translocation in phloemAQA A-Level Biology: Revision notes

Translocation is the transport of organic substances (mainly sucrose, with some amino acids) through the phloem from where they are made or stored to where they are used. Phloem contains two main cell types:

• Sieve tube elements: living cells joined end to end, with sieve plates (perforated end walls) between them. They have very little cytoplasm and no nucleus, so the cytoplasm is continuous from cell to cell and flow is easy.
• Companion cells: each has a nucleus, dense cytoplasm and many mitochondria. They are linked to the sieve tube element by plasmodesmata and provide the ATP needed to load sucrose.

A source is a region where organic substances are produced or released into the phloem (for example photosynthesising leaves). A sink is a region where they are removed for use or storage (for example growing roots, fruits and tubers).

Sucrose made in the leaf must be moved into the sieve tube against a concentration gradient. Companion cells do this by active loading:

1. Companion cells use ATP to actively transport hydrogen ions out into the cell walls, creating a high hydrogen ion concentration outside.
2. Hydrogen ions diffuse back into the companion cell through co-transporter proteins, carrying sucrose with them (co-transport).
3. Sucrose builds up in the companion cell and passes into the sieve tube element through plasmodesmata.

This lowers the water potential of the sieve tube element.

The mass flow hypothesis explains how sucrose moves from source to sink:

• At the source, loading lowers the water potential of the sieve tube, so water enters from the xylem by osmosis. This raises the hydrostatic pressure.
• At the sink, sucrose is removed (used in respiration or converted to starch), which raises the water potential, so water leaves by osmosis and the hydrostatic pressure is lower.
• The pressure gradient moves the solution along the sieve tubes by mass flow, through the pores in the sieve plates.
• Water leaving at the sink may return to the xylem.

In one short answer link the sequence: loading, water enters, high pressure, flow to low pressure, unloading.

A tracer is a radioactive isotope used to follow a substance through the plant. A leaf is supplied with ¹⁴CO₂, which is fixed into organic compounds such as sucrose in photosynthesis. After a time, stem sections are placed against X-ray film; an autoradiograph shows black areas where radioactivity is present.

The film is darkened over the phloem, showing that organic substances made in the leaf are translocated in the phloem. The results can also show the direction and, by timing, the speed of movement. The method does not show the mechanism.

In a ringing experiment, a band of outer bark and phloem is removed from around a woody stem, leaving the xylem intact.

• Above the ring: the stem swells because sucrose and other organic solutes accumulate when they cannot pass the gap.
• Below the ring: tissues are cut off from sucrose and eventually die, because they cannot respire.
• Water still rises because the xylem is intact, so leaves above the ring stay alive for some time.

This shows that organic substances are transported in the phloem, and that they move downwards from the leaves.

Evidence for:

• Sap exudes from a cut stylet, showing the sieve tube contents are under pressure.
• Sucrose concentration is higher in the source than in the sink.
• Metabolic inhibitors reduce translocation, showing that ATP-dependent loading is involved.
• Companion cells have many mitochondria.

Evidence against:

• Different solutes move at different rates and some in opposite directions, whereas mass flow predicts a single rate and direction.
• Sieve plates would hinder flow, and the hypothesis does not explain their function.

A good evaluation weighs both and reaches a justified conclusion: mass flow explains much, but not everything.