Lecture 11: Transport in Plant
1) Transport in plants occurs on three levels:
(a) the uptake and loss of water and solutes by individual cells
(b) short-distance transport of substances from cell to cell at the level of tissues or organs
(c) long-distance transport of sap within xylem and phloem at the level of the whole plant.
The selective permeability of a plant cells plasma membrane controls the movement of solutes between the cell and the extracellular solution.
Molecules tend to move down their concentration gradient, and when this occurs across a membrane it is passive transport and occurs without the direct expenditure of metabolic energy by the cell.
Transport proteins embedded in the membrane can speed movement across the membrane.
In active transport, solutes are pumped across membranes against their electrochemical gradients.
The cell must expend metabolic energy, usually in the form of ATP, to transport solutes uphill - counter to the direction in which the solute diffuses.
Transport proteins that simply facilitate diffusion cannot perform active transport.
Active transporters are a special class of membrane proteins, each responsible for pumping specific solutes
The role of protons pumps in transport is a specific application of the general mechanism called chemiosmosis, a unifying principle in cellular energetics.
In chemiosmosis, a transmembrane proton gradient links energy-releasing processes to energy-consuming processes.
The ATP synthases that couple H+ diffusion to ATP synthesis during cellular respiration and photosynthesis function somewhat like proton pumps.
However, proton pumps normally run in reverse, using ATP energy to pump H+ against its gradient.
2) Differences in water potential drive water transport in plant cells
The survival of plant cells depends on their ability to balance water uptake and loss.
The net uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane.
In the case of a plant cell, the direction of water movement depends on solute concentration and physical pressure, together called water potential, abbreviated by the Greek letter psi.
Plant biologists measure psi in units called megapascals (abbreviated MPa), where one MPa is equal to about 10 atmospheres of pressure.
An atmosphere is the pressure exerted at sea level by an imaginary column of air - about 1 kg of pressure per square centimeter.
A car tire is usually inflated to a pressure of about 0.2 MPa and water pressure in home plumbing is about 0.25 MPa.
If a 0.1 M solution is separated from pure water by a selectively permeable membrane, water will move by osmosis into the solution.
Water will move from the region of higher psi (0 MPa) to the region of lower psi (-0.23 MPa).
The combined affects of pressure and solute concentrations on water potential are incorporated into the following equation:
psi = psiP + psis
Where psiP is the pressure potential and psis is the solute potential (or osmotic potential).
3. Both the symplast and the apoplast function in transport within tissues and
In one route, substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell by same mechanism.
This transmembrane route requires repeated crossings of plasma membranes.
The second route, via the symplast, requires only one crossing of a plasma membrane.
After entering one cell, solutes and water move from cell to cell via plasmodesmata.
The third route is along the apoplast, the extracellular pathway consisting of cell wall and extracellular spaces.
Water and solutes can move from one location to another within a root or other organ through the continuum of cell walls before ever entering a cell.
4. Bulk flow functions in long-distance transport
Water and solutes move through xylem vessels and sieve tubes by bulk flow, the movement of a fluid driven by pressure.
In phloem, for example, hydrostatic pressure generated at one end of a sieve tube forces sap to the opposite end of the tube.
In xylem, it is actually tension (negative pressure) that drives long-distance transport.
Transpiration, the evaporation of water from a leaf, reduces pressure in the leaf xylem.
This creates a tension that pulls xylem sap upward from the roots.
5. Root hairs, mycorrhizae, and a large surface area of cortical cells enhance water and mineral absorption
6. The ascent of xylem sap depends mainly on
and the physical properties of water. Xylem sap ascends by solar-powered bulk flow
Xylem sap flows upward to veins that branch throughout each leaf, providing each with water.
Plants loose an astonishing amount of water by transpiration, the loss of water vapor from leaves and other aerial parts of the plant.
An average-sized maple tree losses more than 200 L of water per hour during the summer.
The flow of water transported up from the xylem replaces the water lost in transpiration and also carries minerals to the shoot system.
The mechanism of transpiration depends on the generation of negative pressure (tension) in the leaf due to unique physical properties of water.
As water transpires from the leaf, water coating the mesophyll cells replaces water lost from the air spaces.
The remaining film of liquid water retreats into the pores of the cell walls, attracted by adhesion to the hydrophilic walls.
Cohesive forces in the water resist an increase in the surface area of the film.
Adhesion to the wall and surface tension causes the surface of the water film to form a meniscus, pulling on the water by adhesive and cohesive forces.
Long-distance transport of water from roots to leaves occurs by bulk flow, the movement of fluid driven by a pressure difference at opposite ends of a conduit, the xylem vessels or chains of tracheids.
The pressure difference is generated at the leaf end by transpirational pull, which lowers pressure (increases tension) at the upstream end of the xylem.
On a smaller scale, gradients of water potential drive the osmotic movement of water from cell to cell within root and leaf tissue.
Differences in both solute concentration and pressure contribute to this microscopic transport.
In contrast, bulk flow, the mechanism for long-distance transport up xylem vessels, depends only on pressure.
Bulk flow moves the whole solution, water plus minerals and any other solutes dissolved in the water.
The plant expends none its own metabolic energy to lift xylem sap up to the leaves by bulk flow.
The absorption of sunlight drives transpiration by causing water to evaporate from the moist walls of mesophyll cells and by maintaining a high humidity in the air spaces within a leaf.
Thus, the ascent of xylem sap is ultimately solar powered.
7. Phloem translocates its sap from sugar sources to sugar sinks. Pressure flow is the mechanism of translocation in angiosperms
In contrast to the unidirectional flow of xylem sap from roots to leaves, the direction that phloem sap travels is variable.
In general, sieve tubes carry food from a sugar source to a sugar sink.
A sugar source is a plant organ (especially mature leaves) in which sugar is being produced by either photosynthesis or the breakdown of starch.
A sugar sink is an organ (such as growing roots, shoots, or fruit) that is a net consumer or storer of sugar.
Phloem sap flows from source to sink at rates as great as 1 m/hr, faster than can be accounted for by either diffusion or cytoplasmic streaming.
Phloem sap moves by bulk flow driven by pressure.
Higher levels of sugar at the source lowers the water potential and causes water to flow into the tube.
Removal of sugar at the sink increases the water potential and causes water to flow out of the tube.
The difference in hydrostatic pressure drives phloem sap from the source to the sink
Plant transport of sugar occurs on three levels.
At the cellular level across membranes, sucrose accumulates in phloem cells by active transport.
At the short-distance level within organs, sucrose migrates from mesophyll to phloem via the symplast and apoplast.
At the long-distance level between organs, bulk flow within sieve tubes transports phloem sap from sugar sources to sugar sinks.