Long-Distance Transport: The Role of Bulk Flow (Lecture 12)

Steps involved

›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.

›Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the day.

›When transpiration is high, xylem sap is usually under tension, rather than under pressure, due to transpirational pull.

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:

*TACT Mechanism

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:

›Transpiration

›Adhesion

›Cohesion

›Tension

i. Transpiration

• involves the pulling of water up through the xylem of a plant
• Utilize the energy of evaporation and the tensile strength of water.

ii. Adhesion

• 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

iii. Cohesion

• 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”.

1. Tension

*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

1.Root Pressure

—Promotes uptake of water in the root

2.Transpiration pull

—Main force to ‘suck’ up the water

3.Capillary Action

—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

1.Using Aphids

• 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