Plant Transport

7.1 Uptake and transport of water and ions

Root hair cells

Root hair cells take up water and mineral ions from the soil. Their structure is related to this function:

• a long, thin extension (the root hair) gives a very large surface area for absorption;
• a thin cell wall and membrane, so water can move in easily;
• a sap vacuole with dissolved sugars and salts gives the cell a lower water potential than the soil water, so water enters by osmosis;
• many mitochondria release the energy that is needed for the active transport of ions.

The pathway of water

Water moves through the plant in this order:

1. root hair cells, from the soil, by osmosis;
2. root cortex cells, moving from cell to cell down a water potential gradient;
3. xylem in the root, then up the stem in the xylem;
4. xylem in the leaf, then into the mesophyll cells;
5. water evaporates from the surface of the mesophyll cells into the air spaces and diffuses out through the stomata (transpiration).

Investigating the pathway of water

Place a cut stem (for example celery or a white flower) in water containing a dye such as red ink or eosin. After a few hours, cut the stem across or lengthways and look: the dye shows the xylem (stained red) running up the stem and into the leaves, so water travels in the xylem.

7.2 Transpiration and translocation

Transpiration

Water evaporates from the surfaces of the mesophyll cells into the air spaces, and then diffuses out of the leaves through the stomata as water vapour.

Factors that affect the rate of transpiration

Investigation (a potometer). A cut shoot is fitted to a tube of water with a small air bubble. As the plant takes up water, the bubble moves along a scale, so the distance the bubble moves in a given time shows the rate of water uptake (which is nearly equal to the rate of transpiration). Repeat the measurement with a fan (wind), a lamp (light or heat), or a plastic bag (high humidity).

Wilting

If a plant loses water by transpiration faster than its roots can take it up, the cells lose water and become flaccid. The plant loses its support from turgor pressure and wilts. Closing the stomata (at night, or when short of water) reduces water loss.

The transpiration stream

As water leaves the leaf, more water is pulled into the leaf from the xylem. The water column in the xylem is pulled up from the roots all the way up the stem. Water entering the roots replaces it. This stream brings water and mineral ions to the leaves, where water is needed for photosynthesis and for keeping the cells turgid, and it also cools the leaf.

Xylem

Xylem vessels are adapted to their function:

• thick walls strengthened with lignin, which makes them strong enough to withstand the pull of the water column and to support the plant;
• no cell contents (no cytoplasm or nucleus), so water can flow through freely;
• cells are joined end to end with no cross walls, forming a long, continuous tube.

Phloem and translocation

Unlike the xylem, the phloem is made of living cells, and it carries substances in both directions in the plant, depending on where the sources and sinks are.

Positions of the tissues

In a transverse section of a non-woody dicotyledonous plant:

• Root: the xylem is in the centre (often in a star or "X" shape), with the phloem between its arms, surrounded by the cortex.
• Stem: the vascular bundles (xylem on the inside, phloem on the outside) are arranged in a ring near the edge, with the cortex outside them.

The phloem carries it. The leaves are the source and the roots (which store or use the sucrose) are the sink.