Transport in plants

Vascular tissue – in rootsThe pericycle and its contents - xylem, phloem and cambium (not visible here) - are known collectively as the stele.

Vascular tissue – in stems

In stems, the xylem, phloem and cambium are arranged in vascular bundles.

Vascular tissue – in leaves

Xylem – transports water and dissolved minerals from the roots, up through the plant and eventually out through the leaf stomata. One-way transport.

Phloem – transports the dissolved products of photosynthesis in various directions around the plant e.g. from the leaf to a fruit, or from a root to a growing tip.

XylemThere are two types of “tube” which together make up the xylem:Tracheids, made from cells whose end walls overlap but do not disappear. Water can move from one to another only through pits;

Xylem vessels, made from stacks of cells whose cell walls either disappear or have large perforations, forming one continuous tube.Both types contain no living material and their walls are strengthened with lignin (they are described as lignified), the major constituent of wood. These walls have thinner areas known as pits, which allow water to “leak” through.

Phloem structure

The individual sieve tube elements that make up the phloem are alive, although they have no nucleus, very few organelles and only strands of cytoplasm.

Unlike in xylem vessels, the end cell walls do not disappear, but instead form structures called sieve plates, through which these strands of cytoplasm can pass.

Because of their greatly reduced contents, sieve tube elements cannot keep themselves alive and have to be aided by companion cells which respire, excrete, etc. on the elements’ behalf. The cytoplasm of the companion cells and their sieve tube elements is joined through pores in the side walls.

Transport in phloemThe transport of soluble organic substances (sometimes called assimilates) within a plant is known as translocation. The solutes are transported in sieve tube elements.

The “Mass Flow Hypothesis” is the theory by which we think solute transport occurs in plants. Any area where sucrose is produced in a plant is known as a source, and any area where it is taken out (usually, used in respiration) is known as a sink.

Sucrose is actively transported into the sieve tubes of the phloem at the source (e.g. root or leaf), lowering the water potential inside the sieve and so water enters the tubes via osmosis, creating a higher pressure inside the sieve tubes at the source.

At the sink, sugars leave the phloem to be used up, increasing the water potential inside the sieve tubes, so water leaves via osmosis, lowering the pressure inside the sieve tubes.The result is a pressure gradient from source to sink, pushing sugars to where they're needed.

Transport in phloem (continued)Supporting evidence for mass flow hypothesis:•When the phloem is cut, sap oozes out, showing a pressure gradient.•Suitable water potential gradient between leaves and other plants, in theory.•Phloem sap has a high pH, which is to be expected since hydrogen ions are actively transported out of the cell.•ATP is present in phloem sieve elements in high numbers since it is required for active transport of hydrogen ions.

Evidence against mass flow hypothesis:•Sugar travels to many different sinks.•Sieve plates are a barrier to mass flow.•Doesn't require living cells, but phloem cells are alive. Why waste the energy?