TRANSPORT IN PLANTS

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Two main types of plant tissue are used in transport - xylem and phloem.

-Xylem transports water and minerals.
 
-Phloem transports organic molecules such as the products of photosynthesis.

Xylem

There are four types of xylem cells:

  • Xylem vessels: Consist of dead hollow cells because the walls are lignified and the cell contents disintegrate. The lignin makes the cell wall impermeable so they are in effect waterproof. It also makes the vessels extremely strong and prevents them from collapsing. They have a wide lumen and are linked end to end to create a long, hollow tube since the end cell walls have one or many perforations in them. This allows the transport of large volumes of water. The sidewalls have bordered pits (unlignified areas) to allow lateral movement of water. Xylem vessels are found in angiosperms.
  • Tracheids: Similar to vessels but with narrower lumens and connected by pits. They have tapered ends so that they dovetail together. Tracheids are found in conifers.
  • Parenchyma: Living cells with thin cellulose walls. They can store water, which makes them turgid and so gives them a supporting role.
  • Fibres: They provide strength because their walls are lignified (and therefore, dead).

 

Movement in the root

Water enters through the root hair cells and then moves across into the xylem tissue in the centre of the root. Water moves in this direction because the soil water has higher water potential, than the solution inside the root hair cells.

This is because the cell sap has organic and inorganic molecules dissolved in it. The root hairs provide a large surface area over which water can be absorbed.

Minerals are also absorbed but, as you should be able to work out, their absorption requires energy in the form of ATP because they are absorbed by active transport. They have to be pumped against the concentration gradient.

Water taken up by the root hairs moves across the cortex of the root either via the cytoplasm of the cells in between the root hair cell and the xylem (the symplast pathway) or through the cell walls of these cells (the apoplast pathway). The root hair cell will have higher water potential than the cell next to it. As always, water moves by osmosis to where the water potential is lower. In this way, as water is always being absorbed by the root hairs, water will always move towards the centre of the root.

When the water reaches a part of the root called the endodermis, it encounters a thick, waxy band of suberin in the cell walls. This is the Casparian strip and it is impenetrable. In order to cross the endodermis, the water that has been moving through the cell walls must now move into the cytoplasm.

Once it has moved across the endodermis, it continues down the water potential gradient until it reaches a pit in the xylem vessel. It enters the vessel and then moves up towards

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