Transport in Plants

Plants do not have a heart or blood, but they have two specialised vascular tissues for transport: xylem and phloem.

Xylem¶

Xylem carries water and dissolved mineral ions from the roots upward through the stem to leaves and other organs.

Xylem vessels are well-suited for this:

• dead cells with no cell contents — form hollow tubes for unobstructed flow
• lignified walls — strong and waterproof; prevent collapse under tension
• no end walls — water moves freely along the tube

Water moves through xylem by a pull from the top (transpiration pull), aided by cohesion between water molecules.

Phloem¶

Phloem carries dissolved sugars (sucrose) from leaves to other parts of the plant — both upward and downward. This movement is called translocation.

Phloem is made of living sieve tube cells connected end-to-end, with companion cells alongside that supply energy for active loading and unloading of sugars.

Transpiration¶

Transpiration is the loss of water vapour from plant leaves, mainly through stomata.

Water is drawn up through the xylem by the tension created at the top as water evaporates from leaf cells — this is the transpiration pull or transpiration stream.

Factors affecting the rate of transpiration¶

Plant adaptations to reduce water loss¶

Plants in dry habitats conserve water through:

• thick waxy cuticle on leaves
• stomata mainly on the lower (cooler, shadier) surface
• sunken stomata (protected from wind)
• leaf rolling (reduces exposed surface)
• few stomata
• reduced leaf surface area (needles, spines)

Storage of Food¶

All organisms store energy for times when food is scarce.

Storage as insoluble molecules (starch, glycogen) avoids osmotic problems — they do not raise the solute concentration of cells. When energy is needed, enzymes break these down into soluble forms (glucose) that can be respired.

FLASHCARD SET • 5 CARDSTransport in Plants