Long-Distance Transport: The Role of Bulk Flow (Lecture 12)
Absorption of water/ Short distance transport
Ascent of sap/long distance transport
Bulk Flow Transport via the Xylem
Bulk Flow Driven by Negative Pressure in the Xylem
The transport of xylem sap involves transpiration
Plants lose a large volume of water from transpiration. This creates a negative pressure at the stomatal opening (where water was lost).
Water is replaced by the bulk flow of water and minerals, called xylem sap, that is transported from the steles of roots to the stems and leaves.
Is sap mainly pushed up from the roots, or pulled up by the leaves?
Pushing Xylem Sap: Root Pressure
Root pressure is osmotic pressure within the cells of a root system that causes sap to rise or forces water up the root xylem through a plant stem to the leaves.
One manifestation of root pressure is Guttation
At night, when stomates are closed, transpiration is very low.
Root cells continue pumping mineral ions into the xylem
of the vascular cylinder through osmosis, lowering the water potential.
Osmosis-driven water uptake can pressurize the
vascular cylinder (0.05 to 0.5 Mpa) and
can push xylem sap up several centimeters to perhaps a meter.
Leaf edges have hydathodes where excess xylem sap can leak.
Pulling Xylem Sap:
ØThe transpirational pull on the xylem sap is transmitted to the soil solution
ØFour forces combine to transport water solutions from the roots through the xylem elements, and into the leaves
ØThese TACT forces are:
- involves the pulling of water up through the xylem of a plant
- Utilize the energy of evaporation and the tensile strength of water.
- is the attractive force between water molecules and other substances.
- both water and cellulose are polar molecules
- there is a strong attraction for water to the hydrophilic walls of xylem cells
- The small diameter of vessels and tracheids is important to the adhesion effec
- is the attractive force between molecules of the same substance.
- high cohesive force due to the 4 hydrogen bonds
- water’s cohesive force within xylem give it a tensile strength equivalent to that of a steel wire of similar diameter.
- Cohesion of water allows for the pulling of water from the top of the plant without breaking the “chain”.
*a stress placed on an object by a pulling force.
*is created by the surface tension which develops in the leaf’s air spaces.
- The upward pull of sap causes tension (negative pressure) in xylem
- decreases water potential and
- allows passive flow of water from soil into stele
The Cohesion-Tension Hypothesis
The cohesion-tension theory explains how water moves up through the xylem.
Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall.
The leaf contains many large intercellular air spaces for the exchange of gases, which is required for photosynthesis.
The wet cell wall is exposed to the internal air space and the water on the surface of the cells evaporates into the air spaces.
This decreases the thin water film on the surface of the mesophyll cells.
The decrease creates a greater tension on the water in the mesophyll cells (negative pressure), thereby increasing the pull on the water in the xylem vessels .
- Water is pulled upwards through osmosis (Transpiration Pull)
1.The waterway in which the water moves from a higher water potential to a lower water potential
- Transpiration Stream
This mechanism of water flow works because of water potential (water flows from high to low potential), and the rules of simple diffusion.
- When water passes up the thin xylem vessels, it adheres to the surface of the vessels, while the force of osmosis gently ‘pushes’ the water molecules, which cohere to each other, upwards
Small perforations between vessel elements reduce the number and size of gas bubbles that form via a process called cavitation.
The formation of gas bubbles in the xylem is harmful since it interrupts the continuous stream of water from the base to the top of the plant, causing a break (embolism) in the flow of xylem sap.
The taller the tree, the greater the tension forces needed to pull water, increasing the number of cavitation events.
Drought stress or freezing can cause cavitation, the formation of a water vapor pocket by a break in the chain of water molecules
Forces that promotes uptake of water
Promotes uptake of water in the root
Main force to ‘suck’ up the water
Pushes the water upwards due to the adhesion between the walls of the xylem vessels and water molecules as well as the cohesion between the water molecules
Xylem Sap Ascent by Bulk Flow: A Review
The movement of xylem sap against gravity is maintained by the transpiration-cohesion-Adhesion-tension mechanism
Bulk flow is driven by a water potential difference at opposite ends of xylem tissue
Bulk flow is driven by evaporation and does not require energy from the plant; like photosynthesis it is solar powered
*Bulk flow differs from diffusion
It is driven by differences in pressure potential, not solute potential
It occurs in hollow dead cells, not across the membranes of living cells
It moves the entire solution, not just water or solutes
It is much faster
Transport of Food
Sugars are transported from sources to sinks via the phloem
The products of photosynthesis are transported through phloem by the process of translocation
In angiosperms, sieve-tube elements are the channels for translocation
Phloem sap is an aqueous solution that is high in sucrose
It travels from a sugar source to a sugar sink
A sugar source is an organ that is a net producer of sugar, such as mature leaves
A sugar sink is an organ that is a net consumer or involved in the storage of sugar, such as a tuber or bulb
A storage organ can be both a sugar sink in summer and sugar source in winter
Sugar must be loaded into sieve-tube elements before being exposed to sinks
Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways
Companion cells enhance solute movement between the apoplast and symplast
In many plants, phloem loading requires active transport
Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose
At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water
Sucrose travels to the phloem companion cells in two ways.
vFrom cell to cell through the plasmodesmata.
vAlong cell walls in the mesophyll.
Carrier proteins in the cell surface membranes of companion cells actively pump sucrose into the cytoplasm.
From here it passes through plasmodesmata into a sieve element.
The accumulation of sucrose and other solutes, such as amino acids, in sieve elements lowers the water potential so that water diffuses in by osmosis from adjacent cells and from the xylem.
This creates pressure in the sieve elements causing the liquid (phloem sap) to flow out of the leaf.
Phloem sieve elements are adapted for transport as it has:
- End walls that have sieve pores allowing sap to flow freely.
- Little cytoplasm to obstruct the flow of sap.
- Plasmodesmata to allow assimilates to flow in from companion cells.
Sieve elements differ form xylem vessels because
- They are alive.
- They have some cytoplasm with organelles.
- They are not lignified, as they do not need to withstand the same forces as exist in the xylem.
- Sucrose is unloaded at sinks.
- This is taken up by the cells and is respired or stored as starch.
- This reduces the concentration of phloem sap and lowers the pressure, so helping to maintain a pressure gradient form source to sink so the sap keeps flowing in the phloem.
How would you show phloem transports food substances?
experiments to show phloem transport food substances
- Aphid penetrates the stem into the phloem using its mouthpart
called stylet and sucks the plant sap
- A feeding aphid can be anaesthetized and the stylet cut off
- The phloem sap flows out through the stylet and can be analyzed.
It is found to contain sugars and other organic substances
2.The Ringing Experiment