translocation

water potential

• Measure of the potential energy of water per uniti of volume of water 

• Influenced by solute concentration (solute potential) and hydrostatic pressure

• Ψw (water potential) = Ψs (solute potential) + Ψp (pressure potential)

• Allows us to predict the direction in which there will be net movement of water moleculesl 

• Water moves from high to low water potential because this minimizes iys potential energy

• Potential energy of water reduces if solutes dissolve in it , this component is solute potential

• With no solute dissolved, the solute potential is zero, only possible solute potentials are zero or negative

• Rises and falls in hydrostatic pressure also change the potential energy of water, this component is water potential

•  The higher the pressure, the more potential energy water has, pressure potential can be -- because it can be greater or less than atmospheric pressure

bathing in hypotonic solution

• Increased water potential of the plant tissue

• Solute potential less negative

• Pressure potential more positive

• Net movement from solution to the tissue

• When water potential = water potential of thes olution → net movement of water stops

bathing in hypertonic solution

• Solute potential is more negative than the tissue

• Pressure potential in the plant more positive

• Both solute and pressure potentials give the cell a higher water potential

• Net movement of water from the cell to the solution

root pressure

• When transport in the xylem due to transpiration is insufficient to meet the needs of the plant, root pressure is generated to enhance water movement through the xylem

• Root pressure is generated by the active pumping of mineral ions from the soil into root hair cells 

• Root cells adjacent to xylem vessels pump mineral ions → water within xylem becomes hypertonic compared to the surrounding cells → water to move to the xylem vessels → an increase int he pressure inside the vessels → Xylem sap is pushed towards against the force of gravity 

phloem

• Sucrose and other organic compounds can be transported from one part of a plant to another via the phloem 

• The phloem tissuse is composed of sieve elements that provide channels through which transport can occur

sieve elements

• Sieve elements are long and narrow cells that are connected togetjt to form the sieve tubee

• Sieve elements are connected by sieve plates, at the traverse ends, which are porous to enable flow between cells 

• Sieve elements have no nucleus and reduced numbers of organelles to maxiize space for the translocation of materials

• The sieve elements also have thick and rigid walls to withstand the hydrostatic pressures which facilitate flow 

• Because sieve elements have few or no mitochondria, they rely on adjacent cells called companion cells for a supply

• Plasmodemata exist betweens ieve elements and companion cells in relatively large numbers: connect the cytoplasm of the two cells and mediate the exchange of metabolites 

translocation

• Movement of organic compounds from source to sink

• Source is where organic compounds are synthesized - this is the photosynthetic tissue (leaves)

• The sink is where the compounds are delivered to for usen or storage (roots, fruits, devel leaves)

• Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells

• Materials can pass into the sieve tube by interconnecting plasmodesmata

• Loading sucrose into the phloem sieve tubes in an active transport process that requires ATP

• The active transport of solutes into the phloem by companion cells makes the sap solution hypertonic 

• This causes the water to be drawn from the xylem via osmosis

• This build up of water in the phloem causes hydrostatic pressure to increase

• This increase in hydrostatic pressure forces the phloe sap to move towards areas of pressure

• Hence, ththe phloem transports solutes away from the source (and towards the sink) where it will be unloaded by active transport