36Resource Acquisition and Transport in Vascular Plants

36 Transport Overview

• 1- uptake and loss of water and solutes by individual cells (root cells)

• 2- short-distance transport from cell to cell (sugar loading from leaves to phloem)

• 3- long-distance transport of sap within xylem and phloem in whole plant

Figure 36.1 The Pathways of Water and Solutes in the Plant

36Whole Plant Transport

• 1- Roots absorb water and dissolved minerals from soil

• 2- Water and minerals are transported upward from roots to shoots as xylem sap

• 3- Transpiration, the loss of water from leaves, creates a force that pulls xylem sap upwards

• 4- Leaves exchange CO2 and O2 through stomata

• 5- Sugar is produced by photosynthesis in leaves

• 6- Sugar is transported as phloem sap to roots and other parts of plant

• 7- Roots exchange gases with air spaces of soil (supports cellular respiration in roots)

36Figure 5.8 Osmosis Modifies the Shapes of Cells

36 Uptake and Movement of Water and Solutes

• For osmosis to occur, a membrane must be permeable to water but not to the solutes.

• Plant cells have a rigid cell wall.

• As water enters the cell, the plasma membrane presses against the cell wall, restricting expansion.

• The opposing force exerted by the rigid cell wall as water enters is called the pressure potential, or turgor pressure.

• Water enters a plant cell until the pressure potential exactly balances the solute potential. The cell is then called turgid.

36 Transport within tissues/organs

• Tonoplast vacuole

membrane

• Plasmodesmata : cytosolic connection

• Symplast route (lateral)cytoplasmic

continuum

• Apoplast route (lateral)continuum of

cell walls

• Bulk flow (long distance) movement of a

fluid by pressure (phloem)

36Figure 36.4 Apoplast and Symplast

36 Uptake and Movement of Water and Solutes

• The endodermis cell walls have Casparian strips—waxy, suberin-containing structures that form a hydrophobic belt sealing the cell and preventing movement of water and ions between the cells.

• The Casparian strips thus separate the apoplast of the cortex from the apoplast of the stele.

• Water and ions can enter the stele only by way of the symplast—by entering and passing through the endodermal cytoplasm.

36Figure 36.5 Casparian Strips

36 Transport of Xylem Sap

• Transpiration: loss of water vapor from leaves pulls water from roots (transpirational pull); cohesion and adhesion of water

• Root pressure: at night (low transpiration), roots cells continue to pump minerals into xylem; this generates pressure, pushing sap upwards; guttation

Figure 36.7 Guttation

36 Transport of Water and Minerals in the Xylem

• Eduard Strasburger cut trees at the base and placed the cut ends into a bucket of water and poison.

• Transport continued until the poison reached the leaves, at which point it stopped.

• His experiment established three important points:

“Pumping cells” are not responsible for transport.

The leaves play a crucial role in transport.

The roots are not the cause of transport.

36 Transport of Water and Minerals in the Xylem

• The transpiration–cohesion–tension mechanism:

• The concentration of water vapor is higher inside the leaf than outside, so water diffuses out of the leaf through the stomata. This process is called transpiration.

• This creates a tension in the mesophyll that draws water from the xylem of the nearest vein into the apoplast surrounding the mesophyll cells.

• The removal of water from the veins, in turn, establishes tension on the entire volume of water in the xylem, so the column is drawn up from the roots.

36Figure 36.8 The Transpiration–Cohesion–Tension Mechanism

36 Transport of Water and Minerals in the Xylem

• Hydrogen bonding between water molecules results in cohesion, the tendency of water molecules to stick to one another.

• The narrower the tube, the greater the tension the water column can stand.

• The water column is also maintained by adhesion of water molecules to the walls of the tube.

• This combination of cohesion and adhesion creates capillary action

36 Transport of Water and Minerals in the Xylem

• The key elements in water transport in xylem:

Transpiration

Tension

Cohesion

• The transpiration–cohesion–tension mechanism does not require energy.

• At each step, water moves passively toward a region with a more negative water potential.

36 Transpirational Control

• Photosynthesis-Transpiration compromise….

• Guard cells control the size of the stomata

• Xerophytes (plants adapted to arid environments)~ thick cuticle; small spines for leaves

36 Transpiration and the Stomata

• Leaf and stem epidermis has a waxy cuticle that is impermeable to water, but also to CO2.

• Stomata, or pores, in the epidermis allow CO2 to enter by diffusion.

• Guard cells control the opening and closing of the stomata.

• Most plants open their stomata only when the light is intense enough to maintain photosynthesis.

• Stomata also close if too much water is being lost.

36Figure 36.11 Stomata (Part 1)

36 Transpiration and the Stomata

• Opening closing and of the stomata are regulated by controlling K+ concentrations in the guard cells.

• Blue light activates a proton pump to actively pump protons out of the guard cells. The proton gradient drives accumulation of K+ inside the cells.

• Increasing K+ concentration makes the water potential of guard cells more negative, and water enters by osmosis.

• The guard cells respond by changing their shape and allowing a gap to form between them.

36Figure 36.11 Stomata (Part 2)

36 Transpiration and the Stomata

• The guard cells close when the process is reversed; when active transport of protons ceases. K+ diffuses out of the cell, and water follows.

• This occurs in the absence of blue light or when abscisic acid is present.

• Abscisic acid is produced by the mesophyll cells on hot, sunny, windy days so that guard cells will close the stomata to prevent water loss.

36Translocation of Phloem Sap• Translocation: food/phloem transport

• Sugar source: sugar production organ (mature leaves)

• Sugar sink: sugar storage organ (growing roots, tips, stems, fruit)

• 1- loading of sugar into sieve tube at source reduces water potential inside; this causes tube to take up water from surroundings by osmosis

• 2- this absorption of water generates pressure that forces sap to flow alon tube

• 3- pressure gradient in tube is reinforced by unloading of sugar and consequent loss of water from tube at the sink

• 4- xylem then recycles water from sink to source

36 Translocation of Substances in the Phloem

• Sugars, amino acids, some minerals, and other solutes are transported in phloem and move from sources to sinks.

• A source is an organ such as a mature leaf or a starch-storing root that produces more sugars than it requires.

• A sink is an organ that consumes sugars, such as a root, flower, or developing fruit.

• These solutes are transported in phloem, not xylem, as shown by Malpighi by girdling a tree.

Figure 36.12 Girdling Blocks Translocation in the Phloem

36 Translocation of Substances in the Phloem

• Plant physiologists have used aphids to collect sieve tube sap from individual sieve tube elements.

• An aphids inserts a specialized feeding tube, or stylet, into the stem until it reaches a sieve tube.

• Sieve tube sap flows into the aphid. The aphid is then frozen and cut away from its stylet, which remains in the sieve tube.

• Sap continues to flow out the sieve tube and can be collected and analyzed by the physiologist.

Figure 36.13 Aphids Collect Sieve Tube Sap

36 Translocation of Substances in the Phloem

• There are two steps in translocation that require energy:

Loading is the active transport of sucrose and other solutes into the sieve tubes at a source.

Unloading is the active transport of solutes out of the sieve tubes at a sink.

36 Translocation of Substances in the Phloem

• Sieve tube cells at the source have a greater sucrose concentration that surrounding cells, so water enters by osmosis. This causes greater pressure potential at the source, so that the sap moves by bulk flow towards the sink.

• At the sink, sucrose is unloaded by active transport, maintaining the solute and water potential gradients.

• This is called the pressure flow model.

Figure 36.14 The Pressure Flow Model

Table 36.1 Mechanisms of Sap Flow in Plant Vascular Tissues