Transport in Plants


Xylem and phloem are the main vessels that transport substances around a plant.

Structure of Xylem & Phloem

"Understand the structure of xylem and phloem tissues in relation to their role in transport."
Xylem
  • Transports water and dissolved ions
  • Composed mainly of xylem vessels
  • These are long and elongated
  • The tubes are lignified increasing its strength
  • Have perforated ends
  • No cytoplasm or organelles to restrict the movement of water (mature xylem is dead)
The movement of water up the xylem results in tension. This is a negative pressure which serves to reduce the diameter of the plants stem.

Phloem
  • Transports mainly sugars
  • Made up of sieve tube elements and companion cells
  • Sieve tube elements lack cytoplasm and a nucleus, they do however have pores in their ends allowing for quicker transport
  • Companion cells support the sieve tube cells and are joined together by the plasmodesta
  • These companion cells have lots of mitochondria for active transport
  • Also have lots of infoldings to increase its surface area for exchange

Transport Pathways

"Understand how water can be moved through plant cells by the apoplastic and symplastic pathways."
  • Symplastic pathway - occurs when water moves through the cytoplasm of cells.
  • Apoplastic pathway - occurs when water moves through the cell wall.
Water can travel through the apoplastic pathway until it reaches the casparian strip which is impermeable to water.

Water is forced to enter the symplastic pathway. This happens via active transport allowing the flow of water to be regulated.

Cohesion-tension Model

"Understand how the cohesion-tension model explains the transport of water from plant roots to shoots."
Translocation is the movement of water around the plant. Transpiration is the loss of water from a plants leaf. Water is drawn up the stem of a plant through cohesion-tension.
  • Water is lost through transpiration
  • Water molecules are polar and are therefore cohesive allowing a tension to be formed
  • This means that water is drawn up the xylem
  • The molecules have a high tensile strength
  • Water molecules are also adhesive

Root Pressure

  • Mineral ions enter the roots through active transport
  • This maintains a gradient for water to move into the cell via osmosis
  • This creates a weak push effect, moving water through the roots and up the stem of the plant

Root Hair Cells

  • These are small extensions from a plants root
  • They increase the surface area to volume ratio
  • This allows for more efficient mineral ion and water uptake

Rate of Transpiration

"Understand how temperature, light, humidity and movement of air affect the rate of transpiration."
This is the loss of water from a plant through the stomata because of evaporation. The following factors all affect the rate of transpiration:
  • Humidity - a higher humidity in the atmosphere lowers the water potential gradient therefore reducing the rate of transpiration.
  • Temperature - at increased temperatures particles will have more kinetic energy therefore increasing transpiration.
  • Air movement - greater movement removes moist air from around the leaves increasing the water gradient therefore increasing the rate of transpiration.
  • Light - more light increases photosynthesis therefore resulting in the stomata opening to allow more C)2 in, consequently releasing more water.
Stomatal Density
A greater density increases the surface area for exchange therefore increasing transpiration.

Mass-flow Hypothesis

"Understand the strengths and weaknesses of the mass-flow hypothesis in explaining the movement of sugars through phloem tissue."
This hypothesis attempts to explain how sugars mive around a plant.
  1. This concerns the movement of sugars around a cell
  2. At the source cell sucrose is formed
  3. Sugar is loaded into the sieve-tube via active transport
  4. This lowers the water potential causing more water to move into the cell increasing hydrostatic pressure
  5. This pressure causes the sugars to move
  6. At the sink cell sucrose is converted into insoluble starch
  7. And hydrostatic pressure falls
However, this model is not supported by many scientists...

Strengths
  • Sucrose concentration is higher at sources than at sinks
  • Sap oozes out suggesting it is under pressure
  • Viruses are only transported from sources to sinks
Weaknesses
  • Solutes move around in different directions
  • Different solutes are moved at different rates