Xylem & Phloem

The particular problem in plants is that the roots can obtain water easily from the soil but they cannot absorb sugars from the soil. The leaves can produce sugars but cannot absorb water from the air.

Q. What do plant cells require water and sugars for?

Water for photosynthesis: CO₂   +   H₂O  ----  C₆H₁₂O₆   +   O₂

Sugars for respiration: C₆H₁₂O₆ (gluocse)  +   O₂    ----- CO₂   +   H₂O                

The transport systems in plants are found in special tissue called vascular tissue.  Plants have evolved two transport systems:

Xylem- Plant transport tissue that carries water from the roots to the rest of the plant.

Phloem- Plant transport tissue that carries the products of photosynthesis to the rest of the plant (from source to sink).

These cells are dead, this means that the flow of liquid does not affect, and is not affected by, osmosis as it moves through these cells.  

The cell walls are coated in a rigid , strong and waterproof substance called Lignin. This has many advantages for plant transport, the Lignin prevents the cells collapsing under the high and low pressures created by the sucking of liquid along the tubes .  

The  cell walls of these cells have open holes called ‘Pits ’, this allows liquids to move sideways (laterally)  in, out and between vessels as required.  

The end walls of these cells have fallen away entirely to form one continuous tube. This means there is absolutely no resistance against the movement of the liquid through the tubing system.

State the function of lignin.

• Strengthens / thickens the xylem wall
• Waterproofing the xylem wall
• Improves adhesion of water molecules
• Spiral pattern allows flexibility / stretching / movement

 Q. Explain why it is important that the xylem vessel becomes lignified.

•  Prevents collapse of the xylem
• Water under tension is at low pressure
• Reduces lateral loss of water through wall
• Increases capillarity
• Prevents stem breaking

 Q. Explain the function of the xylem pits.

• Pits allow water to move in & out of vessels as well as between vessels
• To bypass blockage
• Supply water to other tissues / parts of plant

• The end walls of these cells are heavily perforated to form structures called ‘Sieve Plates’. This allows liquid to more easily move along the tubing system from cell to cell with little resistance.

• The amount of cytoplasm and the number of organelles in these cells is reduced to an absolute minimum. This means that some cells even lack a nucleus. The reduced contents of these cells are a great advantage as it allows more space for transport of fluids and less resistance to the flow.

• Some  of these cells have cytoplasm filled with a very high concentration of mitochondria and ribosomes. This allows these cells to support other cells with little cytoplasm by supplying proteins and ATP.